Handbook on Calibration of Radiation Protection Monitoring Instruments

I N T E R N A T I O N A L A T O M I C E N E R G Y A G E N C Y , V I E N N A . 1 9 7 1

HANDBOOK ON CALIBRATION O F RADIATION P R O T E C T I O N MONITORING I N S T R U M E N T S

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REPUBLIC OF KOREA, REPUBLIC OF TUNISIA COSTA RICA KUWAIT TURKEY CUBA LEBANON UGANDA CYPRUS LIBERIA UKRAINIAN SOVIET SOCIALIST CZECHOSLOVAK SOCIALIST LIBYAN ARAB REPUBLIC REPUBLIC

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NIGERIA

The Agency's Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Headquarters of the Agency are situated in Vienna. Its principal objective is "to acce lerate and enlarge the contribution of atomic energy to peace , health and prosperity throughout the world"

© I A E A , 1971

Permission to reproduce or translate the information contained in this publication may be obtained by writing to the International Atomic Energy Agency, Karntner Ring 11, P.O. Box 590, A-1011 Vienna, Austria.

Printed by the IAEA in Austria

November 1971

T E C H N I C A L R E P O R T S SERIES No. 133

HANDBOOK ON CALIBRATION OF RADIATION PROTECTION MONITORING INSTRUMENTS

I N T E R N A T I O N A L A T O M I C E N E R G Y A G E N C Y VIENNA, 1971

HANDBOOK ON C A L I B R A T I O N O F R A D I A T I O N P R O T E C T I O N MONITORING I N S T R U M E N T S

IAEA, VIENNA, 1971 S T I / D O C / l O / 1 3 3

FOREWORD

T h e i n t e r p r e t a t i o n of p e r s o n n e l and a r e a m o n i t o r i n g r e s u l t s i s a n i m -p o r t a n t p a r t of t h e r a d i a t i o n p r o t e c t i o n of r a d i a t i o n w o r k e r s . C o r r e c t i n t e r p r e t a t i o n i s b a s e d on t h e a c c u r a t e c a l i b r a t i o n of r a d i a t i o n m e a s u r i n g i n s t r u m e n t s . T h e r e l i a b i l i t y and a c c u r a c y of t h e m e a s u r e m e n t s a r e a l s o p a r t l y b a s e d o n t h e p r o c e d u r e s a p p l i e d in c a l i b r a t i o n . T h e I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y , a w a r e of t h e i m p o r t a n c e of t h i s m a t t e r , d e c i d e d t o p r e p a r e a h a n d b o o k d e s c r i b i n g s o m e c a l i b r a t i o n f a c i l i t i e s a n d p r o c e d u r e s w i d e l y u s e d in d i f f e r e n t c o u n t r i e s . T o h e l p in t h e c o l l e c t i o n of i n f o r m a t i o n , abou t 150 q u e s t i o n n a i r e s w e r e s en t to d i f f e r e n t o r g a n i z a t i o n s . T h e r e s p o n s e w a s c o n s i d e r a b l e .

A f i r s t d r a f t of t h e h a n d b o o k w a s p r e p a r e d w i t h t h e h e l p of two c o n s u l -t a n t s , M r . H. V. L a r s o n of B a t t e l l e N o r t h - W e s t , R i c h l a n d , W a s h . , USA, and M r . I. M. G. T h o m p s o n of t h e C E G B B e r k e l e y N u c l e a r L a b o r a t o r i e s , B e r k e l e y , G l o s , U n i t e d K i n g d o m . M r . G. A. D o r o f e e v of t h e IAEA, w i t h t h e i r a s s i s t a n c e , d i r e c t e d t h e w o r k and h i m s e l f w r o t e a s e c t i o n of t h e b o o k . About f i f t y s p e c i a l i s t s i n d i f f e r e n t c o u n t r i e s w e r e a s k e d t o m a k e c o m m e n t s on t h e d r a f t , w h i c h w e r e t h e n u s e d to i m p r o v e t h e h a n d b o o k .

In a d d i t i o n t o t h a n k i n g t h e c o n s u l t a n t s , t h e A g e n c y w i s h e s to e x p r e s s i t s a p p r e c i a t i o n t o a l l o r g a n i z a t i o n s who c o n t r i b u t e d i n f o r m a t i o n , t o a l l s p e c i a l i s t s who c o m m e n t e d on t h e f i r s t d r a f t , and e s p e c i a l l y t o P r o f e s s o r D. N a c h t i g a l l of t h e P a d a g o g i s c h e H o c h s c h u l e R u h r , D o r t m u n d , F e d e r a l R e p u b l i c of G e r m a n y , a n d D r . P . N. K r i s h n a m o o r t h y of t h e B h a b h a A t o m i c R e s e a r c h C e n t r e , B o m b a y , Ind ia , f o r t h e i r s u g g e s t i o n s and v a l u a b l e c o m m e n t s .

CONTENTS

1. I N T R O D U C T I O N 1

1 . 1 . G e n e r a l 1 1 . 2 . P u r p o s e a n d s c o p e 1 1 . 3 . Q u a n t i t i e s , u n i t s and s y m b o l s 1

2. R E Q U I R E M E N T S O F A C A L I B R A T I O N L A B O R A T O R Y AND THE T E C H N I Q U E S T O B E USED 2

2 . 1 . P u r p o s e of c a l i b r a t i o n 2 2 . 2 . T e c h n i q u e s 4 2 . 3 . E l e c t r o m a g n e t i c r a d i a t i o n 4

2 . 3 . 1 . X - r a d i a t i o n 5 2 . 4 . B e t a r a d i a t i o n 21 2 . 5 . N e u t r o n r a d i a t i o n 23

2 . 5 . 1 . G e n e r a l c o n s i d e r a t i o n s 23 2 . 5 . 2 . C a l i b r a t i o n wi th s e a l e d n e u t r o n s o u r c e s 27 2 . 5 . 3 . C a l i b r a t i o n w i t h a n a c c e l e r a t o r s o u r c e 30 2 . 5 . 4 . Slow n e u t r o n f l u x d e n s i t i e s 33 2 . 5 . 5 . P r o b l e m s a s s o c i a t e d wi th the c a l i b r a t i o n of

d i f f e r e n t t y p e s of n e u t r o n i n s t r u m e n t s 34 2 . 6 . A l p h a r a d i a t i o n 35 2. 7. C a l i b r a t e d s o u r c e s 36

3 . C A L I B R A T I O N F A C I L I T I E S 39

3 . 1 . P e r s o n n e l d o s i m e t e r s ' 39 3 . 1 . 1 . B e t a - g a m m a d o s i m e t e r s 40 3 . 1 . 2 . N e u t r o n d o s i m e t e r s 46

3 . 2 . P o r t a b l e s u r v e y m e t e r s 46 3 . 2 . 1 . D o s e o r e x p o s u r e r a t e m o n i t o r i n g i n s t r u m e n t s . . . 47 3 . 2 . 2 . C o n t a m i n a t i o n m o n i t o r i n g i n s t r u m e n t s 50

3 . 3 . S i m i l a r i t y m e t h o d f o r s u b - r a n g e l i n e a r i t y c h e c k s 53

4 . E R R O R S AND Q U A L I T Y C O N T R O L 54

4 . 1 . U n c e r t a i n t y l i m i t s 54 4 . 2. S o u r c e s of e r r o r 55 4 . 3 . F r e q u e n c y of c a l i b r a t i o n 57 4 . 4 . C o n s t a n c y c h e c k s 58 4 . 5. E v a l u a t i o n of m e c h a n i c a l and e l e c t r i c a l p r o p e r t i e s 58

4 . 5 . 1 . M e c h a n i c a l e v a l u a t i o n 60 4 . 5 . 2 . E l e c t r i c a l e v a l u a t i o n 61 4 . 5 . 3 . E n v i r o n m e n t a l e v a l u a t i o n 62

5 . I N S T R U M E N T M A I N T E N A N C E AND R E P A I R 66

A P P E N D I X I: L i s t of p r i m a r y s t a n d a r d l a b o r a t o r i e s 6 7

A P P E N D I X II: USSR S t a t e S t a n d a r d . M o n i t o r i n g i n s t r u m e n t s f o r t h e m e a s u r e m e n t of e x p o s u r e s and e x p o s u r e r a t e s of g a m m a r a d i a t i o n wi th photon e n e r g y r a n g i n g f r o m 30 to 250 f J ( 0 . 2 - 1 . 5 MeV) . C h e c k i n g m e t h o d s 69

A P P E N D I X III: A t y p i c a l l a b o r a t o r y b u i l d i n g and s t a f f 77

R E F E R E N C E S 90

B I B L I O G R A P H Y 93

1. I N T R O D U C T I O N

1 . 1 . G e n e r a l

T h e p r o b l e m s a s s o c i a t e d wi th t h e u s e and c a l i b r a t i o n of r a d i a t i o n m e a s u r i n g i n s t r u m e n t s h a v e r e c e n t l y b e e n t h e s u b j e c t of m u c h n a t i o n a l and i n t e r n a t i o n a l i n t e r e s t and an ICRU r e p o r t i s s h o r t l y t o b e p u b l i s h e d on t h i s t o p i c . T h i s h a n d b o o k h a s a t t e m p t e d to avo id d u p l i c a t i n g t h i s w o r k , but t o d e a l in m o r e d e t a i l wi th t h e c a l i b r a t i o n a p p a r a t u s , a c c o m m o d a t i o n and s t a f f .

1 . 2 . P u r p o s e and s c o p e

T h i s h a n d b o o k i s p r o v i d e d a s a g u i d e f o r t h o s e who a r e e s t a b l i s h i n g o r o p e r a t i n g c a l i b r a t i o n f a c i l i t i e s f o r t h e i r r a d i a t i o n m o n i t o r i n g i n s t r u -m e n t s . T h e s o u r c e s of r a d i a t i o n and a s s o c i a t e d a p p a r a t u s and c a l i b r a t i o n t e c h n i q u e s p r e s e n t e d a r e i n t e n d e d a s e x a m p l e s of wha t e s t a b l i s h e d c a l i b -r a t i o n l a b o r a t o r i e s h a v e d e e m e d a d e q u a t e , but a r e no t to b e c o n s i d e r e d a s t h e o n l y s u i t a b l e m e t h o d s and i n s t r u m e n t s a v a i l a b l e f o r p r o p e r c a l i b r a t i o n . T h e r e a d e r ' s a t t e n t i o n i s d r a w n to t h e e x t e n s i v e b i b l i o g r a p h y found a t t h e end of t h i s h a n d b o o k f o r a l i s t of d o c u m e n t s wh ich i n c l u d e o t h e r s u f f i c i e n t c a l i b r a t i o n t e c h n i q u e s .

I n a d e q u a t e c a l i b r a t i o n f a c i l i t i e s h a v e in t h e p a s t c a u s e d l a r g e e r r o r s i n s o m e d o s e e s t i m a t e s . T h i s i s e s p e c i a l l y t r u e in t h e d e v e l o p i n g c o u n t r i e s w h e r e a d e q u a t e f u n d i n g and e x p e r t i s e a r e o f t e n u n a v a i l a b l e . T h e r e i s a t e n d e n c y f o r t h e b e g i n n i n g u s e r of r a d i a t i o n m o n i t o r i n g i n s t r u m e n t a t i o n to a s s u m e t h a t t h e m a n u f a c t u r e r p o s s e s s e s f a c i l i t i e s c a p a b l e of c a l i b r a t i n g t h e i n s t r u m e n t o v e r i t s c o m p l e t e d o s e - r a t e and e n e r g y r a n g e , and t h a t s p e c i f i c a t i o n s a r e a c c u r a t e l y r e p o r t e d . Such an a s s u m p t i o n c a n l e a d to l a r g e e r r o r s in i n t e r p r e t a t i o n .

B e c a u s e of t h e m u l t i t u d e of a p p l i c a t i o n s , e . g . in m e d i c i n e , r a d i o g r a p h y , a g r i c u l t u r e , f o r r a d i a t i o n m o n i t o r i n g i n s t r u m e n t a t i o n , i t i s i m p o s s i b l e in o n e h a n d b o o k to d e s c r i b e t h e c o m p l e t e c a l i b r a t i o n of a l l i n s t r u m e n t s . H o w e v e r , t h e p r i n c i p l e s d e s c r i b e d h e r e shou ld s e r v e a s a b a s i s f o r c a l i -b r a t i n g r a d i a t i o n p r o t e c t i o n i n s t r u m e n t s .

1 . 3 . Q u a n t i t i e s , u n i t s a n d s y m b o l s

To a s s i s t in t h e u s e of t h i s h a n d b o o k a l i s t of d e f i n i t i o n s of a n u m b e r of f u n d a m e n t a l q u a n t i t i e s r e l e v a n t t o t h e c a l i b r a t i o n of i n s t r u m e n t s a r e g i v e n . M o s t of t h e s e d e f i n i t i o n s h a v e b e e n t a k e n f r o m ICRU R e p o r t 11 [1]:

(1) T h e p a r t i c l e f l u e n c e ($) i s t h e q u o t i e n t of A N b y A a , w h e r e A N i s t h e n u m b e r of p a r t i c l e s w h i c h e n t e r a s p h e r e of c r o s s - s e c t i o n a l a r e a A a .

(2) T h e f l u e n c e r a t e o r f l ux d e n s i t y (<p) = A $ / A t w h e r e A $ i s t h e p a r t i c l e f l u e n c e in t i m e A t .

(3) T h e e x p o s u r e (X) = A Q / A m , w h e r e AQ i s t h e s u m of the e l e c t r i c a l c h a r g e s on a l l t h e i o n s of o n e s i g n p r o d u c e d in a i r when a l l t h e e l e c t r o n s ( n e g a t r o n s and p o s i t r o n s ) l i b e r a t e d by p h o t o n s in a v o l u m e e l e m e n t of a i r w h o s e m a s s i s A m a r e c o m p l e t e l y s t o p p e d in a i r . F o r a f u l l e r d i s c u s s i o n of t h i s u n i t s e e ICRU R e p o r t 11 [1], T h e s p e c i a l u n i t of e x p o s u r e , f o r wh ich t h e m a j o r i t y of g a m m a i n s t r u m e n t s a r e d e s i g n e d to m e a s u r e , i s t h e r o e n t g e n ( R ) .

1

1R = 2 . 5 8 X 1 0 " 4 C / k g

(4) E x p o s u r e r a t e = A X / A t w h e r e AX i s t h e e x p o s u r e in t i m e At . (5) T h e a v e r a g e e n e r g y (W) in a g a s p e r ion p a i r f o r m e d = E / N , w h e r e

N i s t h e a v e r a g e n u m b e r of ion p a i r s f o r m e d when a p a r t i c l e of i n i t i a l k i n e t i c e n e r g y E i s c o m p l e t e l y s t o p p e d by t h e g a s .

(6) T h e a b s o r b e d d o s e (D) = A E D / A m , w h e r e A E D i s t h e e n e r g y i m p a r t e d by i o n i z i n g r a d i a t i o n to t h e m a t t e r in a v o l u m e e l e m e n t and A m i s t h e m a s s of m a t t e r in t h a t v o l u m e e l e m e n t . T h e s p e c i a l u n i t of a b s o r b e d d o s e i s t h e r a d .

1 r a d = T 5 o J / k g = 1 0 0 e T g / s -

(7) T h e a b s o r b e d d o s e - r a t e = A D / A t w h e r e AD i s t h e e n e r g y a b s o r b e d in t i m e At .

(8) T h e d o s e e q u i v a l e n t (DE) in r e m s i s d e f i n e d a s t h e p r o d u c t of a b s o r b e d d o s e D, t h e q u a l i t y f a c t o r (QF) , t h e a b s o r d e d d o s e d i s t r i b u t i o n f a c t o r (DF) and o t h e r n e c e s s a r y m o d i f y i n g f a c t o r s

DE = D • (QF) • (DF) •

(9) T h e s p e c i f i c g a m m a - r a y c o n s t a n t ( r ) of a g a m m a - e m i t t i n g n u c l i d e i s t h e q u o t i e n t of £ 2 (AX/ At) by A, w h e r e A X / A t i s t h e e x p o s u r e r a t e a t a d i s t a n c e & f r o m a po in t s o u r c e of t h i s n u c l i d e h a v i n g an a c t i v i t y A . r c a n h a v e t h e s p e c i a l u n i t s of R . m . h " 1 . C i " 1 , o r in t he c a s e of r a d i u m with a f i l t e r t h i c k n e s s of 0 . 5 m m p l a t i n u m , R . m 2 . h " 1 . g

(10) k V c p : T h e p o t e n t i a l d i f f e r e n c e in k i l o v o l t s of a c o n s t a n t p o t e n t i a l g e n e r a t o r .

(11) Q F i s a q u a l i t y f a c t o r n o r m a l l y d e f i n e d a s in p a r a g r a p h (3), p. 101 of ICRU R e p . 10b [2] a s a f u n c t i o n of L E T .

(12) H V T i s t h a t t h i c k n e s s of m a t e r i a l n e c e s s a r y to p r o d u c e a r e d u c t i o n of 50% of t he i n i t i a l e x p o s u r e r a t e . (Th i s H V T i s a l s o c a l l e d t h e " f i r s t HVT ")

(13) T h e e f f e c t i v e e n e r g y of an X - r a y b e a m wi th a p a r t i c u l a r HVT i s d e f i n e d a s t h e e n e r g y of a m o n o e n e r g e t i c b e a m h a v i n g the s a m e H V T .

(14) T h e a c t i v i t y (A) of a quan t i ty of a r a d i o a c t i v e n u c l i d e i s t he q u o t i e n t of A N b y At w h e r e A N i s t h e n u m b e r of n u c l e a r t r a n s f o r m a t i o n s w h i c h o c c u r in t h i s q u a n t i t y in t i m e At .

2. R E Q U I R E M E N T S O F A C A L I B R A T I O N L A B O R A T O R Y AND T H E T E C H N I Q U E S T O B E USED

2. 1. P u r p o s e of c a l i b r a t i o n

B e f o r e g iv ing a d e s c r i p t i o n of t h e n e c e s s a r y e q u i p m e n t and t e c h n i q u e s t h a t s h o u l d be u s e d f o r c a l i b r a t i n g r a d i a t i o n i n s t r u m e n t s c o n s i d e r a t i o n s h o u l d f i r s t be g i v e n a s t o t h e p u r p o s e of a c a l i b r a t i o n . M e a s u r e m e n t s m a d e wi th t h e t y p e s of i n s t r u m e n t s t h a t a r e d i s c u s s e d in t h i s h a n d b o o k n o r m a l l y h a v e t h e o b j e c t of d e t e r m i n i n g t h e s t a t u s of an a r e a wi th s p e c i a l r e f e r e n c e to r a d i a t i o n d o s e s r e c e i v e d by a p e r s o n in t h e a r e a [3] ,

In a c c o r d a n c e wi th I C R P p u b l i c a t i o n 12 [ 3 a ] , u s u a l l y t h e u p p e r l i m i t s t o o r g a n d o s e s r a t h e r t h a n a c t u a l v a l u e s a r e a s s e s s e d w h e n t h e r e a d i n g s of d o s i m e t e r s a r e r e c o r d e d . N e v e r t h e l e s s , it h a s b e c o m e c o n v e n t i o n a l

2

t o n o t e and r e c o r d a l l r a d i a t i o n d o s e s above t h e d e t e c t i o n t h r e s h o l d of t he p e r s o n a l d o s i m e t e r . T h e r e f o r e I C R P r e c o m m e n d s t h a t i n d i v i d u a l m o n i -t o r i n g and a s s o c i a t e d r e c o r d s a r e not n e e d e d if t h e d o s e s a r e m o s t u n l i k e l y t o e x c e e d 3 / 1 0 of t he a n n u a l m a x i m u m p e r m i s s i b l e l e v e l s . In t h i s c a s e t h e u n c e r t a i n t i e s a c c e p t a b l e in r o u t i n e i n d i v i d u a l m o n i t o r i n g shou ld no t e x c e e d 50%. T h i s u n c e r t a i n t y i n c l u d e s e r r o r s due to v a r i a t i o n s in t h e d o s i m e t e r s e n s i t i v i t y wi th i n c i d e n t e n e r g y and d i r e c t i o n of i n c i d e n c e a s w e l l a s i n t r i n s i c e r r o r s in t h e d o s i m e t e r and i t s c a l i b r a t i o n . A l though t h e u n c e r t a i n t y l i m i t (50%) i s v e r y b r o a d , s p e c i a l c a r e s h o u l d be pa id t o t h e p r o p e r i n t e r p r e t a t i o n of t h e m e a s u r e m e n t r e s u l t s in o r d e r not t o e x c e e d t h e l i m i t .

E v e n wi th s o m e of t h e b e s t a v a i l a b l e d o s i m e t e r s , m e a s u r e m e n t s w i l l o n l y a t b e s t g ive an a p p r o x i m a t i o n of t h e d o s e . In t he c a s e of e l e c t r o -m a g n e t i c r a d i a t i o n , it h a s b e e n shown [4] t h a t e v e n if an i n s t r u m e n t m e a s u r e s e x p o s u r e v e r y a c c u r a t e l y , t h e f a c t o r r e q u i r e d to c o n v e r t t h i s to g ive a n i n d i c a t i o n of t h e d o s e to a p a r t i c u l a r o r g a n cou ld v a r y c o n s i d e r a b l y wi th pho ton e n e r g y and the d i r e c t i o n of t h e r a d i a t i o n u p o n t h e h u m a n body . F o r e x a m p l e , in t h e c a s e of t h e t e s t e s t he f a c t o r c a n v a r y f r o m a p p r o x i m a t e l y 0 . 0 5 f o r 3 0 - k e V p h o t o n s , w h e r e t h e i r r a d i a t i o n i s f r o m t h e b a c k , t o 1. 6 f o r 1 0 0 - k e V p h o t o n s in t he c a s e of a f r o n t a l i r r a d i a t i o n . It i s a l s o i m p o r t a n t t o r e a l i z e t h a t m o s t i n s t r u m e n t s r e s p o n d to a p a r t i c u l a r p h y s i c a l p a r a m e t e r of t h e r a d i a t i o n f i e l d , f o r e x a m p l e f l u e n c e , and w i l l not g ive a d i r e c t m e a s u r e of t h e d o s e .

T h e ch ie f o b j e c t i v e s of a c a l i b r a t i o n f a c i l i t y s h o u l d b e a s f o l l o w s :

(1) To e n s u r e t h a t an i n s t r u m e n t i s w o r k i n g p r o p e r l y ; (2) In t h e c a s e of an i n s t r u m e n t wi th no c a l i b r a t i o n a d j u s t m e n t , t o

e n s u r e t h a t e r r o r s in t h e i n s t r u m e n t r e a d i n g a r e r e v e a l e d ; f o r i n s t r u m e n t s p r o v i d e d wi th a c a l i b r a t i o n a d j u s t m e n t , t h i s m a y l e a d to an i m p r o v e m e n t in t h e o v e r a l l a c c u r a c y of t h e i n s t r u m e n t ;

(3) To u n d e r t a k e d e t a i l e d i n v e s t i g a t i o n s upon d i f f e r e n t d e s i g n s of i n s t r u m e n t s s o t h a t c o n f i d e n c e l i m i t s c a n be p l a c e d upon m e a s u r e m e n t s m a d e wi th t h e s e i n s t r u m e n t s . T h e s e t e s t s shou ld i n c l u d e a m e a s u r e m e n t of t h e i n s t r u m e n t ' s e n e r g y r e s p o n s e , i t s d i r e c t i o n a l r e s p o n s e , e n v i r o n -m e n t a l e f f e c t s , a s we l l a s a n u m b e r of o t h e r m e a s u r e m e n t s wh ich w i l l b e d e s c r i b e d in l a t e r s e c t i o n s of t h e handbook ;

(4) To g ive g u i d a n c e to u s e r s of i n s t r u m e n t s c o n c e r n i n g the b e s t a v a i l a b l e i n s t r u m e n t s f o r t h e i r r e q u i r e m e n t s and t o i n f o r m t h e m of a n y d e s i g n f e a t u r e s which a r e i m p o r t a n t when u s i n g t h e i n s t r u m e n t ;

(5) To p r o v i d e r a d i a t i o n s t a n d a r d s which wi l l a s s i s t in d e s i g n i n g and t e s t i n g n e w i n s t r u m e n t s .

A l t h o u g h t h i s h a n d b o o k d e s c r i b e s a c o m p r e h e n s i v e r a n g e of c a l i b r a t i o n e q u i p m e n t and t e c h n i q u e s , t h e s c o p e of any one p a r t i c u l a r f a c i l i t y wi l l , h o w e v e r , g r e a t l y d e p e n d upon t h e t y p e of i n s t r u m e n t s t h a t i t i s r e q u i r e d t o c a l i b r a t e a s we l l a s upon t h e c o n d i t i o n s u n d e r wh ich t h e i n s t r u m e n t s a r e l i k e l y t o be u s e d . T h e f a c i l i t i e s m a y r a n g e f r o m t h o s e w h e r e a s e r i e s of s i m p l e a s s e m b l i e s f o r r o u t i n e c h e c k i n g a r e e m p l o y e d to one w h e r e d e t a i l e d e n e r g y r e s p o n s e c u r v e s f o r v a r i o u s f o r m s of r a d i a t i o n c a n be m a d e a long wi th e n v i r o n m e n t a l t e s t s . T h e m o r e s o p h i s t i c a t e d of t h e s e f a c i l i t i e s w i l l h a v e a r a n g e of s e c o n d a r y s t a n d a r d s wh ich in g e n e r a l w i l l b e c o m p a r e d to n a t i o n a l p r i m a r y s t a n d a r d s wh ich a r e t h e m s e l v e s p r o b a b l y s u b j e c t to i n t e r n a t i o n a l i n t e r c o m p a r i s o n .

3

A d i s c u s s i o n i s g i v e n l a t e r in t h i s book on e r r o r s a s s o c i a t e d wi th t he c o m p a r i s o n s b e t w e e n t h e p r i m a r y and s e c o n d a r y s t a n d a r d , wi th t h e c a l i b r a t i o n of t h e r a d i a t i o n i n s t r u m e n t a g a i n s t t h e s e c o n d a r y s t a n d a r d and f i n a l l y wi th t h e " in f i e l d " m e a s u r e m e n t wi th t he i n s t r u m e n t .

2 . 2 . T e c h n i q u e s

T h e r e a r e b a s i c a l l y two t e c h n i q u e s f o r c a l i b r a t i n g r a d i a t i o n i n s t r u -m e n t s . T h e s e a r e t h e u s e of r a d i a t i o n f i e l d s w h o s e p r o p e r t i e s a r e w e l l known, and t h e u s e of l e s s w e l l - d e f i n e d r a d i a t i o n f i e l d s w h e r e t h e c a l i b r a -t i on i s by a s u b s t i t u t i o n t e c h n i q u e . In t he f i r s t t e c h n i q u e t h e r a d i a t i o n f i e l d c a n b e d e f i n e d f r o m a k n o w l e d g e of c e r t a i n p a r a m e t e r s of a s t a n d a r d s o u r c e o r by m e a s u r e m e n t s m a d e wi th a s e c o n d a r y s t a n d a r d . C e r t a i n f a c t o r s , s u c h a s a l a r g e a m o u n t of s c a t t e r e d r a d i a t i o n , c a n m a k e r e l i a b l e s p e c i f i c a t i o n of a r a d i a t i o n f i e l d d i f f i c u l t and in t h e s e c a s e s t he c a l i b r a t i o n i s b e s t m a d e by t h e s u b s t i t u t i o n t e c h n i q u e . An i d e n t i c a l t y p e of i n s t r u m e n t t o t h e one u n d e r t e s t , bu t t h a t h a s b e e n c a l i b r a t e d a g a i n s t p r i m a r y s t a n d a r d s , i s p l a c e d in t h e r a d i a t i o n f i e l d and i t s r e a d i n g n o t e d . T h e t e s t i n s t r u m e n t i s t h e n s u b s t i t u t e d f o r t h i s f i r s t i n s t r u m e n t and the c a l i b r a t i o n m a d e by c o m p a r i n g t h e two r e a d i n g s .

It i s d e s i r a b l e t h a t a l l c a l i b r a t i o n m e a s u r e m e n t s s h o u l d be t r a c e a b l e to p r i m a r y s t a n d a r d s , u s u a l l y v i a a s e c o n d a r y s t a n d a r d . A l though t h i s i s d e s i r a b l e i t w i l l b e s e e n f r o m t h e l a t e r s e c t i o n s of t h i s h a n d b o o k t h a t i t i s n o t a l w a y s p o s s i b l e t o do t h i s d i r e c t l y . F o r e x a m p l e , t h e p r i m a r y s t a n d a r d X - r a y e x p o s u r e r a t e s m a y be e i t h e r m u c h h i g h e r o r l o w e r t h a n t h o s e r e q u i r e d f o r t h e c a l i b r a t i o n of a g a m m a d o s i m e t e r and c o n s e q u e n t l y t h e s e c o n d a r y s t a n d a r d c a n n o t be u s e d to r e l a t e t h e d o s i m e t e r c a l i b r a t i o n to t h e p r i m a r y s t a n d a r d u n l e s s i t i s c a p a b l e of m e a s u r i n g a v e r y wide r a n g e of e x p o s u r e r a t e s .

T h e c a l i b r a t i o n of i n s t r u m e n t s d e s i g n e d to r e s p o n d to d i f f e r e n t f o r m s of r a d i a t i o n w i l l b e d i s c u s s e d u n d e r t h e f o l l o w i n g s e c t i o n s d e v o t e d to t h e s e r a d i a t i o n s .

2 . 3 . E l e c t r o m a g n e t i c r a d i a t i o n

To p r o v i d e r a d i a t i o n f i e l d s s u i t a b l e f o r c a l i b r a t i n g X - and g a m m a i n s t r u m e n t s o v e r t h e e n e r g y r a n g e f r o m a f e w keV to a b o u t 10 M e V r e q u i r e s a r a n g e of X - r a y e q u i p m e n t a s we l l a s a n u m b e r of d i f f e r e n t r a d i o n u c l i d e s o u r c e s . A wide r a n g e of s o u r c e s t r e n g t h s a r e a l s o r e q u i r e d s o t h a t t h e l e v e l of t he r a d i a t i o n f i e l d c a n be v a r i e d f r o m n a t u r a l b a c k g r o u n d l e v e l s u p t o v e r y h igh l e v e l s , s a y t h o u s a n d s of R • h"1 . T h e i d e a l r a d i a t i o n f i e l d f o r c a l i b r a t i o n i s one in which the r a d i a t i o n h a s a d i s c r e t e e n e r g y and an a c c u r a t e l y known e x p o s u r e r a t e . A d i s c r e t e e n e r g y i s i d e a l l y r e q u i r e d s i n c e t he s e n s i t i v i t y of m o s t i n s t r u m e n t s v a r i e s c o n s i d e r a b l y wi th photon e n e r g y . F o r e n e r g i e s a b o v e 300 keV, r a d i o n u c l i d e s o u r c e s a r e n o r m a l l y u s e d , but b e l o w t h i s e n e r g y v e r y few s u c h s o u r c e s h a v e long enough h a l f -l i v e s and in m a n y c a s e s t h e y e m i t p h o t o n s of m a n y d i f f e r e n t e n e r g i e s . B e l o w 300 keV, t h e r e f o r e , an X - r a y m a c h i n e i s u s u a l l y m o r e s u i t a b l e f o r c a l i b r a t i o n .

4

2 . 3 . 1 . X - r a d i a t i o n

T h e r e a r e two r a d i a t i o n s o u r c e s which c a n be u s e d f o r a s s e s s i n g the r e s p o n s e of i n s t r u m e n t s at e n e r g i e s f r o m a f e w k e V up to 300 keV; t h e s e a r e t h e f l u o r e s c e n c e X - r a y s and t h e u s e of h e a v i l y f i l t e r e d X - r a y b e a m s . T h e s e c o n d of t h e s e t e c h n i q u e s h a s h i s t o r i c a l l y b e e n u s e d by m a n y p e o p l e [ 5 - 7 e ] , but f l u o r e s c e n c e t e c h n i q u e s a r e ga in ing in p o p u l a r i t y .

^LAL1.!.1 I_ F i l t e r e d X-r«i<3 iji tk )n_

T h e t h e o r e t i c a l s p e c t r u m r e s u l t i n g f r o m o p e r a t i n g an X - r a y tube a t a c o n s t a n t p o t e n t i a l s h o w s a l i n e a r v a r i a t i o n in i n t e n s i t y . In R e f . [8] t h i s v a r i a t i o n i s e x p r e s s e d by t h e e q u a t i o n N ( E ) d E = K • i • (E 0 — E ) d E , w h e r e N(E) i s t h e photon f l ux p e r un i t e n e r g y i n t e r v a l in t h e e n e r g y r a n g e E t o E + d E , i i s t h e t u b e c u r r e n t , K i s a c o n s t a n t f o r t h e t u b e , and E Q i s t h e pho ton e n e r g y c o r r e s p o n d i n g to the c o n s t a n t p o t e n t i a l on t h e t u b e . C u r v e ' a ' in F i g . 1 s h o w s t h i s s p e c t r u m . H o w e v e r , in p r a c t i c e t h e i n h e r e n t f i l t r a t i o n of t h e t a r g e t , t u b e window and t h e c o o l a n t p r e f e r e n t i a l l y a b s o r b t h e l o w e r e n e r g y p a r t of t h e s p e c t r u m . A t y p i c a l s p e c t r u m ob t a ined with' i n h e r e n t f i l t r a t i o n i s shown a s c u r v e ' b ' in F i g . 1. T h i s t y p e of s p e c t r u m i s of l i t t l e p r a c t i c a l v a l u e b e c a u s e of t h e wide d i s t r i b u t i o n of photon e n e r g i e s , and f o r m a n y a p p l i c a t i o n s the e x p o s u r e r a t e s m a y a l s o be too h igh . A m o r e u s e f u l s p e c t r u m c a n be o b t a i n e d by p l a c i n g t h i c k f i l t e r s in the X - r a y b e a m p a t h . T h e s e a b s o r b e r s g r e a t l y r e d u c e t h e wid th of t h e s p e c t r u m , c u r v e ' c ' F i g . 1, by a b s o r b i n g t h e low e n e r g y c o m p o n e n t s . T h i s p r e f e r e n t i a l a b s o r p t i o n of t h e l o w e r e n e r g y pho tons i s due to t h e p h o t o e l e c t r i c a l e f f e c t which i s t h e d o m i n a t i n g a b s o r p t i o n p r o c e s s a t low e n e r g i e s . T h e r e s u l t i n g n a r r o w e r s p e c t r u m a l s o r e s u l t s in l o w e r e x p o s u r e r a t e s wh ich a r e m o r e u s e f u l f o r c a l i b r a t i n g r a d i a t i o n m o n i t o r i n g i n s t r u m e n t s . A r e c o m m e n d e d

UJ t— z

> -

£ z WITH INHERENT FILTRATION CURVE b

E, 0 X-RAY ENERGY

FIG. 1. Typical continuous X-ray spectra.

5

T A B L E I . D E T A I L S O F H E A V I L Y F I L T E R E D X - R A Y SOURCES

P a r t A

Additional filtration other than inherent

(mm)

Tube potential

M e a n 3

energy

(keV)

Width3

(keV)

Effective energy

(keV)

HVT

(mm) Pb Sn Cu A1

Tube potential

M e a n 3

energy

(keV)

Width3

(keV)

Effective energy

(keV)

HVT

(mm)

- - 0 . 2 5 1 . 0 32 29 4 29 2 . 2 3 A1

- - 1 . 0 1 . 0 53 46 8 46 0 . 2 4 5 C u

- - 2 . 5 1 .0 67 59 12 59 0 . 4 9 Cu

- 2 . 0 0 . 5 3 1 . 0 97 85 16 87 1 .33 Cu

- 4 . 0 1 . 0 5 1. 0 129 111 21 110 2 . 2 3 Cu

2 . 0 3 . 0 0 . 5 3 1 . 0 171 152 22 154 3 . 9 0 Cu

3 . 5 2 . 0 0 . 5 3 1 . 0 203 181 28 188 4 . 9 5 Cu

5 . 5 2 . 0 0 . 5 3 1 . 0 224 203 29 209 5 . 5 4 Cu

Inherent filtration of X-ray machine 2 mm A1 at 250 kV. Data taken from Ref. [ 6]

a Results of measurements taken from Ref. [ 12]

P a r t B

Width at Exposure rate Tube potential E m e a n a half height o A c at 1 m Filter

kV„ cp (keV) (AE) (keV) m (mR/h .mA) (mm)

40 29 12 5 . 1 30 360 O. lCu + 4 .0A1

60 48 17 7 . 6 29 1320 0 .6Cu + 4 .0A1

80 66 20 9 . 0 26 240 2 . OCu + 4 .0A1

100 83 23 11 .0 23 138 5. OCu + 4 .0A1

120 99 27 12 .7 22 210 l.OSn + 5.OCu + 4 .0A1

150 119 43 16 .5 29 1000 2. 5Sn + 4 .0A1

200 157 50 2 0 . 4 25 210 l.OPb + 3 .0Sn + 2.OCu + 4 .0A1

250 205 62 2 8 . 5 25 252 3.0Pb + 2 .0Sn + 4 .0A1

300 248 84 3 1 . 8 28 240 5.0Pb + 3 .0Sn + 4 .0A1

Data taken from Ref. [ 1 1 ] a Emean - the energy at the maximum of the spectrum b o = standard deviation of the Gauss distribution c A = AE/kV™ in °)o ("resolving power")

6

P a r t C

Tube potential

k V c p

X-ray current

(mA)

Primary filter3

(g / cm 2 )

Effective energy (keV)

Spectral peak (keV)

Spectral width at half -height

(keV)

Exposure rate at 50 cm

(mR/min)

100 15 2 . 0 Mo 84 86 24 70

130 15 4 . 0 Mo 108 108 30 67

170 15 8. 0 Mo 135 140 36 95

200 15 9. 0 Sn 162 164 39 72

250 10 15. 0 Sn 205 205 46 73

300 10 8. 0 Pb 242 247 60 68

a Filters placed between twoO. 8 - m m - t h i c k aluminium sheets Data taken from Ref. [ 13]

P a r t D

kV setting Filter materials and thickness (cm)

Average theoretical

energy (keV)

Experimental e f f ec t ive

energy (keV)

100 0. 0137 A1 + 0. 0457 Pb + 0. 196 Cu 85 88 - 92

150 0. 254 U + 0. 0368 Cd 115 100-127

200 0. 2208 Cd + 0. 0254 Pt 156 149-164

236 0. 0254 U + 0. 0457 Pb + 0. 0254 Pt + 0. 0368 Cd 185 187

Data taken from Ref. [ 14]

l i s t of s u i t a b l e f i l t e r s wi l l no t be g iven s i n c e t h e s p e c t r u m wi l l v a r y f r o m m a c h i n e to m a c h i n e d e p e n d i n g on the i n h e r e n t f i l t r a t i o n . I n f o r m a t i o n of t h e f i l t e r s u s e d a t v a r i o u s l a b o r a t o r i e s c a n b e found in t he p a p e r s l i s t e d a t t h e b e g i n n i n g of t h i s s e c t i o n and t h e s e h a v e b e e n s u m m a r i z e d in T a b l e I a l o n g wi th o t h e r p a r a m e t e r s of t h e s p e c t r u m s h a p e . T h e r e a r e , h o w e v e r , a n u m b e r of c r i t e r i a t h a t s h o u l d be e x a m i n e d b e f o r e f i n a l s e l e c t i o n of a f i l t e r , and t h e s e a r e now d i s c u s s e d .

S e c o n d a r y l o w e r e n e r g y p e a k s m a y a r i s e f r o m t h e r a d i a t i o n p r o d u c e d a t t h e K e d g e of t h e f i l t e r o r t a r g e t m a t e r i a l [ 9], T h e u s e of a t h i c k l e a d f i l t e r w h e n o p e r a t i n g t h e X - r a y s e t , f o r e x a m p l e a t 200 kV, m a y r e s u l t i n an u n w a n t e d s e c o n d a r y p e a k a t 88 k e V f r o m the K a l p h a r a d i a t i o n of l e a d . S e c o n d a r y f i l t e r s m a y be u s e d to a b s o r b t h i s K a l p h a r a d i a t i o n . A s u i t a b l e s p e c t r u m cou ld be o b t a i n e d by u s i n g a f i l t e r c o n s t r u c t e d f r o m l e a d + t i n + c o p p e r + a l u m i n i u m . T h e t in + c o p p e r + a l u m i n i u m i s u s e d to a b s o r b

7

t h e l e a d K a l p h a r a d i a t i o n , t h e c o p p e r + a l u m i n i u m t o a b s o r b t h e t i n 2 9 - k e V K a l p h a r a d i a t i o n , and t h e a l u m i n i u m f i l t e r i s u s e d t o a b s o r b t h e c o p p e r 9 - k e V K a l p h a r a d i a t i o n . T h e c h a r a c t e r i s t i c r a d i a t i o n f r o m a l u m i n i u m h a s a n e n e r g y of 1 . 6 k e V a n d i s r e a d i l y a b s o r b e d i n a f e w c e n t i m e t r e s of a i r . T h e m a t e r i a l s u s e d f o r f i l t e r s shou ld b e of h igh p u r i t y . T h e e f f e c t of i m p u r i t i e s in f i l t e r m a t e r i a l s h a s b e e n i n v e s t i g a t e d in R e f . [10] . If a f i l t e r c o m b i n a t i o n i s b e i n g u s e d t h a t h a s no t b e e n p r e v i o u s l y i n v e s t i g a t e d t h e n i t i s d e s i r a b l e t o m a k e a m e a s u r e m e n t of t he r e s u l t i n g s p e c t r u m . T h i s i s b e s t done u s i n g a N a l c r y s t a l s p e c t r o m e t e r o r a Ge (L i ) s p e c t r o m e t e r [11] . If t h e f i l t e r c o m b i n a t i o n s l i s t e d in T a b l e I a r e to b e u s e d t h e n t h e t o t a l f i l t r a t i o n , i n h e r e n t f i l t r a t i o n , p l u s a d d i t i o n a l f i l t r a t i o n s h o u l d be t h e s a m e in a l l c a s e s . T h i s i s p a r t i c u l a r l y i m p o r t a n t a t l o w e r X - r a y e n e r g i e s w h e n t h e r a t i o of i n h e r e n t t o a d d i t i o n a l f i l t r a t i o n i n c r e a s e s . If i t i s known f r o m p r e v i o u s s p e c t r u m m e a s u r e m e n t s t h a t t h e t o t a l f i l t r a t i o n , i n h e r e n t p l u s a d d i t i o n a l , w i l l g ive t h e r i g h t s p e c t r u m s h a p e t h e n t h e q u a l i t y of t h e b e a m p r o d u c e d f r o m u s i n g t h i s f i l t e r wi th a d i f f e r e n t X - r a y m a c h i n e i s b e s t d e t e r m i n e d f r o m a b s o r p t i o n m e a s u r e m e n t s . T h e t h i c k n e s s of a s u i t a b l e a b s o r b e r wh ich r e d u c e s t o 50% t h e i n i t i a l e x p o s u r e a s m e a s u r e d wi thou t a n y of t h i s a b s o r b e r p r e s e n t in t h e b e a m i s d e t e r m i n e d ; t h i s t h i c k n e s s i s t e r m e d t h e h a l f - v a l u e t h i c k n e s s (HVT) . S u i t a b l e a b s o r b e r s (and t u b e v o l t a g e s a b o v e w h i c h t h e y m a y b e u s e d ) a r e a l u m i n i u m (10 kV c p ) and c o p p e r (35 k V c p ) . F r o m t h e m e a s u r e d H V T f o r a l u m i n i u m o r c o p p e r t h e e f f e c t i v e e n e r g y of t h e X - r a y b e a m m a y b e d e d u c e d by r e f e r e n c e t o t he c o r r e s p o n d i n g a b s o r p t i o n d a t a .

T h e e f f e c t i v e e n e r g y of an X - r a y b e a m wi th a p a r t i c u l a r H V T i s d e f i n e d a s t h e e n e r g y of a m o n o e n e r g e t i c b e a m h a v i n g the s a m e H V T . T h e t e c h n i q u e f o r m e a s u r i n g t h e H V T h a s b e e n d i s c u s s e d in d e t a i l in ICRU H a n d b o o k 85 [2] a n d by T r o u t [2a ] , W h i l e d e t e r m i n i n g t h e H V T it i s e s s e n t i a l t o e n s u r e t h a t t h e t u b e v o l t a g e and c u r r e n t r e m a i n c o n s t a n t . If a b e a m m o n i t o r i s u s e d t o c h e c k t h e ou tpu t t h e n i t i s i m p o r t a n t t o c h e c k t h a t p l a c i n g t h e a b -s o r b e r s in t h e b e a m d o e s not c h a n g e t h e m o n i t o r r e a d i n g . T h e m o s t a p p r o -p r i a t e p o s i t i o n of t h e a b s o r b e r s i s ha l f w a y b e t w e e n t h e s o u r c e and t h e d e t e c t o r . T h e i o n i z a t i o n c h a m b e r u s e d to d e t e r m i n e t h e H V T s h o u l d be p l a c e d 100 c m f r o m t h e s o u r c e and t h e a b s o r b e r s 50 c m f r o m t h e s o u r c e . T h e v a r i a t i o n of s e n s i t i v i t y of t h i s i o n i z a t i o n c h a m b e r s h o u l d no t e x c e e d ±5% f o r t h e e n e r g y r a n g e of i n t e r e s t and i n a known m a n n e r so t h a t a n y a p p r o p r i a t e e n e r g y c o r r e c t i o n f a c t o r s m a y b e a p p l i e d .

A s c h e m a t i c d i a g r a m of a s u i t a b l e l a y o u t of t h e a p p a r a t u s f o r c a l i b r a -t i n g i n s t r u m e n t s wi th a n X - r a y m a c h i n e i s s h o w n in F i g . 2.

LEAD LEAD COLLIMATOR COLLIMATOR

SURVEY INTRUMENT

PROTECTIVE ifiiir

TV. CAMERA

X-RAY TUBE

SECONDARY STANDARD I0NISATI0N CHAMBER

a (a )

FIG. 2. Equipment used for calibration with filtered X-rays, (a) Schematic diagram

8

( b )

FIG.'2. Equipment used for ca l ib ra t ion with f i l te red X-rays ,

(b) Photograph

T h e u s e of h e a v y t i t r a t i o n s l e a d s t o two p r o b l e m s . T h e s e a r e :

(a) Any i n s t a b i l i t i e s in t h e g e n e r a t i n g v o l t a g e o r w a v e f o r m l e a d to l a r g e f l u c t u a t i o n s in t h e s p e c t r u m and t h e e x p o s u r e r a t e . C h a n g e s in t u b e v o l t a g e , kVcp , h e a t i n g and ag ing wi l l c a u s e t he r a d i a t i o n s p e c t r u m to c h a n g e a s w e l l a s t h e e x p o s u r e r a t e s ; a l t e r i n g t h e t u b e c u r r e n t a s a m e a n s of v a r y i n g t h e e x p o s u r e r a t e wi l l a l t e r t h e t u b e l o ad i n g , r e s u l t i n g a l s o in t h e f i r s t t y p e of c h a n g e . T h e m o s t r e l i a b l e way to ob t a in r e p r o d u c i b l e s p e c t r a i s t o i n s t a l l a d e v i c e f o r m e a s u r i n g t h e t u b e v o l t a g e , kVCp. T h en f o r a p a r t i c u l a r f i l t e r c o m b i n a t i o n t he s a m e s p e c t r u m w i l l be o b t a i n e d when v a r y i n g t h e t u b e c u r r e n t , p r o v i d e d t h a t t h e a p p l i e d v o l t a g e i s v a r i e d t o k e e p the kV c p c o n s t a n t . T h i s i s a m o r e f l e x i b l e s o l u t i o n f o r o b t a i n i n g d i f f e r e n t e x p o s u r e r a t e s t h a n t h a t o b t a i n e d by w o r k i n g wi th d i f f e r e n t d e g r e e s of f i l t r a t i o n .

(b) T h e e x p o s u r e r a t e s o b t a i n e d a r e v e r y m u c h l o w e r t h a n t h o s e u s e d f o r c a l i b r a t i o n a g a i n s t p r i m a r y s t a n d a r d s .

G e n e r a l l y c a l i b r a t i o n s m a d e a t e n e r g i e s f r o m 30 u p to 300 keV a t N a t i o n a l L a b o r a t o r i e s ( s e e A p p e n d i x I — L i s t of P r i m a r y L a b o r a t o r i e s ) a g a i n s t a p r i m a r y s t a n d a r d a r e m a d e a t h igh e x p o s u r e r a t e s . I d e a l l y ,

9

Lead and B r a s s C o l l i m a t o r

F r e e A i r Ion C h a m b e r f o r C a l i b r a t i o n

T o E l e c t r o m e t e r Chart R e c o r d e r

FIG. 3. Fluorescent X-ray exposure assembly.

e x p o s u r e - r a t e m e a s u r e m e n t s i n t h e h e a v i l y f i l t e r e d X - r a y b e a m s d e s c r i b e d a b o v e s h o u l d be m a d e wi th a s e c o n d a r y i n s t r u m e n t t h a t h a s b e e n c a l i b r a t e d a g a i n s t p r i m a r y s t a n d a r d s . H o w e v e r , s i n c e t h e e x p o s u r e l e v e l s u s e d a t t h e p r i m a r y c a l i b r a t i o n l a b o r a t o r y a r e n o r m a l l y v e r y m u c h g r e a t e r t h a n t h o s e b e i n g u s e d a t t h e s e c o n d a r y c a l i b r a t i o n l a b o r a t o r y , t h i s i s no t a l w a y s p o s s i b l e . A s o l u t i o n to t h i s p r o b l e m i s to u s e two i d e n t i c a l d e t e c t o r s , u s u a l l y g r a p h i t e o r t u f n o l c a v i t y c h a m b e r s . One d e t e c t o r i s c o n n e c t e d to an e l e c t r o n i c m e a s u r i n g s y s t e m c a p a b l e of d e a l i n g wi th t h e l a r g e r i o n i z a t i o n c u r r e n t s p r o d u c e d a t t h e p r i m a r y c a l i b r a t i o n l e v e l w h i l s t t h e s e c o n d d e t e c t o r h a s a m o r e s e n s i t i v e e l e c t r o n i c s y s t e m s o t h a t i t m a y b e u s e d to m e a s u r e l o w e r e x p o s u r e r a t e s . T h e f i r s t d e t e c t o r i s c a l i b r a t e d a g a i n s t t h e p r i m a r y s t a n d a r d and t h i s i s t h e n u s e d t o c a l i b r a t e t h e s e c o n d d e t e c t o r by c o m p a r i s o n m e a s u r e m e n t in t h e h e a v i l y f i l t e r e d X - r a y b e a m s . C o r r e c t i o n s s h o u l d b e a p p l i e d f o r t h e v a r i a t i o n in r e s p o n s e of t h e f i r s t d e t e c t o r w i th pho ton e n e r g y , s i n c e t h e p r i m a r y s p e c t r u m w i l l d i f f e r f r o m t h e s e c o n d a r y s p e c t r u m .

2.3.1_!_2 j_ F l u o r e s c e n c e _ X - r a d £ a t i o n

T h e f l u o r e s c e n c e s o u r c e of r a d i a t i o n h a s b e e n u s e d by m a n y a u t h o r s [13, 15 - 17, 17a] a s a r e f e r e n c e r a d i a t i o n b e l o w 100 keV. F i g u r e 3 s h o w s a s c h e m a t i c d i a g r a m of a t y p i c a l c a l i b r a t i o n s e t u p f o r t h e f l u o r e s c e n c e t e c h n i q u e . T h e fo i l u s e d t o p r o d u c e t h e K f l u o r e s c e n c e i s p l a c e d a t 45° t o t h e p r i m a r y b e a m , and t h e r e s u l t i n g f l u o r e s c e n t r a d i a t i o n i s m e a s u r e d a t 45° to t h i s fo i l , t h e t o t a l a n g l e b e t w e e n t h e p r i m a r y and s e c o n d a r y b e a m i s t h e r e f o r e 90° which e n s u r e s t h a t s c a t t e r e d r a d i a t i o n i s kep t to a m i n i m u m . T h e t e c h n i q u e i s a l i t t l e m o r e d i f f i c u l t t h a n t h e h e a v y f i l t r a t i o n m e t h o d and t h e e x p o s u r e r a t e s o b t a i n e d f o r s i m i l a r s o u r c e - d e t e c t o r d i s t a n c e s a r e a l s o l o w e r . A l i s t of s u i t a b l e r a d i a t o r m a t e r i a l s i s s h o w n in T a b l e II t a k e n f r o m R e f . [ 15] .

2 . 3 . 1 . 3 . G a m m a r a d i a t i o n

A b o v e 300 keV r a d i o n u c l i d e s o u r c e s a r e t he b e s t f o r m of r a d i a t i o n s o u r c e f o r c a l i b r a t i o n . An i d e a l r a d i o n u c l i d e s o u r c e s h o u l d e m i t p h o t o n s of a s i n g l e e n e r g y and h a v e a s u i t a b l y l ong h a l f - l i f e ; v e r y f ew s o u r c e s f u l f i l t h e s e r e q u i r e m e n t s bu t m a n y c a n n e v e r t h e l e s s s t i l l be u s e d a s s t a n d a r d s o u r c e s . W h e n p u r c h a s i n g t h e s o u r c e t h e s u p p l i e r s h o u l d be a s k e d f o r a c e r t i f i c a t e of e i t h e r t h e e f f e c t i v e r a d i o a c t i v e c o n t e n t , in g r a m s f o r r a d i u m and in c u r i e s f o r o t h e r i s o t o p e s , o r of t h e e x p o s u r e r a t e a t a c e r t a i n d i s -t a n c e f r o m t h e s o u r c e . If t h e s p e c i f i c g a m m a - r a y c o n s t a n t ( r ) i s to b e u s e d to c a l c u l a t e t h e e x p o s u r e r a t e a t d i f f e r e n t d i s t a n c e s f r o m t h e s o u r c e t h e n t h e d i m e n s i o n s of t he s o u r c e s h o u l d be s m a l l . T h e s o u r c e s h o u l d a l s o be u s e d u n d e r s i m i l a r c o n d i t i o n s a s t h o s e e m p l o y e d in d e t e r m i n i n g t h i s c o n s t a n t ; i n g e n e r a l t h i s m e a n s t h a t t h e r a d i a t i o n f i e l d f r o m t h e s o u r c e s h o u l d be u s e d u n c o l l i m a t e d , t h a t t h e s o u r c e - t o - d e t e c t o r d i s t a n c e s f r o m 30 to 100 c m s h o u l d be u s e d f o r s m a l l d e t e c t o r s , and t h a t t h e r a d i a t i o n f i e l d in u s e shou ld obey the i n v e r s e s q u a r e r e l a t i o n s h i p . T h i s l a t t e r r e q u i r e m e n t c a n b e m e t by s u p p o r t i n g t h e s o u r c e and i n s t r u m e n t by a m i n i m u m of s c a t t e r i n g m a t e r i a l and a t a d i s t a n c e of 1 m o r m o r e a b o v e t h e g r o u n d ;

11

T A B L E II. D E T A I L S O F F L U O R E S C E N T X - R A Y S O U R C E S a

Atomic number Radiator material Radiator^ thickness

( g / cm 2 )

Ktxj (keV)

K a , (keV)

K B !

(keV) KB 2

(keV)

Weighted 0

average energy

(keV)

Al. L-l ine

filter thickness (g / cm 2 )

Scattered radiation

in fluorescent source (%)

Exposure^ rate mR/min

at 50 cm

29 Cu 0. 014 8 .047 8. 027 8 . 9 0 4 8 . 9 7 6 8 . 1 9 0. 006 4 18

40 Zr 0. 045 15. 774 15. 690 17. 666 17. 969 16. 07 0 . 0 5 5 2 47

47 Ag 0. 074 22. 162 21. 988 2 4 . 9 4 2 25. 454 2 2 . 5 9 0. 103 2 60

56 Ba 0 . 1 3 32. 191 31 .815 36. 376 37. 255 3 2 . 8 1 0 . 1 0 3 4 55

64 Gd2 . ° s 0. 23 42. 983 42. 280 48. 718 4 9 . 9 6 1 43. 78 0 . 1 0 3 4 43

66 Dy2 . ° 3 0. 26 4 5 . 9 8 5 45. 193 52. 178 5 3 . 4 9 1 46. 83 0 . 2 1 5 4 38

68 Er, . ° 3

0. 30 49. 099 48. 205 55. 690 5 7 . 0 8 8 49. 99 0 . 2 1 5 5 35

70 Yb ; . ° 3 0 .35 52. 360 51. 326 5 9 . 3 5 2 60 .959 53. 28 0 . 4 4 1 7 24

72 Hf . ° 2

0. 40 55. 757 54. 579 63. 209 64. 936 56. 79 0 . 4 4 1 7 22

77 Ir 0. 55 64. 886 63. 278 73. 549 75. 605 65. 94 0 . 4 4 1 12 19

82 Pb 0. 72 74. 957 72. 794 8 4 . 9 2 2 87. 343 76. 08 0 . 4 4 1 13 18

90 Th 1. 50 93 .334 89. 942 105. 592 108. 67 94. 52 1 .446 - 1 5 12

92 U 1. 50 98 .428 94. 648 111. 289 114. 549 99. 61 1.446 - 1 5 12

a Data taken from Ref. [ 15] k For oxides only the mass of rare earth is included c Assuming relative intensities of 1. 0, 0. 5, 0. 25 and 0. 05 for KtXj, K a „ KB t , KB2 , respectively ^ Primary source 300 kV and 15 mA

F i g . 4 s h o w s t h e t y p e of s t a n d s t h a t a r e s u i t a b l e . D e p a r t u r e s f r o m t h e i n v e r s e s q u a r e l a w r e l a t i o n s h i p due to s c a t t e r e d r a d i a t i o n c a n be i n v e s t i g a t e d in t he fo l l owing w a y . T h e l i n e a r i t y of an i n s t r u m e n t wi th v a r y i n g e x p o s u r e r a t e i s f i r s t c h e c k e d s o t h a t any n o n - l i n e a r i t y in i t c a n b e c o r r e c t e d . T h i s i n s t r u m e n t shou ld h a v e a p h y s i c a l l y s m a l l d e t e c t o r and i t s s e n s i t i v i t y s h o u l d v a r y on ly s l i g h t l y wi th photon e n e r g y . T h e i n s t r u m e n t and s o u r c e a r e p l a c e d on t h e i r s u p p o r t s t a n d s and the d i s t a n c e b e t w e e n t h e m i s kep t f i x e d . S o u r c e s of t h e s a m e i s o t o p e bu t h a v i n g v a r y i n g known s t r e n g t h s , a r e t h e n p l a c e d in t he s o u r c e s t a n d ( s e e s e c t i o n 3. 3) . If t h e i n s t r u m e n t h a s a l i n e a r e x p o s u r e r a t e r e s p o n s e t h e n t h e r e a d i n g s wi l l be p r o p o r t i o n a l t o t h e s o u r c e s t r e n g t h s . U s i n g a s i n g l e s o u r c e t h e d i s t a n c e of t h e i n s t r u m e n t f r o m t h e s o u r c e i s now v a r i e d and f o r t h e i n v e r s e s q u a r e r e l a t i o n s h i p to ho ld , t he i n s t r u m e n t r e a d i n g a f t e r c o r r e c t i o n f o r n o n - l i n e a r i t y s h o u l d b e p r o p o r t i o n a l t o t h e r e c i p r o c a l of t he d i s t a n c e s q u a r e d . T h e d i s t a n c e s o v e r w h i c h t h i s i s o b e y e d a r e no ted and t h e g a m m a s u r v e y i n s t r u m e n t s s h o u l d a l w a y s be c a l i b r a t e d w i th in t h e s e l i m i t s . T h e s o u r c e s t r e n g t h s h o u l d be v a r i e d r a t h e r t h a n d i s t a n c e s b e u s e d w h e r e an a p p r e c i a b l e s c a t t e r c o r r e c t i o n would h a v e to be a p p l i e d . To p r o v i d e a d e q u a t e p r o t e c t i o n f o r t h e c a l i b r a t i o n s t a f f , when u s i n g s o u r c e s w h o s e o u t p u t s a r e in e x c e s s of abou t 10 m C i ,

FIG.4. Calibration stands and equipment for routine gamma calibration (a) Photo of stand

13

FIG. 4. Calibration stands and equipment for routine gamma calibration (b) Outdoor system

Adjus tab le Support J i g

/ R e m o v a b l e

C a l i b r a t e d D i s t a n c e M e a s u r e m e n t Rod

S o u r c e S t o r a g e C a s k

\ T r o l l e y T r a c k

Mobi l e T r o l l e y

FIG. 4. Cal ibra t ion stands and equ ipmen t for rout ine g a m m a ca l ib ra t ion (c) Diagram of equ ipment

T A B L E III. G A M M A - R A Y SOURCES S U I T A B L E F O R C A L I B R A T I O N 3

Radionuclide Effect ive

quantum energy (keV)

H a l f - l i f e Spec i f ic gamma-ray

constant R.m 2 / C i . h

, , 5 i 35 5 9 . 2 d 0 . 0 7 0

A m 60 458 yr 0 . 0 1 2 9 57 -Co 122 269 d 0 . 0 9 7

m m In 192 50 d 0 . 0 4 2 7

203 H g 279 4 7 . 1 d 0 . 1 1 9

192 Ir 300-600 7 4 . 3 d 0 . 4 4

1 9 8AU 412 2 . 7 d 0 . 2 3 1

Cs 662 2 9 . 9 yr 0 . 3 2 3 60 „ Co 1250 5 . 2 3 yr 1 . 3 0 226

Ra 180-2200 1608 yr 0 . 8 2 5 b

24 Na 1380 and 2800 15 h 1 . 8 3

a Data taken from Ref. [ 79] b

0 . 5 m m Pt fi ltration, in equilibrium with daughter products

p n e u m a t i c o r o t h e r r e m o t e s o u r c e p o s i t i o n i n g d e v i c e s s h o u l d be u s e d . F o r v e r y l a r g e ou tpu t s o u r c e s t he r a d i a t i o n f i e l d s f r o m t h e m s h o u l d be m e a s u r e d u s i n g a s e c o n d a r y s t a n d a r d c a v i t y i o n i z a t i o n c h a m b e r and d u r i n g c a l i b r a t i o n s a l l t h e i n s t r u m e n t s s h o u l d be v i e w e d r e m o t e l y .

A l i s t of s u i t a b l e s o u r c e s i s g iven in T a b l e III t o g e t h e r wi th t h e i r r e l e v a n t c a l i b r a t i o n d a t a . A l though 2 2 6 R a , e n c a p s u l a t e d in 0 . 5 m m P t , . e m i t s p h o t o n s of d i f f e r e n t e n e r g i e s i t i s s t i l l a u s e f u l s t a n d a r d s i n c e i t i s u s u a l l y one of t h e m o s t a c c u r a t e l y c a l i b r a t e d s o u r c e s and a l s o any s i g n i f i c a n t c o r r e c t i o n s f o r s o u r c e d e c a y a r e u s u a l l y n e g l i g i b l e . C a r e s h o u l d be t a k e n not to d a m a g e t h e r a d i a t i o n s o u r c e and s e v e r a l c o u n t r i e s h a v e a s t a t u t o r y r e q u i r e m e n t to p e r i o d i c a l l y t e s t s e a l e d s o u r c e s .

T h e v a l u e of t h e s o u r c e c o n t e n t u s e d to e s t i m a t e t h e e x p o s u r e r a t e f r o m t h e s p e c i f i c g a m m a - r a y c o n s t a n t shou l d be t h e e f f e c t i v e s o u r c e c o n t e n t ; t h i s wi l l a l l o w f o r t he e f f e c t of t h e s o u r c e e n c a p s u l a t i o n .

T h e s p e c i f i c g a m m a - r a y c o n s t a n t , F , i s a m e a s u r e of t h e e x p o s u r e r a t e f r o m an u n f i l t e r e d u n i t po in t s o u r c e and t h i s i m p l i e s t h a t t h e pho ton f r o m on ly t h e s o u r c e c o n t r i b u t e s to t he e x p o s u r e r a t e . T h r e e o t h e r s o u r c e s of r a d i a t i o n c a n , h o w e v e r , c o n t r i b u t e to t h e e x p o s u r e r a t e . T h e s e a r e a n n i h i l a t i o n r a d i a t i o n , c h a r a c t e r i s t i c X - r a y s , and b r e m s s t r a h l u n g . M o s t of t h e v a l u e s of F s h o w n in T a b l e III i n c l u d e t h e c o n t r i b u t i o n f r o m t h e a n n i h i l a t i o n r a d i a t i o n .

T h e c h a r a c t e r i s t i c X - r a y s e m i t t e d f o r i n t e r n a l c o n v e r s i o n o r e l e c t r o n c a p t u r e and t h e e f f e c t of b r e m s s t r a h l u n g a r e not i n c l u d e d .

16

2 . 3 . 1 . 4 . S e c o n d a r y s t a n d a r d s , c a l i b r a t i o n and a s s e s s m e n t p r o b l e m s a s s o c i a t e d wi th e l e c t r o m a g n e t ^ r a d i a t i o n

F o r b o t h t h e f l u o r e s c e n c e a n d t h e h e a v y f i l t r a t i o n t e c h n i q u e s t h e X - r a y m a c h i n e s h o u l d b e o p e r a t e d i n a s h i e l d e d r o o m and wi th a p r o p e r s y s t e m of i n t e r l o c k s . I n f o r m a t i o n and r e c o m m e n d a t i o n s on t h e s a f e o p e r a t i o n of X - r a y m a c h i n e s a r e d e a l t w i th i n N B S H a n d b o o k s N o s 76, 93 and N C R P H B 33 [ 18 - 20].

T h e u n i f o r m i t y of t h e b e a m s h o u l d b e c h e c k e d a t v a r i o u s d i s t a n c e s f r o m t h e f o c a l s p o t of t h e X - r a y t a r g e t in a p l a n e a t r i g h t a n g l e s t o t h e b e a m to m a k e s u r e t h a t t h e r e a r e no s i g n i f i c a n t v a r i a t i o n s . A p h y s i c a l l y s m a l l i o n i z a t i o n c h a m b e r wi th a f l a t e n e r g y r e s p o n s e s h o u l d b e u s e d . T h e s e m e a s u r e m e n t s s h o u l d a l s o be u s e d to c h e c k f o r a n y d e p a r t u r e s f r o m t h e i n v e r s e s q u a r e r e l a t i o n s h i p t h a t w i l l i n d i c a t e t h e p r e s e n c e of s c a t t e r e d r a d i a t i o n a n d t h i s c a n p r o b a b l y b e r e d u c e d by i m p r o v i n g t h e c o l l i m a t i n g s y s t e m and s h i e l d i n g t h e X - r a y t u b e . L a r g e r d i s t a n c e s f r o m t h e f o c a l s p o t b a c k -s c a t t e r f r o m t h e end w a l l of t h e r o o m m a y m a k e a c o n t r i b u t i o n t o t h e m a i n X - r a y b e a m and w o r k in s u c h r e g i o n s s h o u l d n o r m a l l y b e a v o i d e d .

A s u b s t i t u t i o n t e c h n i q u e s h o u l d b e u s e d f o r c a l i b r a t i n g i n s t r u m e n t s wi th X - r a y s . T h e e x p o s u r e r a t e i s f i r s t m e a s u r e d u s i n g t h e s e c o n d a r y s t a n d a r d i n s t r u m e n t ; d u r i n g t h i s d e t e r m i n a t i o n t h e m o n i t o r i o n i z a t i o n c h a m b e r i s u s e d to c h e c k t h e l e v e l of t h e X - r a y o u t p u t . T h e t e s t i n s t r u m e n t i s t h e n s u b s t i t u t e d in t h e s a m e p o s i t i o n ( i . e . t h e g e o m e t r i c a l c e n t r e of d e t e c t o r a t s a m e d i s t a n c e f r o m t h e f o c a l s p o t a s t h e c e n t r e of t h e s e c o n d a r y s t a n d a r d ) and i t s r e a d i n g t a k e n , v e r i f y i n g t h a t t h e b e a m m o n i t o r r e a d i n g h a s no t c h a n g e d . T h e u s e of a r e m o t e p o s i t i o n i n g d e v i c e f o r t h e e x c h a n g e of t h e t e s t i n s t r u m e n t and t h e s e c o n d a r y i n s t r u m e n t a l l o w s t h i s c o m p a r i s o n to b e m a d e wi thou t h a v i n g to t u r n off t h e X - r a y s e t and e n t e r t h e s h i e l d e d r o o m . A p a r t f r o m s p e e d i n g u p c a l i b r a t i o n m e a s u r e m e n t , c h a n g e s in t h e X - r a y m a c h i n e ou tpu t a r e l e s s l i a b l e t o o c c u r . D u r i n g t h e i n t e r c o m p a r i s o n t h e kVCp s h o u l d a l s o b e kep t c o n s t a n t . T h e t e s t i n s t r u m e n t r e a d i n g s h o u l d b e o b s e r v e d r e m o t e l y e i t h e r w i th t e l e v i s i o n , t h r o u g h a l e a d g l a s s window, o r v i a a s y s t e m of m i r r o r s .

T h e r e a r e two t y p e s of s e c o n d a r y s t a n d a r d s t h a t a r e s u i t a b l e f o r X - r a y c a l i b r a t i o n ; t h e s e a r e a f r e e - a i r i o n i z a t i o n c h a m b e r [21, 21a] and a c a v i t y i o n i z a t i o n c h a m b e r . Of t h e two , t h e c a v i t y c h a m b e r i s t h e s i m p l e r t o o p e r a t e , f o r m e a s u r e m e n t of X - r a y s f r o m abou t 50 t o 300 keV t h e d i m e n s i o n s of t h e f r e e - a i r i o n i z a t i o n c h a m b e r h a v e to be l a r g e , wh ich p r e s e n t s g r e a t e r d i f f i c u l t i e s w i th r e m o t e p o s i t i o n i n g . I o n i z a t i o n c h a m b e r s w h o s e r e s p o n s e s v a r y by m o r e t h a n 10% o v e r t h e e n e r g y r a n g e 10 to 300 k e V s h o u l d no t be u s e d a s s e c o n d a r y s t a n d a r d s . It i s a l s o a d v i s a b l e to u s e a s t a n d a r d r a d i o -n u c l i d e s o u r c e to m a k e p e r i o d i c c o n s t a n c y c h e c k s of t h e s t a n d a r d b e t w e e n i t s p e r i o d i c c a l i b r a t i o n a g a i n s t t h e p r i m a r y s t a n d a r d . B e l o w 50 k e V e i t h e r a t h i n - w a l l e d i o n i z a t i o n c h a m b e r o r a f r e e - a i r i o n i z a t i o n c h a m b e r s h o u l d be u s e d . W h e r e t h e s t a n d a r d i s a n u n s e a l e d c h a m b e r t h e n c o r r e c t i o n f a c t o r s s h o u l d be a p p l i e d f o r p r e s s u r e , t e m p e r a t u r e and h u m i d i t y if t h e s e d i f f e r f r o m t h o s e r e p o r t e d in t h e c a l i b r a t i o n a g a i n s t t h e p r i m a r y s t a n d a r d [22] ,

F i g u r e 5 i s a d i a g r a m a t i c r e p r e s e n t a t i o n of a l i k e l y r e l a t i o n s h i p b e t -w e e n t h e v a r i o u s r a d i a t i o n s o u r c e s and t h e p r i m a r y and s e c o n d a r y s t a n d a r d s . A l t h o u g h p a r t of t h i s h a s b e e n b a s e d on t h e e x i s t i n g c o n d i t i o n s w i t h i n t h e U n i t e d K i n g d o m t h e e x a c t d e t a i l s of s u c h a c h a r t w i l l v a r y f r o m c o u n t r y to c o u n t r y a s w e l l a s f r o m one c a l i b r a t i o n l a b o r a t o r y to a n o t h e r . A n u m b e r

17

X-ray Primary standard free air ionisation chamber (high exposure rates)

226 Y Primary standard, Ra source 1 226„

Secondary standard, free air ionisation chamber.

1st secondary cavity ionisation chamber standard with insensitive electronics

Secondary standard, Ra source ^ 226 Reference Ra Source

Primary standard 2MV X-rays, cavity ionisation chamber

20 keV

(low exposure rates) I

2nd secondary cavity ionisation chamber standard with sensitive electronics

100 keV

|Fluorescent X-ray source] I Heavily filtered X-ray source]

30 keV

Secondary standard

300 keV

(free air ionisation) ( chamber ^ ) and (2nd secondary cavity )

(ionisation chamber ) (standard with sensitive) (electronics )

\

Reference ^ C O source

Absolute measurement \ 6 0 - . . Co activity

198. 137„ 226_ Au Cs Ra

0.41MeV 0.66MeV 0.8MeV

60 r C o

1,25MeV

Specific gamma ray constant ( r )

Associated particle counting and cavity ionisation chamber with sensitive electronics

Energy response determinations or calibration of instruments

FIG. 5. Schematic diagram of X- and y-sources and their standards.

1-5

10 END CAP ON

0-5 •

0-25 0-5 075 PHOTON ENERGY (MeV)

-^VAW ' 6 MeV

FIG. 6. A typical response curve for an ionization chamber exposure-rate meter.

of n e w p r i m a r y s t a n d a r d s w i th in t h e U K a r e a l s o u n d e r d e v e l o p m e n t and when t h e s e b e c o m e a v a i l a b l e t h e c h a r t would b e c o n s i d e r a b l y s i m p l i f i e d [23] , T h e t y p e of e n e r g y r e s p o n s e t h a t would be o b t a i n e d f o r an i o n i z a t i o n c h a m b e r i n s t r u m e n t f r o m u s i n g t h e s e s o u r c e s i s shown in F i g . 6 and t h e s e t y p i c a l r e s u l t s a r e u s e d to i l l u s t r a t e a n u m b e r of i m p o r t a n t p o i n t s .

O n c e t h e e n e r g y r e s p o n s e of t h e i n s t r u m e n t i s known t h i s i n f o r m a t i o n s h o u l d be u s e d not on ly to a s s i s t in t he i n t e l l i g e n t u s e of t h e i n s t r u m e n t bu t a l s o d u r i n g i t s c a l i b r a t i o n . S i n c e one of t h e m o s t i m p o r t a n t p a r t s of a c a l i b r a t i o n i s t he a d j u s t m e n t of an i n s t r u m e n t , a t t e m p t s s h o u l d a l w a y s b e m a d e to a d j u s t t h e i n s t r u m e n t if t h i s would l e a d to an i m p r o v e m e n t in t h e o v e r a l l r e s p o n s e of t h e i n s t r u m e n t . T h u s , if a n i n s t r u m e n t h a s i t s m a x i m u m r e s p o n s e a t 100 k e V ( s e e F i g . 6) t h e n a d j u s t i n g i t to r e a d c o r r e c t l y a t t h i s e n e r g y would be of l i t t l e v a l u e a s t he i n s t r u m e n t would u n d e r - r e a d a t a l l o t h e r e n e r g i e s . W h e r e a m u l t i - r a n g e i n s t r u m e n t h a s c a l i b r a t i o n a d j u s t m e n t s f o r e a c h i n d i v i d u a l r a n g e t h e n t h e s e s h o u l d a l l be a d j u s t e d . C h e c k s s h o u l d be m a d e to s e e t h a t a d j u s t i n g one r a n g e h a s no t a f f e c t e d t h e c a l i b r a t i o n of o t h e r r a n g e s . F o r a m u l t i - r a n g e i n s t r u m e n t wi th on ly a s i n g l e c a l i b r a t i o n a d j u s t m e n t , t h e n a c h o i c e e x i s t s . If i t i s known t h a t o n e p a r t i c u l a r r a n g e i s m o s t f r e q u e n t l y u s e d t h e n i t i s p r o b a b l y b e s t t o s e t t h i s r a n g e c o r r e c t l y , p r o v i d e d t h a t in do ing s o s e r i o u s e r r o r s a r e not i n t r o d u c e d in t h e o t h e r r a n g e s . If it i s now known on w h i c h r a n g e t h e i n s t r u m e n t i s m o s t l i k e l y to b e u s e d , t h e n t h e i n s t r u m e n t s h o u l d b e a d j u s t e d t o m i n i m i z e t h e e r r o r s o v e r t h e c o m p l e t e e x p o s u r e r a t e r a n g e .

T h e t y p e of s p e c t r u m t h a t t h e i n s t r u m e n t i s t o b e u s e d in m a y a l s o i n f l u e n c e t h e i n i t i a l t e s t s t h a t a r e m a d e on t h e i n s t r u m e n t a s w e l l a s p o s s i b l y t h e e n e r g y at w h i c h it i s c a l i b r a t e d . F i g u r e 6 w i l l b e u s e d to i l l u s t r a t e t h i s p o i n t . M a n y r e a c t o r s p r o d u c e s i g n i f i c a n t a m o u n t s of 6. 1 3 - M e V g a m m a r a d i a t i o n . In g a s - c o o l e d r e a c t o r s t h i s a r i s e s f r o m 3 6 0 e x c i t e d s t a t e w h i c h

19

i s f o r m e d b y t h e f a s t n e u t r o n c a p t u r e in t h e l e O of t h e CO z c o o l i n g g a s f o r m i n g 1 6N and t h e s u b s e q u e n t b e t a d e c a y of 1 6 N . t o 1 6 0 7 6. 1 3 - M e V g a m m a -r a y s a r e e m i t t e d b y t h e o x y g e n n u c l e i a s t h e y r e t u r n to t h e g r o u n d s t a t e , a l o n g wi th a s m a l l e r a m o u n t of 1. 11- and 2. 7 5 - M e V g a m m a s . I t i s c l e a r l y i m p o r t a n t t h a t t h e r e s p o n s e t o 6. 1 3 - M e V g a m m a - r a y s of bo th p h o t o n and n e u t r o n i n s t r u m e n t s u s e d f o r m e a s u r e m e n t s a t s u c h p o w e r s t a t i o n s b e k n o w n . A s u i t a b l e c a l i b r a t i o n s o u r c e i s a p o s i t i v e ion a c c e l e r a t o r m a k i n g u s e of t h e 3 3 5 - k e V p r o t o n r e s o n a n c e on f l u o r i n e . 1 9 F (p, <*7)1 60, t h e r a d i a t i o n i s t h e r e f o r e f o r m e d b y t r a n s i t i o n s f r o m t h e s a m e e x c i t e d s t a t e of 1 6 0 , w i th t h e a d d i t i o n a l e m i s s i o n of an a s s o c i a t e d a l p h a p a r t i c l e [24], T h e r a d i a t i o n f i e l d f r o m t h i s s o u r c e c a n b e s t a n d a r d i z e d f r o m a k n o w l e d g e of t h e r a t e of p r o d u c t i o n of a l p h a p a r t i c l e s and t h i s c a n b e c h e c k e d by m e a s u r e m e n t s m a d e wi th an i o n i z a t i o n c h a m b e r c a l i b r a t e d a g a i n s t t h e p r i -m a r y s t a n d a r d wi th 2 - M V X - r a y s ; c o r r e c t i o n s b e i n g m a d e f o r t h e c h a n g e in s e n s i t i v i t y of t h e c h a m b e r b e t w e e n t h e 2 - M V c a l i b r a t i o n and 6 MeV.

F i g u r e 6 a l s o c l e a r l y d e m o n s t r a t e s a p r o b l e m a s s o c i a t e d wi th d e t e c t o r s h a v i n g th in w a l l s . M a n y d e s i g n s of i n s t r u m e n t s , p a r t i c u l a r l y i o n i z a t i o n c h a m b e r i n s t r u m e n t s , h a v e a t h i c k - w a l l e d c a p p l a c e d o v e r t h e d e t e c t o r . On r e m o v a l t h i s l e a v e s a t h in w a l l to t he c h a m b e r ; if in t h i s s t a t e t h e i n s t r u m e n t r e a d i n g i n c r e a s e s t h e n , a c c o r d i n g to m o s t i n s t r u c t i o n m a n u a l s , t h i s i n d i c a t e s t h e p r e s e n c e of a b e t a - e m i t t i n g s o u r c e . T h e u p p e r c u r v e of F i g . 6 f o r an u n c a p p e d c h a m b e r i l l u s t r a t e s how m i s l e a d i n g t h i s a s s u m p t i o n c a n b e . T h e i n c r e a s e in an i n s t r u m e n t s r e a d i n g on r e m o v a l of s u c h a c a p m a y i n d i c a t e t h a t an u n c o l l i m a t e d s o u r c e of g a m m a r a d i a t i o n i s p r e s e n t [25], F o r s u c h a s o u r c e t h e r e i s u s u a l l y an e x c e s s of s e c o n d a r y e l e c t r o n s f r o m t h e s u r r o u n d i n g a i r . It s h o u l d a l s o b e n o t e d f r o m t h e c u r v e s in F i g . 6 t h a t a s t h e p h o t o n e n e r g y i n c r e a s e s t h e d i f f e r e n c e b e t w e e n t h e c a p - o n and c a p - o f f r e a d i n g a l s o i n c r e a s e s . T h u s a t h i n - w a l l e d i o n i z a t i o n c h a m b e r wi l l u s u a l l y r e a d h igh w h e n e x p o s e d to t h e u n c o l l i m a t e d s o u r c e of 6 - M e V r a d i a t i o n f r o m a p o w e r s t a t i o n .

T h e c a l i b r a t i o n of l o w - l e v e l i n s t r u m e n t s s h o u l d b e m a d e in a l ow b a c k g r o u n d e n v i r o n m e n t . T h i s i s b e s t a c h i e v e d b y u s i n g a s u i t a b l y s h i e l d e d c e l l . C a r e s h o u l d b e t a k e n t o r e m o v e a l l p o s s i b l e s o u r c e s of e x t r a n e o u s r a d i a t i o n ; a l u m i n o u s w a t c h w o r n by t h e c a l i b r a t o r m a y e m i t s u f f i c i e n t r a d i a t i o n to i n c r e a s e t h e b a c k g r o u n d l e v e l . T h e b a c k g r o u n d e x p o s u r e r a t e s h o u l d a l w a y s b e a d d e d t o t h e c a l c u l a t e d e x p o s u r e r a t e f r o m a s o u r c e w h e n c a l i b r a t i n g an i n s t r u m e n t a t /JR. h"1 l e v e l s .

D u r i n g t h e i n i t i a l i n v e s t i g a t i o n s on a n e w t y p e of i n s t r u m e n t , t e s t s s h o u l d b e m a d e to s e e t h a t if t he i n s t r u m e n t h a s a t e s t b a t t e r y f a c i l i t y and w h e t h e r , in f a c t , it t e s t s a l l t h e b a t t e r i e s . If it d o e s no t , t h e n t h e u s e r s h o u l d b e a d v i s e d a l w a y s to c h e c k t h e c o n d i t i o n of t h e i n s t r u m e n t s b a t t e r i e s wi th a n e x t e r n a l v o l t m e t e r b e f o r e m a k i n g a n y m e a s u r e m e n t . T h e b e h a v i o u r of t h e i n s t r u m e n t on m a g n e t i c and R F f i e l d s s h o u l d b e i n v e s t i g a t e d , and t h e s u i t a b i l i t y of t h e i n s t r u m e n t f o r m e a s u r i n g r a d i a t i o n f r o m a p u l s a t o r y s o u r c e s h o u l d a l s o b e a s s e s s e d .

P a r t i c u l a r a t t e n t i o n s h o u l d b e p a i d to t h e c o n d i t i o n s u n d e r w h i c h t h e c a l i b r a t i o n i s c a r r i e d out . P e r s o n n e l d o s i m e t e r s a r e w o r n on t h e body , t h e r e f o r e i d e a l l y t h e y s h o u l d b e c a l i b r a t e d on a p h a n t o m w h i c h w i l l i n t r o d u c e s i m i l a r m o d i f i c a t i o n s t o t he r a d i a t i o n f i e l d a s t h e h u m a n body . If t h e y a r e c a l i b r a t e d f r e e in s p a c e t h e n e x p e r i m e n t s s h o u l d b e m a d e to d e t e r m i n e t h e e n e r g y c o r r e c t i o n f a c t o r t h a t s h o u l d b e a p p l i e d to a l l ow f o r t h e b a c k -s c a t t e r f r o m t h e h u m a n b o d y . S i m i l a r r e a s o n i n g s h o u l d b e a p p l i e d t o t h e c a l i b r a t i o n of i n s t a l l e d m o n i t o r s t h a t a r e to b e p l a c e d on w a l l s .

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2 . 4 . B e t a r a d i a t i o n

T h e c a l i b r a t i o n of i n s t r u m e n t s wi th b e t a r a d i a t i o n i s d i f f i c u l t b e c a u s e , u n l i k e X a n d g a m m a c a l i b r a t i o n s , u n i f o r m r a d i a t i o n f i e l d s c a n n o t b e p r o d u c e d v e r y e a s i l y . T h e c o n c e p t m o s t c o m m o n l y u s e d in b e t a d o s i m e t r y i s t h a t of d o s e . A s s e e n f r o m s e c t i o n 1. 3 t h i s i s a m e a s u r e of t h e e n e r g y i m p a r t e d b y i o n i z i n g r a d i a t i o n , to an e l e m e n t of m e d i u m . W e t h u s h a v e t o s p e c i f y t h e m a t t e r in w h i c h t h e d o s e w a s m e a s u r e d . D o s e in a i r and d o s e in t i s s u e a r e t h e two m o s t f r e q u e n t l y u s e d in b e t a d o s i m e t r y and t h e s e a r e u s u a l l y m e a s u r e d in t e r m s of t h e s p e c i a l un i t of d o s e , t h e r a d . D o s e in a i r i s t h e e a s i e r to m e a s u r e , p a r t i c u l a r l y a t low d o s e r a t e s , bu t d o s e in t i s s u e i s of m o r e p r a c t i c a l s i g n i f i c a n c e in d e t e r m i n i n g t h e r a d i a t i o n h a z a r d t o an i n d i v i d u a l . T h e d o s e in a m e d i u m m a y b e d e d u c e d f r o m a m e a s u r e m e n t of t h e i o n i z a t i o n p r o d u c e d in an a i r c a v i t y s i t u a t e d w i t h i n t h e m e d i u m . T h e m e a s u r e m e n t i s , h o w e v e r , d i f f i c u l t b e c a u s e w h e n a f l ux of b e t a p a r t i c l e s t r a v e r s e s s u c h a c a v i t y s i t u a t e d w i th in a m e d i u m , w h o s e m a s s s t o p p i n g p o w e r i s d i f f e r e n t f r o m a i r , t h e f l u x p a t t e r n w i l l b e d i s t o r t e d u n l e s s t h e c a v i t y i s of v e r y s m a l l d i m e n s i o n s .

T h e d o s e d e l i v e r e d t o t h e m e d i u m m a y b e d e t e r m i n e d f r o m t h e i o n i z a t i o n p r o d u c e d in s u c h a s m a l l c a v i t y [26 — 30] by t h e r e l a t i o n s h i p

w h e r e E M i s t h e e n e r g y i m p a r t e d t o u n i t m a s s of t h e m e d i u m , K j i s t h e r a t i o of t h e m a s s s t o p p i n g p o w e r of t h e m e d i u m to t h a t of t h e c a v i t y g a s , Jg i s t h e n u m b e r of ion p a i r s p r o d u c e d p e r u n i t m a s s of t h e g a s a n d W i s t h e a v e r a g e e n e r g y e x p e n d e d by t h e i o n i z i n g p a r t i c l e s in f o r m i n g an ion p a i r i n t h e c a v i t y .

B e c a u s e of t h e r e s t r i c t i o n s o v e r t h e s i z e of t h e c a v i t y and b e c a u s e of t h e i r w a l l t h i c k n e s s , t h i m b l e i o n i z a t i o n c h a m b e r s u s u a l l y e m p l o y e d in m e a s u r e m e n t s w i t h X - r a y s c a n n o t b e u s e d f o r a b s o l u t e m e a s u r e m e n t s of t h e b e t a d o s e . B y u s i n g a c h a m b e r w h o s e v o l u m e o r g a s p r e s s u r e m a y b e v a r i e d , it i s p o s s i b l e to d e t e r m i n e t h e i o n i z a t i o n p e r un i t v o l u m e of g a s f o r a v e r y s m a l l m a s s of g a s and t h i s v a l u e m a y b e u s e d to e s t i m a t e t h e b e t a d o s e u s i n g t h e a b o v e e q u a t i o n . Such an i o n i z a t i o n c h a m b e r i s c a l l e d an e x t r a p o l a t i o n c h a m b e r [31 — 33],

If t h e d o s e in t i s s u e i s r e q u i r e d t o b e m e a s u r e d t h e c h a m b e r s h o u l d b e c o n s t r u c t e d f r o m m a t e r i a l s w h i c h r e s p o n d to r a d i a t i o n in a s i m i l a r w a y a s t i s s u e . F o r t h e m e a s u r e m e n t of d o s e in a i r t h e c h a m b e r s h o u l d h a v e a t h in a i r - e q u i v a l e n t c o l l e c t o r a n d p o l a r i z i n g p l a t e .

T h e d o s e d i s t r i b u t i o n a b o v e a n u m b e r of b e t a - e m i t t i n g s o u r c e s c a n b e m e a s u r e d w i t h t h e e x t r a p o l a t i o n c h a m b e r .

D u r i n g m e a s u r e m e n t s wi th t h e e x t r a p o l a t i o n c h a m b e r c a r e s h o u l d b e t a k e n to e x c l u d e d r a u g h t s s i n c e t h i s c a n c a u s e d e f l e c t i o n s in t h e t h i n c o l l e c t o r o r p o l a r i z i n g p l a t e . F u r t h e r p r e c a u t i o n s a r e t h e u s e of e l e c t r i c a l s c r e e n i n g and b i o l o g i c a l s c r e e n i n g , m e a s u r e m e n t s s h o u l d b e m a d e to c h e c k t h a t t h e l a t t e r s c r e e n i n g d o e s no t s c a t t e r t h e b e t a r a d i a t i o n b a c k in to t h e c h a m b e r .

U n f o r t u n a t e l y t h e b e t a p a r t i c l e s e m i t t e d f r o m a s o u r c e h a v e a b r o a d s p e c t r u m f r o m E = O up to E m a x ; i t i s u s u a l in b e t a d o s i m e t r y to u s e a m e a n v a l u e f o r t h e e l e c t r o n s e m i t t e d and t h i s t a k e n a s i E m a x . T h e a v e r a g e p a r t i c l e e n e r g y in b e t a d e c a y a s a f u n c t i o n of t h e end po in t k i n e t i c e n e r g y

21

h a s b e e n d e a l t w i t h t h e o r e t i c a l l y in R e f . [ 3 3 a ] , A l i s t of s u i t a b l e b e t a -e m i t t i n g s o u r c e s i s g iven in T a b l e IV t o g e t h e r w i t h i n f o r m a t i o n on t h e i r m e a n e n e r g i e s a n d h a l f - l i v e s . T h e s e s o u r c e s m a y b e u s e d to e v a l u a t e t h e e n e r g y r e s p o n s e of an i n s t r u m e n t . F r o m t h e known d o s e d i s t r i b u t i o n a b o v e t h e s e s o u r c e s it i s p o s s i b l e to e s t i m a t e t h e r e s p o n s e of any i n s t r u m e n t p l a c e d in t h e s e f i e l d s [34 - 36], T h e m a j o r i t y of i n s t r u m e n t s t h a t a r e c a p a b l e of m e a s u r i n g b e t a r a d i a t i o n h a v e b e e n d e s i g n e d a s b e t a - g a m m a i n s t r u m e n t s ( s e e d i s c u s s i o n in s e c t i o n 4. 1. 3). Such i n s t r u m e n t s m a y b e r o u t i n e l y c a l i b r a t e d wi th g a m m a s o u r c e s and s p e c i a l m e a s u r e m e n t s m a d e f o r e a c h p a r t i c u l a r d e s i g n of i n s t r u m e n t to d e t e r m i n e a t y p i c a l b e t a r e s p o n s e . T h e s p a t i a l d i s t r i b u t i o n of d o s e a b o v e m o s t c o m m e r c i a l l y a v a i l a b l e b e t a s o u r c e s s h o w s v e r y l a r g e v a r i a t i o n s . W h e n c a l i b r a t i n g i n s t r u m e n t s w h i c h h a v e l a r g e d e t e c t o r s i t i s t h e r e f o r e n e c e s s a r y t o d e t e r m i n e t h e a v e r a g e d o s e d e l i v e r e d t o t h e p o s i t i o n o c c u p i e d by t h e d e t e c t o r . T h i s i s d o n e b y i n t e g r a t i n g t h e d o s e d i s t r i b u t i o n c u r v e s a s m e a s u r e d by t h e e x t r a p o l a t i o n c h a m b e r .

A. n u m b e r of p r o b l e m s a r i s e in u s i n g m o s t b e t a i n s t r u m e n t s . T h e r e m o v a b l e c a p of m o s t b e t a - g a m m a i n s t r u m e n t s i s u s u a l l y only f i t t e d to one w a l l of t h e d e t e c t o r . B e c a u s e only one w a l l i s t h in enough to a l l o w t h e b e t a p a r t i c l e s to p e n e t r a t e i t , t h e i n s t r u m e n t w i l l h a v e a b e t a r e s p o n s e t h a t w i l l v a r y s i g n i f i c a n t l y wi th t h e d i r e c t i o n of t h e b e t a p a r t i c l e s u p o n t h e d e t e c t o r . A s a l r e a d y m e n t i o n e d t h e l a r g e p h y s i c a l s i z e of m o s t d e t e c t o r s m a k e s it d i f f i c u l t t o c a l i b r a t e and t h i s w i l l a l s o m a k e it d i f f i c u l t t o i n t e r p r e t m e a s u r e m e n t s m a d e wi th i t . V e r y l a r g e e r r o r s m a y a r i s e in t r y i n g to m a k e s u r f a c e d o s e m e a s u r e m e n t s wi th s u c h an i n s t r u m e n t .

T A B L E IV. C A L I B R A T I O N S O U R C E S O F B E T A - R A Y S 3

Radionuclide Eg max. (MeV)

Egav. (MeV)

Hal f - l i f e

3 H 0. 0186 0. 006 12. 26 yr

1 1 C 0. 156 0. 045 5730 yr

35 s 0. 167 0. 049 87. 9 d

4 5 Ca 0. 252 0. 075 165 d

185 W 0. 29 0. 140. 75 d

90 Sr 0 . 4 6 0. 200 27. 7yr

85 Kr 0. 670 0 . 2 2 10 .8yr

204 jj 0. 66 0. 243 3. 81yr

1 U Ag 1. 05 0. 380 7 . 5 d

210 B i 1. 16 0. 398 5. Od

32 p 1. 71 0. 690 14 .3 d

90 y 2. 27 0. 93 64. Oh

42 K 3 . 5 2 1. 42 12 .4 h

3 Data taken from Lederer, C. M . , Hollander, J . M . , and Perlman, I . , Table of Isotopes, Wiley, New York (1967).

22

M e a s u r e m e n t s of t h e s u r f a c e d o s e s h o u l d b e m a d e wi th a v e r y s h a l l o w d e t e c t o r and e v e n t h e n l a r g e e r r o r s w i l l r e s u l t if t h e s i z e of t h e b e t a s o u r c e i s s m a l l c o m p a r e d wi th t h e a r e a of t h e d e t e c t o r .

2 . 5 . N e u t r o n r a d i a t i o n

2. 5. 1. G e n e r a l c o n s i d e r a t i o n s

N e u t r o n s a r e u s u a l l y c l a s s i f i e d a c c o r d i n g to t h e i r e n e r g y and to avo id m i s u n d e r s t a n d i n g o v e r t e r m i n o l o g y t h e f o l l o w i n g s y s t e m w i l l b e u s e d in t h i s s e c t i o n .

(a) S l o w j i e u t r o n s ^ 0 — 0 . 5 eV. At low n e u t r o n e n e r g i e s m o s t m a t e r i a l s h a v e a s e n s i t i v i t y t h a t v a r i e s wi th t h e r e c i p r o c a l of t h e v e l o c i t y of t h e n e u t r o n s . T h e c a d m i u m c u t - o f f f o r a s p e c i f i e d t h i c k n e s s of c a d m i u m , 0. 5 eV, h a s b e e n c h o s e n a s t h e u p p e r l i m i t t o t h e r a n g e , s i n c e b e l o w t h i s e n e r g y d e p a r t u r e s of t h e c r o s s - s e c t i o n f r o m t h e 1 / v l a w f o r m o s t e l e m e n t s a r e s m a l l and a d e t a i l e d k n o w l e d g e of t h e s p e c t r u m i s not u s u a l l y r e q u i r e d , w h i l s t a b o v e 0. 5 eV s u c h c o r r e c t i o n s a r e i m p o r t a n t and r e q u i r e a k n o w l e d g e of t h e n e u t r o n s p e c t r u m .

(b) I n t e r m e d i a t e energy_peu t rons j_ 0. 5 eV — 200 keV. T h e u p p e r l i m i t h a s b e e n c h o s e n s i n c e t h i s c o r r e s p o n d s t o a n e n e r g y b e l o w w h i c h m a n y i n s t r u m e n t s do no t r e s p o n d . It i s a l s o a r e g i o n in w h i c h a l a r g e p r o p o r t i o n of t h e t o t a l n e u t r o n d o s e c a n o c c u r n e a r i n s t a l l a t i o n s l i k e a n u c l e a r r e a c t o r o r a c c e l e r a t o r [37],

(c) F a s t _ n e u t r o n s j _ 200 keV - 20 M e V . 20 MeV i s an e n e r g y a t w h i c h s o m e d e t e c t o r s h a v e a u s e f u l r e s p o n s e on ly a b o v e o r b e l o w t h i s v a l u e .

(d) H i g h _ e n e r g y _ n e u t r o n £ : a b o v e 20 MeV.

L i k e n e u t r o n s , n e u t r o n m e a s u r i n g i n s t r u m e n t s c a n a l s o b e c l a s s i f i e d r o u g h l y in to s e v e r a l g r o u p s .

(a) S l o w _ n e u t r o n m ^ r u m e n t s j T h e s e i n s t r u m e n t s r e s p o n d to s l o w n e u t r o n s , g r o u p (a) , a n d u s u a l l y g ive an i n d i c a t i o n of t h e d o s e e q u i v a l e n t r a t e .

(b) Fa_st_neutron_ins_trume_nt£: T h e d e t e c t o r s of t h e s e i n s t r u m e n t s u s u a l l y u t i l i z e r e c o i l p r o t o n s p r o d u c e d in a h y d r o g e n o u s m a t e r i a l b y t h e n e u t r o n s . T h e r e s p o n s e of s u c h i n s t r u m e n t s w i l l d e p e n d upon t h e g e o m e t r y of t h e r a d i a t o r s a s w e l l a s upon t h e g a s f i l l i n g , b u t t h e y do not r e s p o n d to n e u t r o n s b e l o w an e n e r g y of 100 to 300 keV and t h e i r r e s p o n s e f a l l s off r a p i d l y f o r n e u t r o n s a b o v e 5 - 1 0 M e V . T h i s i n s t r u m e n t t h e r e f o r e r e s p o n d s t o n e u t r o n s in g r o u p (c).

(c) I n t e r m e d i a t e n e u t ^ o i ^ i n s t r u m e n t s ^ T h e r e a r e v e r y f e w i n s t r u m e n t s t h a t h a v e b e e n d e s i g n e d s p e c i f i c a l l y to m e a s u r e i n t e r m e d i a t e n e u t r o n s , bu t an e x a m p l e i s t h e B a s s o n c o u n t e r [38], T h i s i n s t r u m e n t u s e s a s i t s d e t e c t o r a p l a s t i c s c i n t i l l a t o r l o a d e d w i t h ZnS and b o r o n e n r i c h e d w h i c h is s u r r o u n d e d b y a 6. 3 - c m - d i a m . wax s p h e r e . T h e i n s t r u m e n t h a s b e e n

23

d e s i g n e d t o m e a s u r e n e u t r o n s f r o m 1 eV to 15 k e V and i t s r e s p o n s e i s w i t h i n ±12% of t h e d o s e / f l u e n c e r e l a t i o n s h i p o v e r t h i s e n e r g y r a n g e , p a r t of g r o u p (b).

(d) R e m _ c o u n t e r s j _ T h e s e i n s t r u m e n t s u s e a s l o w n e u t r o n d e t e c t o r s u r r o u n d e d b y an a b s o r b e r - m o d e r a t o r a s s e m b l y and a r e d e s i g n e d to d e -t e r m i n e t h e d o s e e q u i v a l e n t r a t e f o r n e u t r o n s f r o m t h e r m a l e n e r g i e s up to a b o u t 1 0 - 2 0 M e V , i . e. n e u t r o n s in g r o u p s a + b + c.

P r o b l e m s a s s o c i a t e d wi th t h e c a l i b r a t i o n of t h e s e v a r i o u s t y p e s of i n s t r u m e n t s w i l l b e d i s c u s s e d in a l a t e r p a r t of t h i s s e c t i o n .

T o c o n c l u d e t h i s s u m m a r y on t h e c l a s s i f i c a t i o n of n e u t r o n s and i n s t r u m e n t s a v e r y b r i e f o u t l i n e w i l l b e g i v e n of t h e r a n g e of c a l i b r a t i o n s o u r c e s t h a t a r e a v a i l a b l e . It s h o u l d b e s t r e s s e d t h a t v e r y f e w c a l i b r a t i o n l a b o r a t o r i e s w i l l r e q u i r e a l l t h e s e s o u r c e s . M u c h s t r a i g h t f o r w a r d c a l i b r a t i o n c a n b e d o n e w i t h s e a l e d s o u r c e s and j i g s . W h e r e d e t a i l e d i n f o r m a t i o n on t h e e n e r g y r e s p o n s e , e n v i r o n m e n t a l e f f e c t s and t h e r e s p o n s e of t h e i n s t r u m e n t s to o t h e r f o r m s of r a d i a t i o n i s r e q u i r e d it i s s u g g e s t e d t h a t t h i s co u l d b e o b t a i n e d b y c o r r e s p o n d e n c e wi th o n e of t h e m o r e f u l l y e q u i p p e d l a b o r a t o r i e s .

(i) Slow_neutro_n £ o u r c e £ : T h e s e r e l y on the m o d e r a t i o n of a h i g h e r e n e r g y n e u t r o n s o u r c e ; s u i t a b l e s o u r c e s a r e a n u c l e a r r e a c t o r , an a c c e l e r -a t o r and r a d i o n u c l i d e n e u t r o n s o u r c e s . T h e c h o i c e of any of t h e s e s o u r c e s d e p e n d s v e r y l a r g e l y upon t h e t y p e of c a l i b r a t i o n t h a t i s r e q u i r e d .

(ii) R a d i o n u c l i d e neutron_s_ources_^ T h e r e a r e a w i d e r a n g e of ( 7 , n) , (a , n) a n d s p o n t a n e o u s f i s s i o n s o u r c e s t h a t a r e c o m m e r c i a l l y a v a i l a b l e . T h e s e s o u r c e s n o r m a l l y e m i t a w i d e s p e c t r u m of n e u t r o n s a n d a k n o w l e d g e i s r e q u i r e d of t h i s s p e c t r u m , and h a l f - l i f e . S e r i o u s l i m i t a t i o n s of m a n y of t h e s e s o u r c e s a r e t h e i r low n e u t r o n e m i s s i o n r a t e and h i g h a s s o c i a t e d g a m m a e m i s s i o n .

(ii i) A c c e l e r a t o r ^ n e u t r o n _ s o u r c e _ s : _ T h i s i s a m o r e c o s t l y f o r m of p r o -v i d i n g a c a l i b r a t i o n n e u t r o n s o u r c e t h a n by u s i n g r a d i o n u c l i d e s o u r c e s . T h e a d v a n t a g e of s u c h a s o u r c e i s t h a t i t c a n p r o v i d e a d i s c r e t e e n e r g y and t h e e m i s s i o n r a t e s a r e u s u a l l y g r e a t e r t h a n t h o s e f r o m a r a d i o n u c l i d e s o u r c e a n d c a n a l s o b e v a r i e d . T h e m e a s u r e m e n t of t h e r a d i a t i o n f i e l d s f r o m an a c c e l e r a t o r i s , h o w e v e r , f a i r l y c o m p l i c a t e d .

T h e u s e of t h e s e s o u r c e s f o r c a l i b r a t i o n p u r p o s e s wi l l b e d i s c u s s e d in d e t a i l in t h e f o l l o w i n g s e c t i o n s . It i s , h o w e v e r , w o r t h n o t i n g t h a t t h e r e i s a r a n g e of n e u t r o n s f o r wh ich t h e r e a r e no s u i t a b l e s o u r c e s , 0. 5 eV to a b o u t 10 k e V . U n f o r t u n a t e l y t h i s i s w i t h i n t h e i n t e r m e d i a t e r a n g e in w h i c h l a r g e p e r c e n t a g e s of t h e t o t a l n e u t r o n d o s e c a n o c c u r [37], A. m e t h o d of p r o d u c i n g m o n o e n e r g e t i c n e u t r o n s in t h i s r e g i o n i s by a t i m e - o f - f l i g h t t e c h n i q u e bu t t h e m e t h o d s of c a l i b r a t i n g wi th s u c h a s o u r c e i s v e r y d i f f i c u l t . A t e c h n i q u e u s i n g r e s o n a n c e n e u t r o n s c a t t e r i n g h a s b e e n p r o p o s e d in R e f . [39],

T h e c a l i b r a t i o n of n e u t r o n i n s t r u m e n t s i s m o r e d i f f i c u l t t h a n t h e c o r r e s p o n d i n g c a l i b r a t i o n of e l e c t r o m a g n e t i c i n s t r u m e n t s . T h i s i s b e c a u s e , u n l i k e X - a n d g a m m a - r a d i a t i o n , w h e r e t h e c o n c e p t of e x p o s u r e c a n b e r e l a t e d w i t h c e r t a i n i n t e r p r e t a t i o n t o a d o s e e s t i m a t i o n , t h e r e i s no s u c h s i m i l a r c o n c e p t in n e u t r o n d o s i m e t r y . T o c a l i b r a t e n e u t r o n i n s t r u m e n t s

24

T A B L E V. V A L U E S O F N E U T R O N F L U E N C E R A T E C O R R E S P O N D I N G T O A D O S E E Q U I V A L E N T R A T E O F 2. 5 m r e m / h

Neutron energy Fluence rate per 2 . 5 m r e m / h (MeV) (cm"2 sec*1)

2 . 5 x lCT^thermal) 680

1 X 10"' 680

1 X lO"6 560

1 X 10"5 560

1 x 10"4 580

1 x 10"3 680

1 X 10"2 700

1 X 1 0 " 115

5 x 10"1 27

1 19

2 . 5 20

5 16

7 17

10 17

M 12

20 11

40 10

60 11

1 x 102 14

2 x 102 13

3 X 102 11

4 X 102 10

t h e r a d i a t i o n f i e l d s a r e m e a s u r e d in t e r m s of p h y s i c a l p a r a m e t e r s of t h i s f i e l d , u s u a l l y f l u e n c e a n d p a r t i c l e e n e r g y , and c o n v e r s i o n m o d i f y i n g f a c t o r s a r e t h e n a p p l i e d t o e s t i m a t e t h e d o s e e q u i v a l e n t r a t e . If t h e d o s e f o r a c a l i b r a t i o n s o u r c e i s m e a s u r e d w i t h a t i s s u e e q u i v a l e n t i o n i z a t i o n c h a m b e r t h e n m o d i f y i n g f a c t o r s a r e s t i l l r e q u i r e d .

F a c t o r s f o r c o n v e r t i n g f l u e n c e r a t e of n e u t r o n s a t d i f f e r e n t e n e r g i e s to d o s e e q u i v a l e n t r a t e a r e g i v e n in T a b l e V; t h e s e v a l u e s h a v e b e e n t a k e n f r o m ICRU (to be p u b l i s h e d ) and a r e f o r m o n o e n e r g e t i c n e u t r o n s i n c i d e n t n o r m a l l y on a c y l i n d e r of u n i t d e n s i t y t i s s u e , 30 c m in d i a m . V a l u e s b a s e d on c a l c u l a t i o n s of k e r m a in u n i t s of 0. 01 J / k g m u l t i p l i e d by a q u a l i t y f a c t o r a r e q u o t e d in R e f . [40],

F o r n e u t r o n s w i t h e n e r g i e s not l i s t e d in T a b l e V v a l u e s of t h e f l u e n c e r a t e f o r a d o s e e q u i v a l e n t r a t e of 2. 5 m r e m . h" 1 c a n b e o b t a i n e d f r o m t h e

25

800 • •800

. 700 lc E

^ 600 -

700

600

500 • 500

<.00 • • <,00

300 • 300

200 200

100 _j • • • " i i i 1(T'

NEUTRON ENERGY (M«V) (b)

FIG. 7(a) and (b). Neutron f luence rate to g i v e 2. 5 m r e m . h"1 .

g r a p h of t h e s e f a c t o r s , F i g . 7. It i s i m p o r t a n t to r e a l i z e t h a t t h e a b o v e f a c t o r s w e r e o b t a i n e d f r o m c a l c u l a t i o n s m a d e f o r m o n o e n e r g e t i c n e u t r o n s i n c i d e n t n o r m a l l y on a c y l i n d e r of u n i t d e n s i t y t i s s u e 30 c m in d i a m . T h e s e c o n d i t i o n s d i f f e r f r o m t h o s e u s e d d u r i n g t h e c a l i b r a t i o n of an i n s t r u m e n t w h e r e d i v e r g e n t b e a m s of n e u t r o n s a r e e m p l o y e d and s o t h e a d o p t i o n of t h e s e f a c t o r s to c o n v e r t t h e c a l i b r a t i o n f l u e n c e r a t e t o d o s e e q u i v a l e n t r a t e wi l l o b v i o u s l y i n t r o d u c e e r r o r s ( ICRU, t a s k g r o u p IIIA). E v e n if t h e c a l i -b r a t i o n c o n d i t i o n s did a p p r o x i m a t e to t h e m , a s w e l l a s t h e c o n d i t i o n s u n d e r

26

w h i c h t h e i n s t r u m e n t wou ld b e u s e d , t h e n t h e i n s t r u m e n t would s t i l l on ly m e a s u r e t h e d o s e e q u i v a l e n t r a t e t o p a r t s of t h e b o d y w h e r e t h e o r g a n f a c t o r w a s u n i t y . It cou ld no t , t h e r e f o r e , b e u s e d to m e a s u r e t h e d o s e e q u i v a l e n t (DE) t o t h e e y e s i n c e t h e o r g a n f a c t o r f o r t h e e y e i s only 1 w h e n i n c i d e n t r a d i a t i o n h a s a Q F of 1. W h e r e t h e c a l i b r a t i o n c o n d i t i o n s a r e s i m i l a r t o t h o s e u s e d in d e r i v i n g t h e d a t a in T a b l e V, b u t t h e c o n d i t i o n s in w h i c h it w i l l b e u s e d f o r f i e l d m e a s u r e m e n t s a r e d i f f e r e n t , t h e n t h e i n s t r u m e n t r e a d i n g w i l l g i v e a n o v e r e s t i m a t i o n of t h e D E r a t e and wi l l t h e r e f o r e , f o r t u n a t e l y , e r r on t h e s a f e s i d e .

It h a s b e e n s t a t e d e a r l i e r t h a t w h e r e p o s s i b l e a l l c a l i b r a t i o n s s h o u l d b e m a d e a g a i n s t s e c o n d a r y s t a n d a r d s w h i c h t h e m s e l v e s c a n b e c o m p a r e d wi th p r i m a r y s t a n d a r d s . T h e r e a r e g e n e r a l l y v e r y f e w p r i m a r y s t a n d a r d s a v a i l a b l e w h i c h c a n b e u s e d f o r n e u t r o n i n s t r u m e n t c a l i b r a t i o n . T h e m o s t w i d e l y a v a i l a b l e p r i m a r y n e u t r o n s t a n d a r d s a r e v a r i o u s v e r s i o n s of t h e m a n g a n e s e b a t h f o r m e a s u r i n g t h e e m i s s i o n r a t e s f r o m r a d i o n u c l i d e n e u t r o n s o u r c e s a n d s t a n d a r d t h e r m a l n e u t r o n f l u x a s s e m b l i e s s u i t a b l e f o r t h e c a l i b -r a t i o n of f o i l s . P r i m a r y s t a n d a r d s of n e u t r o n f l u e n c e r a t e s a t v a r y i n g e n e r g i e s a r e b e i n g e s t a b l i s h e d w i t h i n t h e UK a t t h e N a t i o n a l P h y s i c a l L a b o r a -t o r y [41] , a n d w h e r e t h e s e t y p e s of s t a n d a r d s a r e no t r e a d i l y a v a i l a b l e , t h e n it w i l l b e n e c e s s a r y t o e s t a b l i s h o n e ' s own s t a n d a r d s and t o i n t e r c o m p a r e t h e s e wi th , s a y , a p r i m a r y m a n g a n e s e b a t h s t a n d a r d .

2. 5. 2. C a l i b r a t i o n wi th s e a l e d n e u t r o n s o u r c e s

A s e a l e d n e u t r o n s o u r c e s u i t a b l e f o r c a l i b r a t i o n p u r p o s e s s h o u l d i d e a l l y h a v e t h e f o l l o w i n g q u a l i t i e s - t h e s o u r c e s h o u l d h a v e a r e a s o n a b l y l o n g h a l f - l i f e , t h e p h y s i c a l s i z e of t h e s o u r c e s h o u l d b e s m a l l , t h e n e u t r o n e m i s s i o n r a t e shou ld b e h i g h , i t s h o u l d h a v e a known s p e c t r u m and s h o u l d e m i t n e u t r o n s of a d i s c r e t e e n e r g y . T h e s o u r c e s h o u l d a l s o not e m i t f o r m s of r a d i a t i o n o t h e r t h a n n e u t r o n s and t h e e n e r g y s p e c t r u m and a n g u l a r d i s t r i b u t i o n of t h e e m i s s i o n r a t e s h o u l d b e known. T h e e m i s s i o n r a t e and s p e c t r u m s h o u l d a l s o not b e a f f e c t e d b y r a d i o a c t i v e g r o w t h in t h e s o u r c e o r b y c h a n g e s in t h e p h y s i c a l m i x i n g of t h e s o u r c e . T h e r e i s no s o u r c e t h a t w i l l p o s s e s s a l l t h e s e q u a l i t i e s and t h o s e l i s t e d in T a b l e VI m a y b e u s e d p r o v i d e d t h a t t h e i r u n d e s i r a b l e c h a r a c t e r i s t i c s a r e k n o w n and a l l o w e d f o r .

When a p r o p e r l y c a l i b r a t e d n e u t r o n s o u r c e c a n n o t b e p u r c h a s e d t h e n e u t r o n e m i s s i o n r a t e s f r o m t h e s e s o u r c e s s h o u l d b e a b s o l u t e l y d e t e r m i n e d . T h e m o s t w i d e l y u s e d m e t h o d f o r t h i s d e t e r m i n a t i o n i s t h e m a n g a n e s e b a t h t e c h n i q u e . T h e s o u r c e i s p l a c e d a t t h e c e n t r e of a l a r g e s p h e r i c a l s h a p e d t a n k w h i c h c o n t a i n s a s a t u r a t e d s o l u t i o n of M n S 0 4 . T h e s o l u t i o n a c t s a s a m o d e r a t o r and t h e s l o w n e u t r o n s p r o d u c e d a r e c a p t u r e d b y t h e s t a b l e 5 5 M n to g ive 5 6 M n w h i c h d e c a y s w i t h a h a l f - l i f e of 2. 578 h . T h e d i m e n s i o n s of t h e b a t h a r e c h o s e n s o t h a t t h e e s c a p e of n e u t r o n s f r o m t h e b a t h i s k e p t a s low a s p o s s i b l e . S a m p l e s of t h e s o l u t i o n a r e r e m o v e d a n d an a b s o l u t e d e -t e r m i n a t i o n of t h e 5 6 M n a c t i v i t y i s m a d e . T h i s r e q u i r e s t h a t t h e a b s o l u t e s e n s i t i v i t y f o r 5 6 M n of t h e d e t e c t o r s u s e d f o r t h i s m e a s u r e m e n t s h o u l d b e known. T h e d e t e c t o r s m o s t c o m m o n l y u s e d a r e t h i n - w a l l e d GM t u b e s o r N a l c r y s t a l s . A l a r g e n u m b e r of c o r r e c t i o n s h a v e t o b e m a d e in t h e e s t i m a t i o n of t h e e m i s s i o n r a t e . N e u t r o n s m a y u n d e r g o f a s t r e a c t i o n s in t h e s u l p h u r and o x y g e n if t h e i r e n e r g i e s a r e a b o v e t h e t h r e s h o l d s f o r t h e s e r e a c t i o n s ; o t h e r n e u t r o n s a r e s c a t t e r e d b a c k f r o m t h e s o l u t i o n and r e -a b s o r b e d in t h e s o u r c e . A n u m b e r of o t h e r c o r r e c t i o n f a c t o r s a l s o h a v e

27

T A B L E VI. C H A R A C T E R I S T I C S O F N E U T R O N S O U R C E S [ 42, 4 2 a , 42b ]

Source Hal f - l i f e Average

neutron energy (MeV)

Approximate y ie ld

Remarks

Sb/Be ( y f n) 60 d 0. 024 1. 6 x 106 n / sec . C i a

N a / D 2 0 ( y , n ) 14 .8 h 0. 22 0. 22 x 104 n /sec . C i a Too short-lived for reference source

RdTh/DjO ( y , n ) 1 . 9 yr 0 . 2 0 1 . 2 X 106 n / sec . C i a

Na/Be ( y , n) 14. 8 h 0. 83 1 . 3 X ] 0 s n / sec . C i a Too short-lived for reference source

RdTh/Be ( y , n) 1 . 9 yr 0. 80

Ra/Be ( y , n ) 1622 yr 0 . 3 0 1 . 2 x 106 n / sec . C i a

A m / B e ( a , n) 458 yr 4 . 4 2 x 106 n / sec . Ci Few gamma-rays. Suitable as standard source if the purity of 2 4 1Am is high

Ra/Be ( a , n) 1622 yr 3. 9 1 . 5 X 107 n / sec . Ci Increase in emission rate owing to growth of Po. High gamma emission

Pu/Be ( a , n) 24 ,400 yr 4 . 5 1. 5 x 10 s n /sec . Ci Few gamma-rays . Has radioactive growth

Ac/Be ( a , n ) 21. 8 yr 4. 6

Po/Be ( a , n) 1 3 8 . 4 d 4 . 2 2 . 5 x 106 n /sec . Ci Few gamma-rays, short h a l f - l i f e

240 Pu (spontaneous fission)

6600 yr ( a decay) 7 X 102 n /sec . g Expensive, f ew gamma-rays. Low yield

244 Cm (spontaneous fission)

18 .4 yr ( a decay)

107 n / sec . g Expensive, low mass

252 Cf (spontaneous fission)

2 .654 yr ( a decay)

2. 5 X 1012 n /sec . g Point sources may be prepared

a Neutron yield for a 1 Ci gamma source with 1 g target material placed 1 cm away from the gamma source [ 4 2 ] . NB. All photo-neutron sources possess intense gamma-ray backgrounds of at least 103 gamma-rays per neutron.

t o b e a p p l i e d and t h e s e a r e d i s c u s s e d , a l o n g wi th a m o r e d e t a i l e d d e s c r i p t i o n of t h e t e c h n i q u e , in R e f s [ 4 2 - 4 6 b ] , P r e c i s i o n l o n g c o u n t e r s m a y b e u s e d f o r t h e a b s o l u t e m e a s u r e m e n t s of t h e f a s t n e u t r o n e m i s s i o n r a t e s if t h e y a r e c o n s t r u c t e d e x a c t l y in a c c o r d a n c e wi th t h e d e s i g n d e s c r i b e d in R e f . [52],

C a l i b r a t i o n u s i n g a n e u t r o n s o u r c e , a r a d i o n u c l i d e o r a c c e l e r a t o r s o u r c e , i s m a d e d i f f i c u l t by t h e u n c e r t a i n t i e s a r i s i n g f r o m s c a t t e r e d r a d i -a t i o n . T h e d e g r e e of s c a t t e r c a n b e r e d u c e d b y u s i n g t h e s o u r c e s a t t h e c e n t r e of a l a r g e r o o m wi th a d e q u a t e s h i e l d i n g f o r t h e o p e r a t o r a n d b y k e e p i n g t h e a r e a c l o s e to t h e s o u r c e and i n s t r u m e n t a s f r e e a s p o s s i b l e f r o m any s c a t t e r i n g m a t e r i a l . E v e n in a v e r y l a r g e r o o m it w i l l s t i l l b e n e c e s s a r y t o m a k e s c a t t e r c o r r e c t i o n s if a n a c c u r a t e m e a s u r e m e n t i s r e q u i r e d .

T h e f l u x d e n s i t y cp c a n b e e s t i m a t e d f r o m a k n o w l e d g e of t h e s o u r c e e m i s s i o n r a t e .

w h e r e B i s t h e c o r r e c t i o n f a c t o r f o r s c a t t e r e d r a d i a t i o n , QQ i s t h e n e u t r o n e m i s s i o n r a t e f r o m t h e s o u r c e ( n e u t r o n s , sec" 1 ) , r i s t h e d i s t a n c e in c m f r o m t h e s o u r c e c e n t r e a n d £ r i s t h e t o t a l a t t e n u a t i o n c o e f f i c i e n t f o r a i r (cm" 1 ) . T h e c o r r e c t i o n f o r a b s o r p t i o n in a i r i s s m a l l and c a n u s u a l l y b e n e g l e c t e d . C o r r e c t i o n s s h o u l d b e m a d e f o r t h e a n g u l a r d i s t r i b u t i o n of t h e s o u r c e ' s ou tpu t a n d t h e v a l u e of Qo u s e d a b o v e s h o u l d b e t h e c o r r e c t e d v a l u e c o r r e s p o n d i n g to t h e g e o m e t r y of t h e s o u r c e b e i n g u s e d .

T h e f a c t o r B c a n b e e s t i m a t e d in two w a y s :

(a) D e p a r t u r e s f r o m t h e f o l l o w i n g s i m p l i f i e d r e l a t i o n s h i p u s i n g a p h y s i c a l l y s m a l l n e u t r o n d e t e c t o r , 9 = Q 0 / 4 7 r r 2 i s d e t e r m i n e d . A s m a l l d e t e c t o r i s u s e d t o avo id a m b i g u i t i e s abou t t h e e x a c t p o s i t i o n of i t s e f f e c t i v e c e n t r e w h o s e d i s p l a c e m e n t f r o m t h e g e o m e t r i c a l c e n t r e c a n b e e n e r g y d e p e n d e n t . T h i s m e t h o d i s not v e r y r e l i a b l e s i n c e it r e q u i r e s an i n s t r u m e n t w h o s e d i r e c t i o n a l r e s p o n s e i s good a s w e l l a s h a v i n g a f l a t e n e r g y f l u e n c e r e s p o n s e .

(b) T h e m o s t r e l i a b l e m e t h o d f o r e s t i m a t i n g t h e s c a t t e r c o r r e c t i o n i s b y u s i n g a " s h a d o w c o n e " [47 , 48], T h e d e s i g n of t h e c o n e s h o u l d b e s u c h t h a t it g e o m e t r i c a l l y b l o c k s out t h e d i r e c t f l u e n c e f a l l i n g upon t h e d e t e c t o r f r o m t h e s o u r c e . T h e c o n e s h o u l d b e c o n s t r u c t e d f r o m p a r a f f i n w a x o r p o l y t h e n e . T h i s i s u s e d to m o d e r a t e t h e n e u t r o n s and t h e y c a n t h e n b e a b s o r b e d by a c a d m i u m o r b o r o n a b s o r b e r p l a c e d a t t h e b a s e of t h e c o n e , o r b y l o a d i n g t h e w h o l e c o n e u n i f o r m l y w i t h b o r o n 5% b y w e i g h t . T h e h e i g h t of t h e c o n e s h o u l d b e s u f f i c i e n t t o m o d e r a t e t h e h i g h e s t e n e r g y n e u t r o n s f r o m t h e s o u r c e . H o w e v e r , it i s n e c e s s a r y t o b e a r in m i n d t h a t t h e c o n e i t s e l f c a n i n f l u e n c e t h e s c a t t e r i n g c o n d i t i o n s in t h e l a b o r a t o r y .

R a d i o n u c l i d e s o u r c e s h a v e m a x i m u m e m i s s i o n r a t e s of t h e o r d e r 1 0 7 n / s e c , b u t t h i s i s no t s u f f i c i e n t f o r c a l i b r a t i n g i n s t r u m e n t s on t h e i r h i g h e r r a n g e s w h i l s t s t i l l m a i n t a i n i n g r e a s o n a b l e s o u r c e - t o - d e t e c t o r s e p a r a t i o n . F r o m t h e c a l c u l a t e d f lux d e n s i t y t h e d o s e e q u i v a l e n t r a t e c a n b e c a l c u l a t e d u s i n g T a b l e V; t h e d o s e f l u e n c e r e l a t i o n s h i p m u s t b e i n t e g r a t e d o v e r t h e s o u r c e ' s n e u t r o n s p e c t r u m to o b t a i n t h e a v e r a g e d o s e e q u i v a l e n t r a t e [49],

29

2. 5. 3. C a l i b r a t i o n wi th a n a c c e l e r a t o r s o u r c e

B e c a u s e of t h e two p r o b l e m s m e n t i o n e d a t t h e end of t h e l a s t s e c t i o n , t h e c a l i b r a t i o n of i n s t r u m e n t s i s b e s t c a r r i e d out w h e r e p r a c t i c a l u s i n g an a c c e l e r a t o r s o u r c e ; t h i s wi l l h a v e a l a r g e r , v a r i a b l e output and wi l l e m i t m o n o e n e r g e t i c n e u t r o n s . T h e e n e r g y of t h e n e u t r o n s wi l l d e p e n d upon t h e r e a c t i o n u s e d , t h e b o m b a r d i n g p a r t i c l e e n e r g y and t h e a n g l e a t w h i c h t h e n e u t r o n s a r e b e i n g d e t e c t e d . T h e r e a r e t w o r e a c t i o n s t h a t m a y b e u s e d wi th a 100 — 400 keV p o s i t i v e ion a c c e l e r a t o r . T h e s e a r e t h e D(d, n ) 3 He r e a c t i o n , r e f e r r e d t o a s t h e (D ,D) r e a c t i o n f o r s h o r t ; and t h e T ( d , n ) 4 He r e a c t i o n , a b b r e v i a t e d to ( D , T ) r e a c t i o n . N e u t r o n s in t h e e n e r g y r e g i o n of 2. 5 M e V c a n b e p r o d u c e d b y t h e (D ,D) r e a c t i o n . If t h e b e a m i s no t p r o p e r l y f o c u s s e d a l o n g t h e f l i gh t t u b e s e c o n d a r y t a r g e t s c a n b e f o r m e d a l o n g t h e f l i g h t t u b e w h e r e t h e b e a m s t r i k e s i t , u s u a l l y a t d i a p h r a g m s . T h i s p r o b l e m a l s o a r i s e s f o r t h e ( D , T ) r e a c t i o n ; t h i s r e a c t i o n c a n b e u s e d to p r o d u c e n e u t r o n s in t h e r e g i o n of 14 MeV. If a 3 - M e V a c c e l e r a t o r i s a v a i l a b l e f u r t h e r r e a c t i o n s m a y b e u s e d , and of t h e s e t h e two m o s t f r e q u e n t l y u s e d a r e t h e 7 L i ( p , n ) ' B e r e a c t i o n and t h e T (p , n ) 3 He r e a c t i o n . M o n o e n e r g e t i c n e u t r o n s f r o m 20 keV to 1. 30 MeV c a n b e p r o d u c e d b y t h e 7 L i ( p , n ) 7 B e r e a c t i o n . C o r r e c t i o n s h a v e to b e a p g l i e d f o r a s e c o n d g r o u p of n e u t r o n s a r i s i n g f r o m t h e e x c i t e d s t a t e of 7 B e ' w h e n t h e b o m b a r d i n g p r o t o n e n e r g y e x c e e d s 2. 378 M e V . T h e T ( p , n) 3 H e r e a c t i o n c a n b e u s e d to g ive m o n o e n e r g e t i c n e u t r o n s f r o m 700 keV to 2. 1 MeV.

E m i s s i o n r a t e s f r o m t h e s e r e a c t i o n s a r e b e s t m e a s u r e d u s i n g a c a l i b -r a t e d l o n g c o u n t e r [ 5 0 - 5 2 ] , T h e r e a r e a l a r g e n u m b e r of d i f f e r e n t d e s i g n s of l o n g c o u n t e r s b u t . t h e d e s i g n m o s t c o m m o n l y c o m i n g in to u s e i s t h a t of d e P a n g h e r [52], T h e chief a d v a n t a g e s of t h i s c o u n t e r a r e : (a) T h e c o u n t e r c a n b e c h e c k e d b y u s e of a r e t r a c t a b l e s m a l l P u / B e (a, n) s o u r c e . H o w e v e r , an A m / B e ( o , n ) s o u r c e is a b e t t e r o n e t o u s e a s , u n l i k e t h e P u / B e s o u r c e , it i s not s u b j e c t t o r a d i o a c t i v e g r o w t h ; (b) P o l y t h e n e h a s b e e n u s e d a s t h e m o d e r a t o r a n d t h e c o u n t e r i s t h e r e f o r e l e s s l i a b l e t o d a m a g e t h a n a p a r a f f i n wax c o u n t e r ; (c) T h e d e s i g n and c o n s t r u c t i o n of t h e c o u n t e r g i v e s r e p r o d u c i b l e r e s u l t s c o m p a r i n g one c o u n t e r t o a n o t h e r . D e s i g n d r a w i n g s f o r t h e c o u n t e r m a y b e o b t a i n e d by w r i t i n g t o W. C . R o e s c h , B a t t e l l e - N o r t h w e s t , R i c h l a n d , W a s h i n g t o n , USA.

S t a n d a r d i z a t i o n of t h e l ong c o u n t e r i s c a r r i e d out u s i n g s o u r c e s s e l e c t e d f r o m T a b l e VI w h o s e m e a n e n e r g i e s v a r y f r o m 30 keV t o 4. 5 MeV and w h o s e o u t p u t s h a v e b e e n m e a s u r e d by r e f e r e n c e to a m a n g a n e s e b a t h . T h e r e a r e two m e t h o d s of s t a n d a r d i z a t i o n :

(a) In t h e f i r s t m e t h o d t h e l ong c o u n t e r i s p o s i t i o n e d in a s s c a t t e r -f r e e c o n d i t i o n s a s p o s s i b l e a n d t h e v a r i o u s s o u r c e s a r e t h e n p l a c e d at v a r y i n g d i s t a n c e s , x c m , f r o m t h e f r o n t f a c e of t h e l ong c o u n t e r . T h e c o u n t - r a t e s , c p s , c o r r e s p o n d i n g to t h e s e d i s t a n c e s a r e n o t e d and c o r r e c t e d f o r d e a d t i m e of t h e c o u n t e r and e l e c t r o n i c c i r c u i t . T h e r e s p o n s e of t h e l o n g c o u n t e r c a n b e e x p r e s s e d a s

C P S = a * + ( x + d 0 ) S

w h e r e d0 i s t h e e f f e c t i v e c e n t r e of t he c o u n t e r in c m f r o m t h e f r o n t f a c e a n d v a r i e s w i t h i n c i d e n t e n e r g y , a x i s t h e r e s p o n s e of t h e l ong c o u n t e r t o s c a t t e r e d r a d i a t i o n (at t h e d i s t a n c e x) and b i s a s o u r c e f a c t o r = n • e / s e c - 47r w h e r e e i s t h e e f f i c i e n c y of t h e c o u n t e r . T h e v a l u e of a x c a n t h e n b e

30

e s t i m a t e d f r o m s h a d o w c o n e m e a s u r e m e n t s and t h e e f f e c t i v e c e n t r e , do, a n d e f f i c i e n c y of t h e l ong c o u n t e r d e t e r m i n e d . In R e f . [52] it i s s h o w n t h a t t h e e f f e c t i v e c e n t r e , d 0 , i s g iven a p p r o x i m a t e l y by t h e r e l a t i o n d 0 = 7. 8 + 1. I E , w h e r e E i s t h e n e u t r o n e n e r g y in MeV. T h e e f f i c i e n c y of t h e l o n g c o u n t e r i s t h e n p l o t t e d a g a i n s t t h e m e a n e n e r g y of t h e n e u t r o n s .

F r o m t h e e f f i c i e n c y c u r v e t h e f a c t o r c o r r e s p o n d i n g to a p a r t i c u l a r e n e r g y p r o d u c e d b y t h e a c c e l e r a t o r c a n b e s e l e c t e d . U s i n g t h i s f a c t o r t h e l o n g c o u n t e r c a n t h e n b e u s e d to m e a s u r e t h e f l u x d e n s i t y f r o m t h e a c c e l e r a t o r a t t h e s a m e p o s i t i o n t h a t the i n s t r u m e n t r e q u i r i n g c a l i b r a t i o n w i l l b e p l a c e d . T h e a c c e l e r a t o r ou tpu t shou ld b e m o n i t o r e d b y a f u r t h e r d e t e c t o r o r by t h e a s s o c i a t e d p a r t i c l e d e t e c t o r t o e n s u r e t h a t t h e output h a s r e m a i n e d c o n s t a n t d u r i n g t h e c o m p a r i s o n . W h e r e t h e s c a t t e r i n g c o n -d i t i o n s a r e b a d it m a y b e n e c e s s a r y t o app ly a c o r r e c t i o n f o r t h e c o n t r i b u t i o n f r o m s c a t t e r e d r a d i a t i o n .

(b) T h e s e c o n d m e t h o d d o e s no t r e q u i r e a k n o w l e d g e of t h e e f f e c t i v e c e n t r e of t he l o n g c o u n t e r o r of i t s r e s p o n s e t o s c a t t e r e d r a d i a t i o n . T h e l o n g c o u n t e r i s p l a c e d o p p o s i t e a t t h e s a m e a n g l e and p o s i t i o n r e l a t i v e t o t h e a c c e l e r a t o r a s t h e i n s t r u m e n t u n d e r g o i n g c a l i b r a t i o n . T h e v a r i o u s r a d i o n u c l i d e n e u t r o n s o u r c e s a r e t h e n p l a c e d a t t h e a c c e l e r a t o r t a r g e t p o s i t i o n and t h e l o n g - c o u n t e r c o u n t - r a t e s no t ed . A g r a p h i s t h e n d r a w n of t h e c o u n t - r a t e , c o r r e c t e d f o r d e a d t i m e , d iv ided by t h e s o u r c e e m i s s i o n r a t e a g a i n s t t h e m e a n n e u t r o n e n e r g y . T h e e m i s s i o n r a t e m u s t b e c o r r e c t e d f o r t h e a n g u l a r d i s t r i b u t i o n of t h e s o u r c e ou tput . F r o m p u b l i s h e d d a t a t h e e n e r g y of t h e n e u t r o n s c o r r e s p o n d i n g to t h e a n g l e at w h i c h t h e l ong c o u n t e r and i n s t r u m e n t a r e s i t u a t e d and t h e e n e r g y of t h e b o m b a r d i n g p a r t i c l e m a y b e d e d u c e d . T h e l o n g - c o u n t e r f a c t o r c o r r e s p o n d i n g t o t h i s e n e r g y i s o b t a i n e d f r o m t h e l o n g - c o u n t e r c a l i b r a t i o n g r a p h . T h i s f a c t o r m a y t h e n b e u s e d to e s t i m a t e t h e a c c e l e r a t o r ou tpu t . T h e f lux d e n s i t y a t t h e i n s t r u m e n t u n d e r c a l i b r a t i o n c a n b e c a l c u l a t e d by t h e s a m e m e t h o d a s w a s d e s c r i b e d f o r c a l i b r a t i o n wi th r a d i o n u c l i d e n e u t r o n s o u r c e s . E f f e c t s of s c a t t e r e d n e u t r o n s upon t h e i n s t r u m e n t u n d e r c a l i b r a t i o n c a n b e d e d u c e d f r o m s h a d o w c o n e m e a s u r e m e n t s .

F o r m a n y of t h e a c c e l e r a t o r r e a c t i o n s an a s s o c i a t e d c h a r g e d p a r t i c l e i s e m i t t e d . If t h e s e a r e m e a s u r e d a f u r t h e r e s t i m a t i o n of t h e a c c e l e r a t o r ou tpu t c a n b e o b t a i n e d [53, 54],

T h e a s s o c i a t e d p a r t i c l e t e c h n i q u e m a y b e u s e d f o r t h e f o l l o w i n g r e -a c t i o n s : D(d, n) 3 He, T (d , n ) 4 He and T ( p , n) 3He. S e v e r a l p r o b l e m s a r i s e , h o w e v e r . F o r t h e (D, D) r e a c t i o n t h e 3 He p a r t i c l e h a s a p p r o x i m a t e l y t h e s a m e r a n g e a s a s c a t t e r e d d e u t e r o n and f o r h i g h t a r g e t c u r r e n t s c a n n o t b e s a t i s f a c t o r i l y r e s o l v e d . F o r t h i s r e a c t i o n it i s t h e r e f o r e b e s t t o count t h e p r o t o n f r o m t h e c o m p e t i n g r e a c t i o n D ( d , p ) 3 H . In t h e (D, T) r e a c t i o n two p r o b l e m s a r i s e . T h e r e i s a g r a d u a l b u i l d - u p on t h e t a r g e t of d e u t e r o n s and c o n s e q u e n t l y t h e a s s o c i a t e d p a r t i c l e d e t e c t o r , u s u a l l y a s o l i d s t a t e d e v i c e , w i l l s t a r t t o d e t e c t t h e 3 H e p a r t i c l e and p r o t o n f r o m t h e two (D,D) r e a c t i o n s . T h e s e c o n d p r o b l e m o c c u r s with old t a r g e t s . A. 3 H e p a r t i c l e i s o b t a i n e d f r o m t h e d e c a y of t r i t i u m and a 3 H e (d, P ) 4 H e r e a c t i o n o c c u r s . T h e a l p h a p a r t i c l e p r o d u c e d h a s m u c h the s a m e e n e r g y a s t h a t f r o m t h e ( D , T ) r e a c t i o n [55], T o avo id t h i s new t a r g e t s s h o u l d b e u s e d o r t h e b o m -b a r d i n g d e u t e r o n e n e r g y shou ld b e k e p t at a p p r o x i m a t e l y 110 k e V . At t h i s e n e r g y t h e (D, T) r e a c t i o n h a s i t s m a x i m u m c r o s s - s e c t i o n and t h e (D, 3He) r e a c t i o n h a s a l ow c r o s s - s e c t i o n , i t s r e s o n a n c e b e i n g a t 440 k e V .

31

Primary standard thermal neutron flux assembly

Primary standard, radio-nuclide source emission rates by manganese bath

Primary standard, 3 MeV Van de Craaff and, or 400 KeV positive

ion accelerator

Secondary standard manganese bath

0,025eV Radionuclide neutron sources (y,n and a,n)

Primary long counter

• Secondary long counter

24 KeV Sb/Be

I 400 KeV Ra/Be

800 KeV Th/Be

Secondard standard thermal neutron flux assembly

3.9 MeV Ra/Be

4.4 MeV Am/Be

Accelerator produced thermal flux

Accelerator neutron sources

3 MeV Van de Graaff ^Li(p,n)^ Be reaction 20 KeV to 1.3 MeV

nd3H(p,n)3 He reaction 700 KeV to 2.0 MeV

400 KeV positive ion accelerator

D/D reaction 1.7 MeV to 6 MeV

3 MeV Van de Graaff

D/T reaction 12 MeV to 20 MeV

Secondary long counter Associated particle counting and Secondary long counter

Energy response determinations and calibration of instruments

FIG. 8. Schematic diagram of neutron sources and their standards.

W h e r e a l a r g e r o o m c a n n o t b e s e t a s i d e f o r t h e c a l i b r a t i o n and c o n -s e q u e n t l y t h e s c a t t e r e d r a d i a t i o n c o n t r i b u t i o n t o t h e r e a d i n g i s g r e a t e r t h a n 25% of t h e t o t a l , a s u b s t i t u t e m e t h o d s h o u l d b e u s e d . T h e d o s e e q u i v a -l e n t r a t e f r o m t h e s o u r c e i s m e a s u r e d wi th an i n s t r u m e n t t h a t h a s b e e n c a l i b r a t e d a t a w e l l - e q u i p p e d l a b o r a t o r y . T h e i n s t r u m e n t u n d e r t e s t i s t h e n s u b s t i t u t e d in t h e s a m e p o s i t i o n . B o t h i n s t r u m e n t s s h o u l d b e of t h e s a m e d e s i g n s o t h a t e r r o r s a r i s i n g f r o m f a c t o r s l i k e e n e r g y r e s p o n s e a r e no t i n t r o d u c e d . If t h e r a d i a t i o n s o u r c e b e i n g u s e d i s an a c c e l e r a t o r t h e n a m o n i t o r s h o u l d b e u s e d t o e n s u r e t h a t o p e r a t i n g c o n d i t i o n s h a v e not a l t e r e d d u r i n g t h e s u b s t i t u t i o n .

An a l t e r n a t i v e a p p r o a c h w h e n b a d s c a t t e r i n g c o n d i t i o n s p r e v a i l i s t o u s e a c o l l i m a t e d s o u r c e [56, 57], T h i s h a s t h e a d v a n t a g e t h a t t h e b e a m c o n d i t i o n s a r e c l o s e r t o t h o s e u s e d in c a l c u l a t i n g d o s e / f l u e n c e r e l a t i o n -s h i p s . T h e f l u e n c e r a t e in t h e b e a m c a n b e c a l c u l a t e d f r o m t h e c h a r a c t e -r i s t i c s of t h e s o u r c e o r t h e s u b s t i t u t i o n t e c h n i q u e j u s t d e s c r i b e d c a n b e u s e d .

2. 5. 4. Slow n e u t r o n f l u x d e n s i t i e s

Slow n e u t r o n f l u x d e n s i t i e s c a n b e p r o d u c e d b y m o d e r a t i n g t h e n e u t r o n s f r o m a h i g h e r e n e r g y n e u t r o n s o u r c e . S o u r c e s t h a t c a n b e u s e d f o r t h i s i n c l u d e n u c l e a r r e a c t o r s , r a d i o n u c l i d e s o u r c e s and a c c e l e r a t o r s . T h e c h o i c e of a p a r t i c u l a r s o u r c e w i l l b e g o v e r n e d by t h e t y p e of m e a s u r i n g i n s t r u m e n t r e q u i r i n g c a l i b r a t i o n and c o s t . W h e r e t h e d e t e c t o r r e q u i r i n g c a l i b r a t i o n i s s m a l l , s u c h a s f o i l s , s m a l l B F 3 t u b e s , f i l m b a d g e s and s o l i d - s t a t e d e v i c e s t h e n t h e b e s t r e s u l t s c a n ' b e o b t a i n e d f r o m an a s s e m b l y in w h i c h r a d i o n u c l i d e n e u t r o n s o u r c e s a r e p l a c e d . M o s t p r i m a r y s t a n d a r d t h e r m a l n e u t r o n f l u x d e n s i t i e s a r e of t h i s t y p e [58, 59],

A l a r g e s o u r c e o r a n u m b e r of s o u r c e s a r e p l a c e d in a m o d e r a t o r , u s u a l l y g r a p h i t e , a n d t h e m o d e r a t o r i s s u r r o u n d e d b y a t h e r m a l n e u t r o n r e f l e c t o r . T h e s t a n d a r d t h e r m a l n e u t r o n f l u x d e n s i t y i s m e a s u r e d in a s m a l l a i r c a v i t y u s u a l l y s i t u a t e d a t t h e c e n t r e of t h e m o d e r a t o r . A m / B e ( o , n) s o u r c e s a r e s u i t a b l e s i n c e t h e y h a v e a l o n g h a l f - l i f e and a r e r e l a t i v e l y f r e e f r o m u n w a n t e d r a d i a t i o n a l t h o u g h o t h e r l o n g - l i v e d s o u r c e s m a y a l s o b e u s e d . A l i m i t a t i o n wi th t h e s e a s s e m b l i e s i s t h e low v a l u e s of f l u e n c e r a t e o b t a i n e d ; t h e s e v e r y r a r e l y e x c e e d 10 4 n /cm 2 . s e c . F o r h i g h e r f l u e n c e r a t e s t h e t h e r m a l f l ux d e n s i t y i n s i d e a r e a c t o r m a y b e u s e d bu t b e c a u s e of t h e a c c o m p a n y i n g h i g h l e v e l s of o t h e r r a d i a t i o n s t h i s t y p e of a s s e m b l y i s n o r m a l l y u s e d only t o c a l i b r a t e f o i l s .

A c c e l e r a t o r p r o d u c e d f l u x e s o r t h e t h e r m a l c o l u m n f r o m a r e a c t o r [ 41, 63, 64] a r e b e s t s u i t e d f o r c a l i b r a t i n g l a r g e - s i z e d e t e c t o r s . T h e f l u e n c e r a t e c a n b e s t a n d a r d i z e d b y a c t i v a t i n g m e t a l l i c f o i l s and t h e n m e a s u r i n g t h e i r i n d u c e d a c t i v i t y , u s u a l l y wi th a 4ir b e t a o r 47r b e t a - g a m m a c o i n c i -d e n c e c o u n t e r . V a r i a t i o n s in t h e f l u e n c e r a t e l e v e l c a n b e o b t a i n e d in t h e s e s y s t e m s a n d it i s s i m p l e r to m o n i t o r t h e l e v e l w i t h a B F 3 o r a b o r o n -l i n e d i o n i z a t i o n c h a m b e r t u b e t h a t h a s i t s e l f b e e n s t a n d a r d i z e d b y f o i l m e a s u r e m e n t s .

In t h e i n t r o d u c t i o n t o t h i s s e c t i o n s l o w n e u t r o n s w e r e d e f i n e d a s t h o s e h a v i n g a n e n e r g y b e l o w 0. 5 eV, t h e c a d m i u m c u t - o f f , a n d t h e r e a s o n s f o r t h i s c h o i c e w e r e a l s o g iven . T h u s , in d e t e r m i n i n g t h e f l u e n c e r a t e l e v e l in t h e a b o v e s y s t e m s , m e a s u r e m e n t s shou ld b e m a d e w i t h t h e f o i l i r r a d i a t e d b a r e and e n c a s e d in 1 m m c a d m i u m , s o t h a t t h e f l u e n c e r a t e b e l o w 0. 5 eV

33

c a n b e c a l c u l a t e d . It t h e r e f o r e f o l l o w s t h a t t h e v a r i o u s d e v i c e s c a l i b r a t e d in t h e s e f l u e n c e s s h o u l d a l s o b e i r r a d i a t e d b a r e and e n c a s e d in c a d m i u m s o t h a t t h e i r r e s p o n s e t o t h e s u b - c a d m i u m f l u e n c e r a t e c a n b e d e t e r m i n e d . W h e r e t h e i n s t r u m e n t r e s p o n d s to g a m m a r a d i a t i o n a c o r r e c t i o n f a c t o r m a y h a v e t o b e m a d e to a l l o w f o r t h e g a m m a r a d i a t i o n p r o d u c e d b y t h e s l o w n e u t r o n c a p t u r e in c a d m i u m . M o r e c o m p l e t e d e s c r i p t i o n s of t h e t e c h n i q u e s a n d a n a l y s i s of t h e m e a s u r e m e n t of s l o w n e u t r o n f l u x e s a r e g iven in t h e v a r i o u s r e f e r e n c e s m e n t i o n e d in t h i s s e c t i o n and in ICRU Handbook 85 [65] , S u i t a b l e m e t a l l i c f o i l s f o r t h e s e m e a s u r e m e n t s a r e i n d i u m , m a n g a n e s e o r gold . C o r r e c t i o n s t o t h e f o i l m e a s u r e m e n t s m u s t b e m a d e f o r t h e f o l l o w i n g : s e l f - s h i e l d i n g , f l ux d e p r e s s i o n , a b s o r p t i o n of r e s o n a n c e n e u t r o n s in t h e c a d m i u m , d e p a r t u r e s of t h e fo i l c r o s s - s e c t i o n f r o m t h e l / v l a w in t h e low e n e r g y r e g i o n , and w h e r e t h e a b s o l u t e a c t i v i t y i s m e a s u r e d b y a 47t b e t a c o u n t e r a c o r r e c t i o n s h o u l d b e m a d e f o r t h e s e l f -a b s o r p t i o n of t h e b e t a p a r t i c l e s in t h e fo i l . T o c o n c l u d e t h i s s e c t i o n a d i a g r a m a t i c r e p r e s e n t a t i o n i s g iven , F i g . 8, of t h e r e l a t i o n s h i p b e t w e e n t h e v a r i o u s n e u t r o n s o u r c e s and t h e p r i m a r y and s e c o n d a r y s t a n d a r d s .

2. 5. 5. P r o b l e m s a s s o c i a t e d wi th t h e c a l i b r a t i o n of d i f f e r e n t t y p e s of n e u t r o n i n s t r u m e n t s

M a n y of t h e p r o b l e m s a s s o c i a t e d wi th t h e c a l i b r a t i o n of i n s t r u m e n t s d e s i g n e d f o r X - and g a m m a - r a y m e a s u r e m e n t s a r e a l s o a p p l i c a b l e t o n e u t r o n i n s t r u m e n t s ; t h u s , f o r a m u l t i - r a n g e i n s t r u m e n t t h e c r i t e r i o n f o r a d j u s t i n g t h e r a n g e s a r e s i m i l a r to t h o s e d i s c u s s e d in s e c t i o n 2. 3. 1. 4. F o r an i n s t r u m e n t d e s i g n e d to m e a s u r e n e u t r o n s o v e r a w i d e e n e r g y r a n g e , a c o m p l e t e e n e r g y r e s p o n s e d e t e r m i n a t i o n m u s t b e m a d e . A k n o w l e d g e of t h i s w i l l e n a b l e t h e i n s t r u m e n t t o b e a d j u s t e d a t t h e c a l i b r a t i o n e n e r g y s o t h a t i t s o v e r a l l a c c u r a c y a t o t h e r e n e r g i e s i s i m p r o v e d .

M a n y n e u t r o n i n s t r u m e n t s wi th l a r g e d e t e c t o r s y s t e m s r e s p o n d a s if t h e y w e r e a po in t d e t e c t o r . T h i s po in t i s t e r m e d t h e e f f e c t i v e c e n t r e and i t s p o s i t i o n c a n v a r y wi th n e u t r o n e n e r g y . M e a s u r e m e n t s m u s t b e m a d e to d e t e r m i n e t h e p o s i t i o n of t h e e f f e c t i v e c e n t r e a t d i f f e r e n t e n e r g i e s s o t h a t t h e d o s e e q u i v a l e n t r a t e a t t h i s p o s i t i o n c a n b e c a l c u l a t e d d u r i n g c a l i b r a t i o n . So t h a t c o n f i d e n c e l i m i t s c a n b e p l a c e d on m e a s u r e m e n t s m a d e wi th t h e i n s t r u m e n t t h e a n g u l a r r e s p o n s e of t h e i n s t r u m e n t s h o u l d a l s o b e d e t e r m i n e d ; t h i s w i l l v a r y wi th t h e n e u t r o n e n e r g y .

E s t i m a t i o n s s h o u l d a l s o b e m a d e of t h e r e s p o n s e of t h e i n s t r u m e n t t o s c a t t e r e d r a d i a t i o n . T h i s i s l i k e l y t o b e m o r e i m p o r t a n t w h e n c a l i b r a t i n g a t l ow d o s e e q u i v a l e n t r a t e s wi th h igh output s o u r c e s and c o n s e q u e n t l y l a r g e s o u r c e - t o - d e t e c t o r d i s t a n c e s . In t h e s e c o n d i t i o n s t h e r a t i o of d i r e c t to s c a t t e r e d f l u e n c e r a t e d e c r e a s e s v e r y r a p i d l y . R a t h e r t h a n c a l i b r a t e u n d e r t h e s e c o n d i t i o n s it i s b e t t e r t o u s e a l o w e r ou tpu t s o u r c e and s m a l l e r s o u r c e - t o - d e t e c t o r d i s t a n c e s . E x c e p t f o r v e r y a c c u r a t e c a l i b r a t i o n w o r k o r w h e r e t h e c o n t r i b u t i o n f r o m s c a t t e r e d r a d i a t i o n i s l a r g e , t h e e f f e c t of s c a t t e r e d r a d i a t i o n c a n o f t e n b e n e g l e c t e d . T h i s i s b e c a u s e t h e r e s p o n s e of m a n y n e u t r o n i n s t r u m e n t s - i n t e r m s of d o s e e q u i v a l e n t - i s l e s s t o t h e l o w e r e n e r g y s c a t t e r e d r a d i a t i o n t h a n to t h e d i r e c t n e u t r o n f l u e n c e r a t e . A l s o t h e d o s e / f l u e n c e c u r v e i s w e i g h t e d a g a i n s t t h e l o w e r e n e r g y n e u t r o n s , and t h e f o l l o w i n g h y p o t h e t i c a l c a s e i l l u s t r a t e s t h i s . L e t u s a s s u m e t h a t w e h a v e a s o u r c e e m i t t i n g 2. 5 - M e V n e u t r o n s and t h a t a t t h e d e t e c t o r 90% of t h e n e u t r o n f l u e n c e r a t e i s of t h i s e n e r g y a n d t h e r e m a i n i n g 10% s c a t t e r e d

34

n e u t r o n s h a v e an e n e r g y of 100 k e V . F o r f u r t h e r s i m p l i f i c a t i o n w e w i l l t a k e t h e t o t a l f l u e n c e r a t e a s 100 cm"2 s ec" 1 . F r o m t h e d o s e / f l u e n c e r e l a t i o n s h i p ( T a b l e V) i t c a n b e s e e n t h a t t h e d o s e e q u i v a l e n t r a t e f r o m t h e 2. 5 - M e V n e u t r o n s i s 1 1 . 2 5 m r e m / h w h i l s t t h e s c a t t e r e d n e u t r o n s c o n t r i b u t e a p -p r o x i m a t e l y 0. 22 m r e m / h , i . e. t h e e f f e c t i s l e s s t h a n 2%. F o r c e r t a i n a c c e l e r a t o r r e a c t i o n s t h e r e v e r s e a p p l i e s s i n c e p a r t of t h e i n d i r e c t f l u e n c e m a y b e of h i g h e r e n e r g y t h a n t h e d i r e c t f l u e n c e . T h i s c a n o c c u r w h e n t h e e n e r g y of t h e n e u t r o n s , f o r a f i x e d b o m b a r d i n g e n e r g y , v a r i e s s i g -n i f i c a n t l y wi th t h e a n g l e of t h e e m i t t e d n e u t r o n s . C o n s i d e r a t i o n m u s t t h e n b e g i v e n to t h e r e s p o n s e of t h e l o n g c o u n t e r t o s c a t t e r e d n e u t r o n s r e l a t i v e t o t h e d i r e c t f l u e n c e c o m p a r e d to t h i s r a t i o f o r t h e t e s t i n s t r u m e n t . I t m a y a l s o b e n e c e s s a r y , in o r d e r t o e n s u r e t h a t b o t h i n s t r u m e n t s a r e e x p o s e d to t h e s a m e d i r e c t s p e c t r u m , tha t t h e y m u s t b o t h s u b t e n d i d e n t i c a l s o l i d a n g l e s f r o m t h e a c c e l e r a t o r t a r g e t .

P r o t o n r e c o i l c o u n t e r s do not r e s p o n d to n e u t r o n s b e l o w a p p r o x i m a t e l y 200 keV and s o s h o u l d no t b e c a l i b r a t e d wi th n e u t r o n s o u r c e s w h i c h e m i t n e u t r o n s b e l o w t h e i r c u t - o f f e n e r g y . T h e i r d i s c r i m i n a t i o n t o g a m m a r a d i a -t i on d o s e s s h o u l d a l s o b e c h e c k e d to s e e t h a t i t i s w i th in t h e i n s t r u m e n t ' s d e s i g n s p e c i f i c a t i o n s .

A d d i t i o n a l p r o b l e m s a s s o c i a t e d wi th r e m c o u n t e r s a r e t h e i r d i s c r i m i n a -t i o n a g a i n s t g a m m a r a d i a t i o n and t h e m e a s u r e m e n t of t h e i r s l ow n e u t r o n r e s p o n s e . T h e c o u n t e r s s h o u l d b e c h e c k e d to s e e if t h e y s t i l l r e s p o n d to n e u t r o n s in i n t e n s e g a m m a f i e l d s . T h i s i s p a r t i c u l a r l y i m p o r t a n t f o r r e m c o u n t e r s t h a t u s e L i l ( E u ) c r y s t a l s a s t h e i r d e t e c t o r s ; f o r t h e s e i n s t r u m e n t s t h e d i s c r i m i n a t i o n a g a i n s t h i g h e n e r g y g a m m a r a d i a t i o n s h o u l d a l s o b e c h e c k e d . S i n c e r e m c o u n t e r s r e s p o n d to n e u t r o n s f r o m t h e r m a l e n e r g i e s u p to a b o u t 10 M e V , a v e r y p u r e t h e r m a l n e u t r o n f l u x d e n s i t y s h o u l d b e u s e d to d e t e r m i n e t h e i r t h e r m a l n e u t r o n r e s p o n s e . F r o m t h e d o s e f l u e n c e r e l a t i o n s h i p , T a b l e V, i t can b e s e e n t h a t on ly a f l u e n c e r a t e of 30 n / c m 2 s e c a t 500 keV w i l l g ive t h e s a m e i n s t r u m e n t r e a d i n g a s a t h e r m a l n e u t r o n f l u e n c e r a t e of 670 n / c m 2 s e c . T h e m o s t r e l i a b l e t h e r m a l n e u t r o n r e s p o n s e i s t h e r e f o r e d e t e r m i n e d b y t h e u s e of a t h e r m a l c o l u m n f r o m a r e a c t o r .

I n R e f . [66] t h e - p r o b l e m s of c a l i b r a t i n g and u s i n g a s l o w n e u t r o n i n s t r u m e n t a r e d i s c u s s e d . A s m e n t i o n e d e a r l i e r m o s t s l o w n e u t r o n d e t e c t o r s h a v e a l / v c r o s s - s e c t i o n ; a c h a n g e in n e u t r o n e n e r g y bu t no t in t h e f l u e n c e r a t e w i l l t h e r e f o r e c a u s e a c h a n g e in t h e i n s t r u m e n t ' s r e a d i n g . T h e d o s e f l u e n c e r e l a t i o n s h i p , h o w e v e r , d o e s not v a r y s i g n i f i c a n t l y in t h i s r e g i o n and s o t h e i n s t r u m e n t w i l l not r e a d a c c u r a t e l y if u s e d in a d i f f e r e n t s p e c t r u m to t h e o n e in wh ich it w a s c a l i b r a t e d . F o r a M a x w e l l i a n f l u e n c e r a t e d i s t r i b u t i o n t h e e r r o r s w i l l , h o w e v e r , b e s m a l l .

If t h e s l o w n e u t r o n i n s t r u m e n t u s e s a b o r o n - l i n e d i o n i z a t i o n c h a m b e r a s i t s d e t e c t o r t h e n it s h o u l d on ly b e c a l i b r a t e d u s i n g s l o w n e u t r o n s . A t t e m p t s to c a l i b r a t e it w i t h a g a m m a s o u r c e w i l l not g i v e an i n d i c a t i o n of i t s s l ow n e u t r o n r e s p o n s e , a s i t w i l l not d e t e r m i n e if in f a c t t h e i n s t r u m e n t s t i l l i s , o r e v e r w a s , b o r o n l i n e d .

2 . 6 . A l p h a r a d i a t i o n O w i n g to t h e v e r y s h o r t r a n g e of an a l p h a p a r t i c l e in a m e d i u m , i n s t r u -

m e n t s t o m e a s u r e a l p h a r a d i a t i o n s h o u l d only b e u s e d to m e a s u r e c o n t a m i -n a t i o n and to e s t i m a t e i n h a l a t i o n h a z a r d s . S o u r c e s t h a t h a v e b e e n s t a n d a r d -i z e d , in t e r m s of d i s i n t e g r a t i o n s p e r s e c o n d e m i t t e d w i t h i n a 2tt s o l i d a n g l e , b y m e a n s of a s t a n d a r d d e t e c t o r s h o u l d b e u s e d f o r c a l i b r a t i n g t h e i n s t r u m e n t s .

35

T A B L E VII. C A L I B R A T I O N S O U R C E S O F A L P H A - R A Y S

Radionuclide Alpha-particle energy

(MeV) (<7o) Hal f - l i f e

Z35y 4 . 1 8 - 4 . 56 7 X 108 yr

239 P u 5. 096 10 .7 2 . 4 4 X 10" yr

5. 147 72. 5

5. 134 16 .8

210 Po(RaF) 5 . 3 0 5 100 138 .4 d 241 Am 5 . 4 4 13 458 yr

5 . 4 8 85'

238pu 5 . 4 9 5 72 86 yr

5 . 4 5 2 28

5. 352 0 . 0 9

242 Cm 6. 110 7 3 . 7 163 d

6. 066 2 6 . 3

5. 965 0 . 0 3 5

210 Pb(RaD+ E) 5. 06 1 .7 X 10"4 22 yr

238U 4. 2 100 4 . 5 1 X 10® yr

244 Cm 5. 801 76. 7 18 yr

5. 759 2 3 . 3

5. 661 0. 017

234 U 4. 763 72 2. 35 x 105 yr

4. 717 28

A r a n g e of r a d i o n u c l i d e s e m i t t i n g a l p h a p a r t i c l e s of d i f f e r e n t e n e r g i e s s h o u l d b e u s e d ( T a b l e VII g i v e s a l i s t of s u i t a b l e i s o t o p e s ) s i n c e , owing t o t h e l a r g e r a n g e v a r i a t i o n wi th a l p h a e n e r g y , t h e s e n s i t i v i t y of an i n s t r u -m e n t w i l l v a r y c o n s i d e r a b l y wi th e n e r g y , t h i s v a r i a t i o n d e p e n d i n g upon t h e w a l l t h i c k n e s s o r d e p t h of t h e d e t e c t o r . T h e l i m i t e d r a n g e of an a l p h a p a r t i c l e in a i r n e c e s s i t a t e s t h a t t h e i n s t r u m e n t b e c a l i b r a t e d and u s e d w i t h t h e d e t e c t o r a s p h y s i c a l l y c l o s e t o t h e r a d i a t i o n s o u r c e a s p o s s i b l e . T h i s in t u r n m a k e s t h e i n s t r u m e n t r e s p o n s e s t r o n g l y d e p e n d e n t u p o n t h e s o u r c e and d e t e c t o r s i z e .

A l p h a s o u r c e s can e i t h e r b e p u r c h a s e d c o m m e r c i a l l y o r b e m a n u f a c t u r e d b y e l e c t r o p l a t i n g a m e t a l l i c a l p h a - r a y e m i t t e r on t o a s t a i n l e s s - s t e e l b a c k i n g o r o t h e r m e t a l d i s c . T h e s t a n d a r d i z a t i o n of t h e s o u r c e c a n b e p e r f o r m e d w i t h a s t a n d a r d c o u n t e r , u s u a l l y e i t h e r a w i n d o w l e s s p r o p o r t i o n a l c o u n t e r o r a c o u n t e r w i t h a v e r y t h i n window.

2. 7. C a l i b r a t e d s o u r c e s

In i o n i z i n g r a d i a t i o n t h e r e p r o d u c t i o n ( f o r c a l i b r a t i o n p u r p o s e s ) of t h e t r u e v a l u e of t h e q u a n t i t y b e i n g m e a s u r e d i s f r e q u e n t l y d o n e b y m e a n s

36

of s t a n d a r d r a d i a t i o n s o u r c e s . A s a r u l e , s t a n d a r d a l p h a - and b e t a - s o u r c e s a r e m a d e in f o r m of p l a t e s ( f r o m 1 c m 2 u p to 900 c m 2 ) c o v e r e d b y a l a y e r of r a d i o a c t i v e m a t e r i a l [67 , 68], G a m m a and n e u t r o n s o u r c e s a r e m a d e in f o r m of s m a l l r i g h t c y l i n d e r s o r s p h e r e s ( 2 - 5 m m d i a m . X 2 - 5 m m h i g h f o r g a m m a - s o u r c e s and 1 0 - 3 0 m m d i a m . X 1 0 - 3 0 m m h i g h f o r n e u t r o n s o u r c e s ) s i m u l a t i n g " p o i n t " s o u r c e s of r a d i a t i o n [67, 68],

T h e r e a r e d i f f e r e n t c a t e g o r i e s of s t a n d a r d s o u r c e s — p r i m a r y , s e c o n -d a r y , l a b o r a t o r y and o p e r a t i o n a l [68a] , A p r i m a r y s t a n d a r d s o u r c e i s a s o u r c e t h a t h a s b e e n c a l i b r a t e d u s i n g a m e t h o d of a b s o l u t e m e a s u r e m e n t s and c e r t i f i e d a t a n a t i o n a l l a b o r a t o r y in t e r m s of f u n d a m e n t a l u n i t s . Such s t a n d a r d s a r e s u b j e c t t o i n t e r n a t i o n a l i n t e r c o m p a r i s o n . W h e n a s o u r c e i s c a l i b r a t e d w i t h r e f e r e n c e t o a p r i m a r y s t a n d a r d s o u r c e , it i s c a l l e d a s e c o n d a r y s t a n d a r d s o u r c e .

C a l i b r a t e d s o u r c e s s h o u l d s a t i s f y s o m e s p e c i a l r e q u i r e m e n t s . F o r g a m m a and n e u t r o n s o u r c e s t h e m a i n r e q u i r e m e n t i s a n g u l a r u n i f o r m i t y of t h e g a m m a o r n e u t r o n e m i s s i o n . F o r a l p h a and b e t a s o u r c e s t h e m a i n f e a t u r e i s s u r f a c e u n i f o r m i t y of t h e a l p h a and b e t a e m i s s i o n . S o m e c o u n t r i e s m a i n t a i n s p e c i f i e d s t a n d a r d s t h a t t h e m a n u f a c t u r e r m u s t f o l l o w [69], T h e s e s t a n d a r d s p r e s c r i b e a l l f e a t u r e s of t h e s o u r c e s and m e t h o d s of t h e i r c a l i -b r a t i o n and t h u s u n i f y c a l i b r a t i o n p r o c e d u r e s a t d i f f e r e n t l a b o r a t o r i e s and p r o v i d e h i g h and r e p r o d u c i b l e a c c u r a c y of c a l i b r a t i o n .

A l p h a and b e t a s o u r c e s a r e c a l i b r a t e d in t e r m s of a c t i v i t y and a c t i v i t y p e r u n i t a r e a of t h e s o u r c e . In a d d i t i o n , b e t a - s o u r c e s u s e d f o r d o s i m e t r y p u r p o s e s a r e c a l i b r a t e d i n t e r m s of d o s e r a t e at c e r t a i n d i s t a n c e s f r o m t h e s o u r c e s u r f a c e .

I s o t o p i c g a m m a s o u r c e s a r e c a l i b r a t e d in t e r m s of e x p o s u r e r a t e a t a c e r t a i n d i s t a n c e f r o m t h e s o u r c e and i s o t o p i c n e u t r o n s o u r c e s a r e c a l i b r a -t e d in t e r m s of t h e n u m b e r of n e u t r o n s e m i t t e d p e r u n i t t i m e .

C a l i b r a t i o n of s o u r c e s in a s e c o n d a r y s t a n d a r d l a b o r a t o r y i s m a i n l y c a r r i e d out b y t h e m e t h o d of r e l a t i v e m e a s u r e m e n t s of t h e q u a n t i t y u n d e r q u e s t i o n . T h e r e f o r e , t h e l a b o r a t o r y s h o u l d b e e q u i p p e d w i t h s p e c i a l c a l i b -r a t i o n j i g s and a p p r o p r i a t e a l p h a , b e t a , g a m m a and n e u t r o n i n s t r u m e n t s . T h e s e c o n d a r y s t a n d a r d j i g s and i n s t r u m e n t s m a y b e c e r t i f i e d b y a n a t i o n a l l a b o r a t o r y . T h e m e t h o d of r e l a t i v e m e a s u r e m e n t s i s a p p l i e d in a c a s e w h e n t h e s o u r c e s b e i n g c o m p a r e d a r e of t h e s a m e n u c l i d e con t en t a s w e l l a s of t h e s a m e s i z e , s h a p e , m a t e r i a l , and s u p p o r t s .

T h e c a l i b r a t i o n j i g s h o u l d p r o v i d e s i m i l a r g e o m e t r y c o n d i t i o n s d u r i n g t h e c o m p a r i s o n of s t a n d a r d and c h e c k e d s o u r c e s . T h e j i g s h o u l d h a v e a s o c k e t f o r p u t t i n g in a l t e r n a t e l y t h e s t a n d a r d and c h e c k e d s o u r c e s . It i s a d v i s a b l e t h a t t h e c o m p a r e d s o u r c e s h a v e s i m i l a r a c t i v i t i e s to av o i d a p p l y i n g c o r r e c t i o n f a c t o r s t h a t wou ld l e a d to a d d i t i o n a l e r r o r s d u e t o m e a s u r e m e n t s a t d i f f e r e n t s u b - r a n g e s of t h e m e a s u r i n g i n s t r u m e n t , o r t o d e a d t i m e of a p u l s e c o u n t i n g un i t .

B e f o r e c a l i b r a t i o n , t h e s o u r c e i n q u e s t i o n m u s t b e c h e c k e d in r e s p e c t of s u r f a c e r a d i o a c t i v e c o n t a m i n a t i o n b y s m e a r t e c h n i q u e o r a n o t h e r m e t h o d . If a s o u r c e c o n t a m i n a t i o n is h i g h e r t h a n t h e p e r m i s s i b l e l e v e l p r e s c r i b e d by r e g u l a t i o n s , it s h o u l d b e c l e a n e d o r r e t u r n e d to t h e s e n d e r .

A s an e x a m p l e , l e t u s c o n s i d e r t h e p r o c e d u r e of 6 0 C o - g a m m a s o u r c e c a l i b r a t i o n b y t h e r e l a t i v e m e t h o d u s e d in t h e USSR [67],

(a) T h e s e c o n d a r y s t a n d a r d 6 0 C o - s o u r c e s a r e c y l i n d e r s of c o b a l t m e t a l (4 m m d i a m . X 4 m m high) l i n e d wi th s t a i n l e s s s t e e l (6 m m d i a m . b y

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6. 5 m m h igh) . N o m i n a l v a l u e s of e x p o s u r e r a t e a t a d i s t a n c e of 1 m a r e f r o m 7. 2 m R - h"1 t o 2. 16 R- h"1 . T h e s o u r c e s a r e m a r k e d b y s e r i a l n u m b e r s e n g r a v e d on t h e s u r f a c e of t h e o u t s i d e c y l i n d e r s . T h e s e m a r k i n g s m a y a l s o b e a r i n f o r m a t i o n of t h e n o m i n a l v a l u e s .

(b) F o r c a l i b r a t i o n p u r p o s e s t h e s o u r c e s a r e p l a c e d in a t y p i c a l c o l l i -m a t o r s h o w n in A p p e n d i x II. ( T h i s c o l l i m a t o r i s a l s o u s e d f o r i n s t r u m e n t c a l i b r a t i o n a t a l l s e c o n d a r y s t a n d a r d l a b o r a t o r i e s . ) T h e d i a m e t e r of t h e c o l l i m a t o r m a y b e e q u a l t o 30, 60 o r 90 m m d e p e n d i n g on t h e s i z e of t h e d e t e c t o r u s e d .

(c) An i o n i z a t i o n c h a m b e r of t h e m e a s u r i n g i n s t r u m e n t i s p l a c e d in t h e m i d d l e of t h e r a d i a t i o n b e a m at s u c h a d i s t a n c e f r o m t h e s o u r c e t h a t t h e d i a m e t e r of t h e b e a m at t h e po in t of t h e d e t e c t o r i s t w i c e a s l a r g e a s t h e b i g g e s t l i n e a r s i z e of t h e c h a m b e r i n t h e p l a n of t h e b e a m c r o s s - s e c t i o n .

(d) T h e i n s t r u m e n t i s s w i t c h e d on and a f t e r a s u f f i c i e n t w a r m - u p t i m e a r e a d i n g f r o m a c o n s t a n c y c h e c k s o u r c e i s r e c o r d e d . T a k i n g in to a c c o u n t t h e e n v i r o n m e n t a l c o n d i t i o n s ( t e m p e r a t u r e and a t m o s p h e r e p r e s s u r e ) t h e r e a d i n g s h o u l d b e t h e s a m e a s t h a t m e n t i o n e d i n t h e c e r t i f i c a t e .

(e) T h e p r i m a r y s t a n d a r d s o u r c e s i m i l a r t o t h e s o u r c e in q u e s t i o n i s pu t i n to t h e s p e c i a l h o l d e r and p l a c e d w i t h i n t h e c o l l i m a t o r and t h e r e a d i n g N o j i s r e c o r d e d .

(f) Nex t , t h e s o u r c e in q u e s t i o n i s s u b s t i t u t e d f o r t h e p r i m a r y s o u r c e and t h e r e a d i n g N x j i s r e c o r d e d . It i s i m p o r t a n t t o b e s u r e t h a t t h e s o u r c e i s p l a c e d e x a c t l y in t h e s a m e p o s i t i o n w i th in t h e c o l l i m a t o r .

(g) Nex t t h e p r i m a r y s o u r c e i s a g a i n p l a c e d in t h e c o l l i m a t o r and t h e r e a d i n g No 2 i s r e c o r d e d .

(h) O p e r a t i o n s m e n t i o n e d in (e), (f) and (g) a r e r e p e a t e d 3 - 5 t i m e s .

(i) T h e r e a d i n g f r o m t h e c o n s t a n c y c h e c k s o u r c e i s a g a i n t a k e n . It s h o u l d b e t h e s a m e a s in (d); o t h e r w i s e t h e i n s t r u m e n t i s r e t u r n e d f o r r e p a i r .

(j) E x p o s u r e r a t e Px a t a d i s t a n c e of 1 m f o r t h e s o u r c e b e i n g c a l i b -r a t e d i s d e t e r m i n e d b y t h e r e l a t i o n :

k . P 0 . N x x " N 0

w h e r e IJ i s t h e e x p o s u r e r a t e a t a d i s t a n c e 1 m f o r t h e p r i m a r y s t a n d a r d s o u r c e ( i t i s s t a t e d f o r d i a m e t e r s of t h e c o l l i m a t o r equa l 30, 60 and 90 m m ) and is t a k e n f r o m t h e c e r t i f i c a t e ;

N0 i s t h e a v e r a g e d v a l u e of t h e i n s t r u m e n t r e a d i n g s f o r t h e p r i m a r y s t a n d a r d s o u r c e ;

Nx i s t h e a v e r a g e d v a l u e of t h e i n s t r u m e n t r e a d i n g s f o r t h e s o u r c e in q u e s t i o n ;

k i s a f a c t o r t a k i n g in to a c c o u n t t h e d i s i n t e g r a t i o n of t h e p r i m a r y s t a n d a r d s o u r c e , d i f f e r e n c e in d i s t a n c e s f o r t h e p r i m a r y s o u r c e and t h e s o u r c e in q u e s t i o n , if any , and a n a t t e n u a t i o n f a c t o r a s w e l l a s e n v i r o n m e n t a l c o n d i t i o n s .

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(k) T h e r e l a t i v e a c c u r a c y AP X /P X ( in %) of t h e e x p o s u r e - r a t e m e a s u r e -m e n t f o r t h e s o u r c e in q u e s t i o n i s d e t e r m i n e d b y t h e r e l a t i o n :

w h e r e AP0 /P0 i s t h e r e l a t i v e a c c u r a c y of t h e e x p o s u r e r a t e a t a d i s t a n c e of 1 m s t a t e d in t h e c e r t i f i c a t e f o r t h e p r i m a r y s t a n d a r d s o u r c e ;

A N O / N Q and A N X / N X a r e t h e r e l a t i v e a c c u r a c i e s of t h e a v e r a g e d r e a d i n g s f o r t h e p r i m a r y s t a n d a r d s o u r c e and t h e s o u r c e in q u e s t i o n s u b s e q u e n t l y ;

6 N / N i s t h e a d d i t i o n a l r e l a t i v e a c c u r a c y d u e t o m e a s u r e m e n t s a t d i f f e r e n t s u b - r a n g e s , if any , o r a t d i f f e r e n t d i s t a n c e s .

T h e USSR r e g u l a t i o n s r e q u i r e t h a t t h e r e l a t i v e a c c u r a c y m u s t no t d e v i a t e f r o m ±4%,

T h e m e a s u r e m e n t r e s u l t s , w i th d e t a i l s of t h e s o u r c e and a l i s t of t h e i n s t r u m e n t s u s e d a n d e n v i r o n m e n t c o n d i t i o n s , a r e r e c o r d e d in a l o g b o o k and a c e r t i f i c a t e c o n t a i n i n g t h i s i n f o r m a t i o n i s i s s u e d . T h e s o u r c e in q u e s t i o n c a n now b e c a l l e d a s e c o n d a r y s t a n d a r d s o u r c e , and c a n b e u s e d f o r i n s t r u m e n t c a l i b r a t i o n a s w e l l a s f o r t e r t i a r y s t a n d a r d s o u r c e c a l i b -r a t i o n .

S i m i l a r p r o c e d u r e s a r e a p p l i e d f o r o t h e r t y p e s of s o u r c e s . T o f a c i l i t a t e c a l i b r a t i o n o p e r a t i o n s s p e c i a l w o r k i n g p r o c e d u r e s a r e p r e p a r e d f o r e a c h t y p e of c a l i b r a t i o n .

D e t a i l e d i n f o r m a t i o n on t h e c e r t i f i c a t i o n of s t a n d a r d i z e d r a d i o a c t i v e s o u r c e s m a y b e found in ICRU R e p . 12 [70],

S t a n d a r d i z e d a l p h a a n d b e t a s o u r c e s a r e , in g e n e r a l , s u p p l i e d in p a i r s . O n e s o u r c e i s u s e d f o r c a l i b r a t i o n o p e r a t i o n s , t h e o t h e r i s c o n s e r v e d in a s p e c i a l box and u s e d on ly f o r r e l a t i v e c h e c k i n g of a p o s s i b l e l o s s of r a d i o -a c t i v e s u b s t a n c e f r o m t h e f i r s t one .

3. C A L I B R A T I O N F A C I L I T I E S

To c a r r y ou t t h e c a l i b r a t i o n p r o g r a m , a l a b o r a t o r y r e q u i r e s p r o p e r l y d e s i g n e d and i n s t r u m e n t e d a s s e m b l i e s ( s o m e t i m e s c a l l e d j i g s o r r i g s ) to p o s i t i o n t he t e s t d o s i m e t e r s , o r p o r t a b l e i n s t r u m e n t s in p r o p e r r e l a t i o n s h i p t o t h e s o u r c e . The n u m b e r , s i z e and c o m p l e x i t y of t h e l a b o r a t o r y a s -s e m b l i e s wi l l d e p e n d on t h e t y p e s of i r r a d i a t i o n s , i n s t r u m e n t s and d o s i -m e t e r s n e e d e d , and on t h e a m o u n t of c a l i b r a t i o n c a r r i e d ou t . T h e r e f o r e , s e v e r a l e x a m p l e s of a s s e m b l i e s a r e g i v e n in e a c h c a t e g o r y of i n s t r u m e n t s and d o s i m e t e r s .

3 . 1 . P e r s o n n e l d o s i m e t e r s

T h e m o s t c o m m o n l y u s e d p e r s o n n e l d o s i m e t e r s i n c o r p o r a t e f i l m o r an i o n i z a t i o n c h a m b e r a s t he d e t e c t o r . H o w e v e r , t h e r m o l u m i n e s c e n t and r a d i o p h o t o l u m i n e s c e n t d e t e c t o r s a r e r a p i d l y r e p l a c i n g f i l m in p e r s o n n e l

39

d o s i m e t e r s . In d e s i g n i n g a s s e m b l i e s f o r c a l i b r a t i n g t h e s e d o s i m e t e r s t h e o b j e c t i v e s a r e to : (1) E n s u r e a c c u r a t e and r e p r o d u c i b l e p o s i t i o n i n g of t e s t d o s i m e t e r s , i n s t r u m e n t s and r a d i a t i o n s o u r c e s ; (2) A c c o m m o d a t e d o s i m e t e r s and i n s t r u m e n t s of v a r i o u s s i z e s and s h a p e s ; (3) M i n i m i z e r a d i a t i o n s c a t t e r by u s i n g l i g h t w e i g h t a s s e m b l i e s f a b r i c a t e d of l ow a t o m i c n u m b e r m a t e r i a l s , and p o s i t i o n e d w e l l a w a y f r o m f l o o r s , w a l l s and c e i l i n g s ; (4) M i n i m i z e e x p o s u r e to p e r s o n n e l and p r o v i d e s h i e l d i n g f o r t h e r a d i a t i o n s o u r c e when not in u s e ; (5) P r o v i d e a m e t h o d of m o v i n g t h e r a d i a t i o n s o u r c e q u i c k l y {a f e w s e c o n d s ) in to t h e d e s i r e d e x p o s u r e p o s i t i o n , and in to t h e s h i e l d e d p o s i t i o n when the c a l i b r a t i o n i s c o m p l e t e d ; (6) P r o v i d e r a d i a t i o n f i e l d s a s u n i f o r m a s p o s s i b l e ; (7) P r o v i d e f o r a c c u r a t e t i m e m e a s u r e m e n t s .

3 . 1 . 1 . B e t a - g a m m a d o s i m e t e r s

A t y p i c a l i r r a d i a t i o n s y s t e m f o r c a l i b r a t i n g b e t a - g a m m a d o s i m e t e r s and f o r p o s i t i o n i n g and r o t a t i n g a s m a l l r a d i o n u c l i d e r a d i a t i o n s o u r c e i s s h o w n in F i g . 9. T h i s s y s t e m u s e s an e l e c t r o n i c t i m e r to s w i t c h on and

FIG. 9. Gamma radiation exposure assembly for personnel dosimeters.

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FIG. 10. Gamma radiation spiral exposure assembly for personnel dosimeters.

off t he s m a l l v a c u u m p u m p which r a i s e s t he r a d i a t i o n s o u r c e in to t he e x -p o s u r e p o s i t i o n by v a c u u m . T h e a i r p a s s i n g the s o u r c e s h o u l d be f i l t e r e d and d i s c h a r g e d e x t e r n a l to t h e bu i ld ing to e n s u r e t h a t c o n t a m i n a t i o n f r o m a p o s s i b l y r u p t u r e d s o u r c e d o e s no t e n t e r t he b u i l d i n g . R o u t i n e m o n i t o r i n g of t h e i n - l i n e f i l t e r c a n s e r v e to d e t e c t any p o s s i b l e r a d i a t i o n s o u r c e l e a k a g e . It i s not a d v i s a b l e t o u s e a i r p r e s s u r e to m o v e the s o u r c e b e c a u s e p o s s i b l e c o n t a m i n a t i o n f r o m a r u p t u r e d r a d i a t i o n s o u r c e cou ld e s c a p e t h e a i r s y s t e m and e n t e r t h e bu i ld ing a t m o s p h e r e if a i r in t h e s o u r c e p o s i t i o n i n g s y s t e m

41

w e r e a t a p o s i t i v e p r e s s u r e wi th r e s p e c t t o t h e b u i l d i n g a t m o s p h e r e . At t he end of the, e x p o s u r e t i m e , t h e v a c u u m p u m p i s s h u t off by t h e e l e c t r o n i c t i m e r and t h e s o u r c e s e t t l e s qu i ck ly , by g r a v i t y , to t h e w e l l - s h i e l d e d p o s i t i o n a t t h e b a s e of t h e a i r c o l u m n . T h i s m e t h o d i s f a i l - s a f e in t h e even t of a p o w e r f a i l u r e , and c a n be bu i l t in to an i n t e r l o c k s y s t e m , wh ich p e r m i t s t h e p o s i t i o n i n g and e x p o s u r e of a l a r g e n u m b e r (50 t o 100) of d o s i m e t e r s a t a t i m e . A c a l i b r a t e d s t a n d a r d c h a m b e r s u c h a s a n " R " m e t e r i s u s e d to m e a s u r e t h e a c t u a l e x p o s u r e , and a r e m o t e r a d i a t i o n m o n i t o r p r o v i d e s a c h a r t r e c o r d of t h e e x p o s u r e d u r a t i o n and i n t e n s i t y a s a v e r i f i c a t i o n of t h e e x p o s u r e . A i r p a s s i n g by t h e r a d i a t i o n s o u r c e n a t u r a l l y i m p a r t s a r o t a t i o n t o i t and e n s u r e s t h a t any a n i s o t r o p y of t h e s o u r c e i s a v e r a g e d o u t . T h e a s s e m b l y c a n a l s o be d e s i g n e d so t h a t t h e d o s i m e t e r s r o t a t e a b o u t t h e s o u r c e and t h i s e n s u r e s tha t t h e y a l l r e c e i v e t h e s a m e i n t e g r a t e d e x p o s u r e . .

A n o t h e r v e r s i o n ( F i g . 10) of t h i s a s s e m b l y u s e s a s p i r a l a s s e m b l y wh ich p o s i t i o n s d o s i m e t e r s a t v a r i o u s d i s t a n c e s f r o m t h e s o u r c e . U s i n g t h i s s y s t e m i t i s p o s s i b l e t o c a l i b r a t e s e v e r a l d o s i m e t e r s to d i f f e r e n t d o s e l e v e l s a t t h e s a m e t i m e , wi th no d o s i m e t e r s h i e l d i n g any o t h e r .

42

FIG. 12. Beta radiation exposure assembly - broad source.

T h e a s s e m b l y s h o w n in F i g . 4c c a n be u s e d when a s m a l l e r n u m b e r of d o s i m e t e r s r e q u i r e c a l i b r a t i o n . T h e s o u r c e p o s i t i o n and t h e m o b i l e t r o l l e y p o s i t i o n a r e m a n u a l l y c o n t r o l l e d f r o m the c a l i b r a t i o n c o n t r o l s t a t i o n . L e a d s h i e l d i n g i n c l u d i n g a l e a d window c a n be a d d e d . F o r s u r v e y m e t e r s , t h e d i a l r e a d i n g s c a n b e r e a d by a m i r r o r - t e l e s c o p e s y s t e m o r a TV c a m e r a a r r a n g e m e n t .

T h e i r r a d i a t i o n e x p o s u r e s in t h e i n t e r m e d i a t e e n e r g y r a n g e (50 to 200 keV) c a n be m a d e wi th h e a v i l y f i l t e r e d X - r a y r a d i a t i o n s [7, 12] wi th t h e t y p i c a l e x p o s u r e s y s t e m s h o w n in F i g . 2. A r o t a t i n g a s s e m b l y a s s h o w n in F i g . 3 m a y be u s e d which p r o v i d e s f o r s i m u l t a n e o u s m e a s u r e m e n t of t h e e x p o s u r e a t t h e t i m e of d o s i m e t e r i r r a d i a t i o n . T h e " R " m e t e r and e x p o s e d d o s i m e t e r s a r e c o n t i n u o u s l y r o t a t e d to e n s u r e i d e n t i c a l e x p o s u r e s . T h e p r o g r e s s of t h e e x p o s u r e i s e s t i m a t e d f r o m t h e p r e v i o u s l y c a l i b r a t e d c o n s t a n t b e a m m o n i t o r . T h e ou tpu t f r o m t h i s m o n i t o r i s r e c o r d e d to p r o v i d e a r e c o r d of t h e a c t u a l e x p o s u r e . T h e c o l l i m a t o r s y s t e m shou ld be d e s i g n e d to p r o v i d e a b r o a d b e a m s u i t a b l e f o r t he s i m u l t a n e o u s e x p o s u r e of s e v e r a l d o s i m e t e r s .

F o r low e n e r g y photon e x p o s u r e s a K - f l u o r e s c e n t r a d i a t i o n s o u r c e s y s t e m [17] s u c h a s t h e t y p i c a l one i l l u s t r a t e d in F i g . 3 c a n be u s e d . T h i s s y s t e m c o n s i s t s of t h e " K " X - r a y r a d i a t o r s wh ich c a n be r e a d i l y c h a n g e d to p r o v i d e a r a n g e of r a d i a t i o n e n e r g i e s , a c o n s t a n t b e a m m o n i t o r and a r o t a t i n g d o s i m e t e r e x p o s u r e j i g . A s t a n d a r d c a v i t y o r f r e e - a i r c h a m b e r c a n be u s e d f o r t he p r i m a r y c a l i b r a t i o n . T h e c o n s t a n t b e a m m o n i t o r i s u s e d a s a s e c o n d a r y c a l i b r a t i o n d e v i c e f o r d e t e r m i n i n g the d o s e d e l i v e r e d

43

Film Dosimeter

Precis ion "Long Counter" for Calibration

Film to Source Measuring Device

Constant Neutron Monitor

. PuBe Fast Neutron Source

Aluminum Support

Jig

. J ^ s ,

A m A

0

' L

Chart Recorder

B F 3 Tube Detector

Polyethylene Moderator

FIG. 13. Fast-neutron exposure assembly.

Plywood Outer Cover

Constant Slow Neutron Monitor

4 4

0 ' 0 '

- c L

Chart R e c o r d e r

Graphi te ' E x p o s u r e S t r i n g e r

C a d m i u m She l l

FIG.14. Slow-neutron exposure assembly - sigma pi le .

at e a c h e x p o s u r e . A c h a r t r e c o r d e r p r o v i d e s a r e c o r d of t he e x p o s u r e f r o m t h e c o n s t a n t b e a m m o n i t o r r e a d o u t .

F o r b e t a r a d i a t i o n e x p o s u r e s s e v e r a l s y s t e m s a r e a v a i l a b l e . A t y p i c a l po in t s o u r c e c a l i b r a t i o n s y s t e m u s i n g 9 0 S r - 9 0 Y i s s h o w n in F i g . 11 . In -i t i a l l y , an e x t r a p o l a t i o n c h a m b e r [31] i s u s e d to m e a s u r e t he b e t a d o s e a s a f u n c t i o n of d i s t a n c e and to d e t e r m i n e v a r i a t i o n s wi th ang l e a t s e v e r a l p o s i t i o n s on the e x p o s u r e a r c . Once t h i s h a s b e e n done , a GM t u b e m e a s u r e -m e n t a t a g i v e n p o s i t i o n can be u s e d to v e r i f y e a c h t e s t e x p o s u r e . An a d j u s t a b l e a n g l e m e a s u r i n g d e v i c e i s u s e d to m e a s u r e t h e d i s t a n c e and a n g l e of e a c h d o s i m e t e r wi th r e s p e c t to t he b e t a r a d i a t i o n s o u r c e . A c h a r t r e c o r d e r f r o m t h e c o n s t a n t m o n i t o r p r o v i d e s t h e r e c o r d of t he e x p o s u r e .

A s u r f a c e b e t a r a d i a t i o n d o s e c a l i b r a t i o n c a n a l s o be m a d e wi th p o l i s h e d u r a n i u m d i s c s . A t y p i c a l s y s t e m f o r t h i s e x p o s u r e i s shown in F i g . 12. D o s i m e t e r s a r e p o s i t i o n e d in a s m a l l r a c k wh ich i s q u i c k l y r a i s e d in to p o s i t i o n s o t h a t t h e d o s i m e t e r s c o m e in c o n t a c t w i th t h e u r a n i u m d i s c ( r i g h t s i d e of p i c t u r e ) . O n c e in t he r a i s e d p o s i t i o n an e l e c t r o n i c t i m e r i s a u t o m a t i c a l l y s t a r t e d . At t h e d u r a t i o n of t he p r e - s e t t i m e , t he d o s i m e t e r s a r e q u i c k l y r e l e a s e d f r o m t h e u r a n i u m and p o s i t i o n e d (as shown in t he r i g h t s i d e of p i c t u r e ) a w a y f r o m the u r a n i u m . T h e s u r f a c e d o s e - r a t e f r o m n a t u r a l u r a n i u m r e p o r t e d in the l i t e r a t u r e r a n g e s f r o m about 200 to 250 m r a d / h ; 225 m r a d / h i s p r o b a b l y t he b e s t a v a i l a b l e c o m p r o m i s e v a l u e .

3 . 1 . 2 . N e u t r o n d o s i m e t e r s

A t y p i c a l s y s t e m f o r m a k i n g the f a s t n e u t r o n e x p o s u r e s i s shown in F i g . 13. T h e f a s t n e u t r o n s o u r c e i s p o s i t i o n e d m a n u a l l y wi th an a p p r o p -r i a t e s o u r c e h a n d l i n g t o o l a f t e r a l l t he d o s i m e t e r s a r e in p l a c e . A p r e -c i s i o n l ong c o u n t e r [52] ( s e e s e c t i o n 2 . 5 . 3) i s u s e d to c a l i b r a t e t h e s y s t e m . C o n s t a n t m o n i t o r i n g and a r e c o r d of t h e e x p o s u r e i s p r o v i d e d by a c h a r t r e c o r d e r r e s p o n d i n g to t h e ou tpu t of a p o l y e t h y l e n e m o d e r a t e d B F 3 n e u t r o n d e t e c t o r t u b e . T h e r e l a t i v e l y low i n t e n s i t y of f a s t n e u t r o n s o u r c e s r e s u l t s in long e x p o s u r e t i m e s ( h o u r s ) wh ich a r e m e a s u r e d m a n u a l l y . E r r o r s a r i s i n g f r o m t h e t i m e i n v o l v e d in m a n u a l l y p l a c i n g and r e m o v i n g the n e u t r o n s o u r c e a r e not s i g n i f i c a n t p r o v i d e d c a r e i s t a k e n when p o s i t i o n i n g and r e m o v i n g t h e s o u r c e no t t o h a v e it c l o s e to t h e d o s i m e t e r s .

F o r t h e r m a l n e u t r o n c a l i b r a t i o n s a s i g m a p i l e s y s t e m a s shown in F i g . 14 c a n be u s e d . T h e s i g m a p i l e shou ld be f a b r i c a t e d of r e a c t o r g r a d e g r a p h i t e and shou ld m e a s u r e not l e s s t h a n 180 c m on a s i d e . T h e c o n s t r u c t i o n of a s i m i l a r m o d e r a t o r of p a r a f f i n o r p o l y e t h y l e n e and m e a s u r i n g abou t 90 c m on a s i d e m a y b e s u b s t i t u t e d f o r t he s i g m a p i l e in t h e i n t e r e s t of e c o n o m y . But i t i s a d v i s a b l e t o c h e c k t h e g a m m a d o s e s c a u s e d by t h e r e a c t i o n H(n, 7)D in p o l y e t h y l e n e . F o r t h e r m a l n e u t r o n c a l i b r a t i o n s a c t i v a t i o n f o i l s a r e e x p o s e d wi th t h e d o s i m e t e r s to p r o v i d e t h e t h e r m a l n e u t r o n f l u e n c e d e t e r m i -n a t i o n . A BF 3 t u b e p r o v i d e s t h e s i g n a l f o r t h e c h a r t r e c o r d of t h e e x p o s u r e . T h e long e x p o s u r e t i m e s r e q u i r e d (hou r s ) p e r m i t m a n u a l h a n d l i n g of t he f a s t n e u t r o n s o u r c e w i thou t t h e i n t r o d u c t i o n of s i g n i f i c a n t t i m i n g o r e x p o s u r e e r r o r s . A d i s c u s s i o n of t he t e c h n i q u e s of s l o w n e u t r o n f l u e n c e r a t e m e a s u r e -m e n t s w a s i n c l u d e d in s e c t i o n 2 . 5 . 4 .

3. 2. P o r t a b l e s u r v e y m e t e r s

T h e s e i n s t r u m e n t s a r e c l a s s i f i e d bo th by the t y p e s of r a d i a t i o n t h e y d e t e c t , e. g. b e t a , g a m m a , n e u t r o n , and b y t h e t y p e of m e a s u r e m e n t , e. g.

46

d o s e - r a t e , e x p o s u r e r a t e , d o s e e q u i v a l e n t r a t e , c o n t a m i n a t i o n . In d e s i g n i n g t h e c a l i b r a t i o n a s s e m b l i e s f o r t h e s e i n s t r u m e n t s t h e o b j e c t i v e s a r e to :

(1) E n s u r e a c c u r a t e and r e p r o d u c i b l e p o s i t i o n i n g of t e s t i n s t r u m e n t s , and r a d i a t i o n s o u r c e s ;

(2) A c c o m m o d a t e i n s t r u m e n t s of v a r i o u s s i z e s and s h a p e s ; (3) P r o V i d e s h i e l d i n g f o r c a l i b r a t i o n p e r s o n n e l and of t he r a d i a t i o n

s o u r c e w h e n no t in u s e . A s y s t e m s h o u l d be u s e d w h e r e n e c e s s a r y f o r r e m o t e r e a d i n g of i n s t r u m e n t s c a l e s , i. e. m i r r o r s , T V etc . ;

(4) P r o v i d e a m e t h o d of m o v i n g the r a d i a t i o n s o u r c e q u i c k l y in to t he d e s i r e d e x p o s u r e p o s i t i o n , and in to t h e s h i e l d e d p o s i t i o n when t h e r a d i a t i o n s o u r c e i s no t in u s e ;

(5) Supp ly known r a d i a t i o n f i e l d s a t t he c a l i b r a t i o n p o s i t i o n .

3. 2. 1. D o s e o r e x p o s u r e r a t e m o n i t o r i n g i n s t r u m e n t s

D o s e o r e x p o s u r e r a t e m o n i t o r i n g e q u i p m e n t i s u s e d t o c o n t r o l r a d i a t i o n e x p o s u r e s e n c o u n t e r e d by p e r s o n n e l . F o r b e t a and g a m m a r a d i a t i o n , t he d o s e - r a t e m e a s u r i n g i n s t r u m e n t s m a y o f t e h e m p l o y an i o n i z a t i o n c h a m b e r f o l l o w e d by a d . c . a m p l i f i e r . S c i n t i l l a t o r s and GM d e t e c t o r s a r e a l s o u s e d . N o r m a l l y , a g a m m a s o u r c e i s u s e d to c a l i b r a t e t h e s e i n s t r u m e n t s , bu t t h e c a l i b r a t i o n c a n be c h e c k e d by e i t h e r a g a m m a o r a b e t a s o u r c e . F o r l a b o r a t o r i e s t h a t c a l i b r a t e l a r g e n u m b e r s of t h e s a m e type of b e t a -g a m m a s u r v e y i n s t r u m e n t s , c a l i b r a t i o n w e l l s [72] ( F i g . 15), h a v e t he a d -v a n t a g e s of low e x p o s u r e t o t h e o p e r a t o r s , of u t i l i z i n g l e s s f l o o r s p a c e and of v e r y l i t t l e h a n d l i n g of t h e s o u r c e . T h e r e q u i r e m e n t s of s u c h a n i n s t a l l a t i o n a r e :

(1) A w e l l l i ned wi th a s t e e l p ipe s t a r t i n g f r o m a po in t abou t 140 c m a b o v e f l o o r l e v e l (eye l e v e l ) and e x t e n d i n g down f o r a t o t a l l eng th of 600 c m (the d i m e n s i o n s d e p e n d on the s i z e of s o u r c e and t h e r a n g e of d o s e - r a t e s r e q u i r e d ) ;

(2) A r a i l i n s i d e t h i s p ipe c a r r y i n g a l igh t t r o l l e y f o r t he s o u r c e ; (3) A s o u r c e t r o l l e y w h i c h c a n be m o v e d v e r t i c a l l y wi th in t h e p i p e

by m e a n s of a s u r v e y o r ' s m e t a l t a p e ; (4) A r e e l d e v i c e f o r t a k i n g up t h i s t a p e and c o n t r o l l i n g t he s o u r c e h e i g h t ; (5) M a s s i v e s h i e l d i n g a b o v e and be low t h e f l o o r l e v e l e q u i v a l e n t t o

8 c m l e a d p l u s 45 c m of c o n c r e t e ; (6) T h e t h i c k n e s s of t h e l e a d s h i e l d shou ld be i n c r e a s e d to 14 c m f o r

a d i s t a n c e of 20 c m b e l o w the t o p of t h e w e l l . T h e 8 - c m - t h i c k l e a d s h i e l d s h o u l d e x t e n d 75 c m be low f l o o r l e v e l d e c r e a s i n g to 40 - 50 c m ;

(7) A l i gh t t r o l l e y wh ich m o v e s a c r o s s t h e o p e n i n g of t he w e l l and c a r r i e s t h e i n s t r u m e n t s o t h a t i t c a n be r e m o v e d f r o m the b e a m wi thou t e x p o s i n g t h e o p e r a t o r ' s h a n d s t o t he r a d i a t i o n ;

(8) A d e v i c e f o r a r r e s t i n g t he r a p i d d o w n w a r d m o v e m e n t of t h § s o u r c e t r o l l e y in t h e e v e n t of t a p e f a i l u r e , e . g. a d a s h p o t o r s p r i n g a r n a n g e m e n t ;

(9) A r e m o t e h a n d l i n g d e v i c e f o r r e m o v i n g t h e s o u r c e f r o m t h e b o t t o m of t h e wel l ;

(10) A s t o p to p r e v e n t t h e s o u r c e f r o m be ing m o v e d n e a r e r t h a n 20 c m f r o m t h e t o p e d g e of t he wel l ;

(11) A s w i t c h to o p e r a t e a w a r n i n g l igh t when t h e s o u r c e i s r a i s e d f r o m t h e b o t t o m of t h e w e l l .

47

\ / \ / \ /

0 100 cm 1

0 20 40 in 1 I I

Approximate Scale

FIG. 15. Diagram of calibration wel l .

B e c a u s e of t h e p r o b l e m s of i n c r e a s e d s c a t t e r i n g , t h e r a d i a t i o n f i e l d f r o m t h e r a d i u m s o u r c e c a n no l o n g e r be s t a n d a r d i z e d in t e r m s of i t s s p e c i f i c g a m m a - r a y e m i s s i o n c o n s t a n t . S t a n d a r d c a v i t y i o n i z a t i o n c h a m b e r s wi th a c c u r a c i e s of 1 - 2 % a r e a v a i l a b l e f o r t h i s t y p e of c a l i b r a t i o n . Of c o u r s e o t h e r s o u r c e s , e . g . 6 0 C o , 1 3 7 C s , 1 2 2 I r , o r n e u t r o n s o u r c e s m a y be u s e d i n t h e v e r t i c a l w e l l s . A l s o , t h e e n t i r e a s s e m b l y c a n b e a u t o m a t e d a s s h o w n in F i g . 16, wh ich i l l u s t r a t e s a t y p i c a l n e u t r o n i n s t r u m e n t b e i n g c a l i b r a t e d . I n s t r u m e n t a d j u s t m e n t s a r e s i m p l y m a d e t u r n i n g t h e p o t e n t i -o m e t e r s wi th a l o n g - h a n d l e d s c r e w d r i v e r .

S e v e r a l a s s e m b l i e s t h a t w e r e d i s c u s s e d u n d e r p e r s o n n e l d o s i m e t e r s , c a n a l s o b e u s e d f o r t h e c a l i b r a t i o n of p o r t a b l e s u r v e y m e t e r s . F o r i n s t a n c e , w i th s l i g h t m o d i f i c a t i o n s , t h e a s s e m b l i e s s h o w n in F i g s 2, 3, 4c and 11 c a n be u s e d t o c a l i b r a t e s m a l l n u m b e r s of s u r v e y m e t e r s to g a m m a , X - r a y and b e t a r a d i a t i o n s .

F o r s l o w n e u t r o n d e t e c t o r s , s u c h a s a BF3 c o u n t e r , a s i g m a p i l e shown in F i g . 14 m a y b e u s e d f o r c a l i b r a t i o n . F o r r o u t i n e c a l i b r a t i o n s , a m u c h s i m p l e r p r o c e d u r e i s to c a l i b r a t e t h e a s s e m b l y s h o w n in F i g . 17. T h i s u n i t c o n s i s t s of an a l u m i n i u m t a n k f i l l ed wi th p a r a f f i n . P a r a f f i n h a s t h e a d v a n t a g e of e f f i c i e n t l y s l o w i n g down n e u t r o n s but a g a i n t h e r e a c t i o n H(n,Y)D in i t m a y c o n t r i b u t e to t he g a m m a b a c k g r o u n d . When a f a s t n e u t r o n s o u r c e , s u c h a s a P u B e , a PuF 4 o r a A m B e s o u r c e , i s p o s i t i o n e d in t h e w e l l

48

FIG. 16. A u t o m a t e d assembly for c a l i b r a t i n g neu t ron in s t rumen t s .

( the P U F 4 s o u r c e i s shown i n s i d e t he c a n wi th t he r i n g on t o p in F i g . 1 7 )

a f l u e n c e of s l ow n e u t r o n s i s f o r m e d wi th in t h e p a r a f f i n m o d e r a t o r . A l u -m i n i u m - l i n e d BF 3 p r o b e c a l i b r a t i o n h o l e s a r e p o s i t i o n e d a t v a r i o u s d i s t a n c e s f r o m t h e s o u r c e w e l l . S i n c e t he s l o w n e u t r o n f l u e n c e l e v e l f a l l s off a s a f u n c t i o n of i n c r e a s e d d i s t a n c e f r o m t h e s o u r c e , e a c h c a l i b r a t i o n h o l e p r o v i d e s a d i f f e r e n t s l o w n e u t r o n f l ux . C a l i b r a t i o n of t h i s a s s e m b l y i s a c c o m p l i s h e d by c a l i b r a t i n g a BF 3 t u b e in a known s l o w n e u t r o n f lux , e . g . in t he s i g m a p i l e , and t h e n p l a c i n g the BF3 t u b e in e a c h of t h e c a l i b r a t i o n h o l e s in t h e a s s e m b l y wi th t h e f a s t n e u t r o n s o u r c e in p o s i t i o n .

49

FIG. 17. Calibration assembly for portable slow-neutron survey meters.

F o r f a s t n e u t r o n s , p o r t a b l e s u r v e y m e t e r s a r e c a l i b r a t e d wi th a s s e m b l i e s a s c o m p l i c a t e d a s a p o s i t i v e ion a c c e l e r a t o r to s o m e t h i n g a s s i m p l e a s a f a s t n e u t r o n s o u r c e , e . g . P u B e , t h a t h a s b e e n c a l i b r a t e d a g a i n s t a s t a n d a r d s o u r c e . T h e a s s e m b l y shown in F i g . 4c c a n be u s e d f o r t h i s c a l i b r a t i o n .

3 . 2 . 2 . C o n t a m i n a t i o n m o n i t o r i n g i n s t r u m e n t s

C o n t a m i n a t i o n m o n i t o r i n g i n s t r u m e n t s a r e u s e d to d e t e r m i n e t h a t p e r s o n n e l and e q u i p m e n t a r e e s s e n t i a l l y f r e e of r a d i o a c t i v i t y . T h e s e i n s t r u -m e n t s u s u a l l y e m p l o y a d e t e c t o r , e . g . G e i g e r t u b e , s c i n t i l l a t i o n c o u n t e r , f o l l o w e d by an a m p l i f i e r , d i s c r i m i n a t o r and c o u n t i n g - r a t e c i r c u i t .

N o r m a l l y a G e i g e r c o u n t e r i s u s e d to d e t e c t b e t a - g a m m a c o n t a m i n a t i o n . S o m e m a n u f a c t u r e r s c a l i b r a t e t h e m e t e r of t he i n s t r u m e n t wi th a p u l s e s h a p e r and g e n e r a t o r t o r e a d d i r e c t l y in c o u n t s p e r m i n u t e o r p e r s e c . T h e u s e r wi l l t h e n s e l e c t a c a l i b r a t i o n s o u r c e t h a t t y p i f i e s h i s c o n t a m i n a t i o n s p e c t r u m , and c a l i b r a t e s t h e c o u n t e r . F o r e x a m p l e , t he i n s t r u m e n t cou ld be c a l i b r a t e d to r e a d 3200 c o u n t s p e r m i n u t e o r p e r s e c o n d in a 1 - m R / h r a d i a t i o n f i e ld i n a l o w - s c a t t e r e x p o s u r e to a r a d i u m g a m m a s o u r c e . A f t e r t h e i n i t i a l c a l i b r a t i o n , a c h e c k s o u r c e s u c h a s t h e one s h o w n in F i g . 18 c a n b e u s e d to c h e c k t h i s c a l i b r a t i o n . T h i s a s s e m b l y [73] c o n s i s t s of a l u c i t e b l o c k wi th h o l e s d r i l l e d i n to i t . Into t h e s e h o l e s a r e i n s e r t e d c l o s e l y f i t t i n g s l e e v e s of b a k e l i t e , w h o s e i n s i d e d i a m e t e r a c c e p t s t he G e i g e r p r o b e .

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FIG. 18. Exposure assembly for calibrating contamination monitoring instruments for beta and gamma radiation.

E a c h s l e e v e h a s an a n n u l a r g r o o v e m a c h i n e d in to i t s o u t e r s u r f a c e . Into t h e s e g r o o v e s a r e f i l l e d s t r i p s of 0 . 0 2 5 - m m 2 3 8 U f o i l . T h e width of t h e fo i l and t h e g r o o v e s v a r i e s by an o r d e r of m a g n i t u d e f o r e a c h h o l e , and t h e n u m b e r of h o l e s c o r r e s p o n d to t h e n u m b e r of r a n g e s on t h e G e i g e r c o u n t e r . T h e c h e c k i s m a d e by p l a c i n g the G e i g e r c o u n t e r p r o b e in t he h o l e and n o t i c i n g if t he d i a l r e a d i n g h a s r e m a i n e d c o n s t a n t s i n c e t he l a s t c h e c k .

F o r a l p h a c o n t a m i n a t i o n m o n i t o r i n g , a s c i n t i l l a t i o n d e t e c t o r , i s n o r m a l l y u s e d . If t he i n s t r u m e n t i s c a l i b r a t e d by the m a n u f a c t u r e r i n ' c o u n t s p e r m i n u t e ( cpm) t h e u s e r wi l l t h e n s e l e c t a c a l i b r a t i o n s o u r c e tha t t y p i f i e s h i s c o n t a m i n a t i o n s p e c t r u m , and c a l i b r a t e s t he c o u n t e r . F o r e x a m p l e , t h e i n s t r u m e n t cou ld be c a l i b r a t e d t o r e a d 4000 c p m u s i n g a t h in 700-pCi 2 4 1 Am s o u r c e o r u r a n i u m s o u r c e , s h o w n i n F i g . 19a , u n i f o r m l y d i s t r i b u t e d o v e r an a r e a e q u a l t o t h a t of t h e p r o b e . A f t e r i n i t i a l c a l i b r a t i o n , a c h e c k s o u r c e s u c h a s t h e one s h o w n in F i g . 19b c a n be u s e d to c h e c k t h i s c a l i b r a t i o n . T h i s a s s e m b l y [74] c o n s i s t s of a p l a s t i c d i s c wi th g r a d u a t e d s i z e h o l e s t h a t

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( b ) FIG. 19. Exposure assembly for ca l ib ra t ing con tamina t ion moni tor ing instruments for alpha rad ia t ion .

(a) Cal ibra t ion of con tamina t ion m e t e r by uranium source (b) An assembly with check alpha source

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i s r o t a t e d b e t w e e n t h e p r o b e and s o u r c e and c a n b e a d j u s t e d to g ive c o u n t i n g r a t e s o v e r s e v e r a l o r d e r s of m a g n i t u d e . T h e c h e c k i s m a d e by p l a c i n g t h e s c i n t i l l a t i o n d e t e c t o r on t h e j ig and n o t i c i n g if t h e d i a l r e a d i n g h a s r e m a i n e d c o n s t a n t s i n c e t h e l a s t c h e c k .

3. 3. S i m i l a r i t y m e t h o d f o r s u b - r a n g e l i n e a r i t y c h e c k s

High a c t i v i t y r a d i a t i o n s o u r c e s a r e not a l w a y s a v a i l a b l e f o r c a l i b r a t i n g i n s t r u m e n t s t h a t o p e r a t e o v e r a w ide r a n g e of e x p o s u r e o r d o s e - r a t e s . A t e c h n i q u e h a s b e e n d e v e l o p e d to c a l i b r a t e t h e s e m u l t i - r a n g e d i n s t r u m e n t s u s i n g low a c t i v i t y r a d i a t i o n s o u r c e s [ 75, 7 6 ] , T h e m e t h o d i s b a s e d u p o n t h e f a c t t h a t t h e p a r t i c l e f l u x d e n s i t y o r e x p o s u r e r a t e a t a f i x e d poin t a t a c e r t a i n d i s t a n c e f r o m p h y s i c a l l y i d e n t i c a l s o u r c e s ( l o c a t e d a t t h e s a m e p o s i t i o n ) , i . e . s a m e i s o t o p e and g e o m e t r i c a l d i m e n s i o n s , but of d i f f e r e n t a c t i v i t i e s i s d i r e c t l y p r o p o r t i o n a l t o t h e s o u r c e a c t i v i t y r e g a r d l e s s of t h e l o c a t i o n a n d c o m p o s i t i o n of t h e s u r r o u n d i n g m e d i u m . F o r e x a m p l e , if t h e i n s t r u m e n t h a s a s e r i e s of s u b - r a n g e s 1, 10, 100 two s o u r c e s w h o s e a c t i v i t i e s d i f f e r by a b o u t a f a c t o r of 10 s h o u l d b e c h o s e n . T h e f o l l o w i n g p r o c e d u r e i s t h e n f o l l o w e d :

(1) T h e i n s t r u m e n t ' s b o t t o m r a n g e i s i n i t i a l l y c a l i b r a t e d u s i n g p r o p e r l y c e r t i f i c a t e d s o u r c e s and j i g s ;

(2) T h e s o u r c e wi th t h e s m a l l e s t a c t i v i t y i s p l a c e d a t a d i s t a n c e f r o m t h e i n s t r u m e n t so t h a t t he d i a l r e a d i n g i s 1 / 3 of t h e f i r s t s u b - r a n g e ;

(3) T h e s m a l l s o u r c e i s t h u s r e p l a c e d by t h e l a r g e r s o u r c e and t h e i n s t r u m e n t i s c a l i b r a t e d a t 1 / 3 f u l l s c a l e of s e c o n d s u b - r a n g e ;

(4) S t e p s 1 and 2 a r e r e p e a t e d f o r 1 / 2 and 2 / 3 of t h e f i r s t and s e c o n d s c a l e s ;

(5) N e x t t h e s o u r c e wi th t h e s m a l l e s t a c t i v i t y i s p l a c e d a t a d i s t a n c e f r o m the i n s t r u m e n t s o t h a t t h e d i a l r e a d i n g i s 1 / 3 of t h e s e c o n d s u b - r a n g e .

(6) T h e s m a l l s o u r c e i s t h e n r e p l a c e d by t h e l a r g e r s o u r c e and t h e i n s t r u m e n t c a l i b r a t e d f o r 1 / 3 f u l l s c a l e of t h e t h i r d r a n g e .

(7) S t e p s 1 and 2 a r e r e p e a t e d f o r 1 / 2 and 2 / 3 of t h e s e c o n d and t h i r d s c a l e s .

T h i s p r o c e d u r e c a n be r e p e a t e d f o r t h e f o u r t h , f i f t h , e t c . s u b - r a n g e s .

T h i s m e t h o d a l l o w s t h e i n s t r u m e n t to be p l a c e d v e r y n e a r t he s o u r c e , t h u s o b t a i n i n g l a r g e r e a d i n g s w i t h r a d i a t i o n s o u r c e s of low a c t i v i t y . When u s i n g s o u r c e s a t a s h o r t d i s t a n c e , h o w e v e r , it i s n e c e s s a r y to b e s u r e t h a t t h e r e i s no r e c o m b i n a t i o n of i o n s in t he d e t e c t o r c a u s e d by a n o n - u n i f o r m d i s t r i b u t i o n of i o n p r o d u c t i o n in t h e w o r k i n g v o l u m e . M o r e o v e r , f o r d e t e c t o r s c o n s i s t i n g of f e w GM o r p r o p o r t i o n a l c o u n t e r s , i t i s i m p o r t a n t t o be s u r e t h a t t h e r e a r e no l o s s e s of p u l s e s owing to d e a d t i m e and p a r a l y s i s i n t h e c o u n t e r c l o s e s t to t h e s o u r c e .

R a t i o of a c t i v i t i e s o r e m i s s i o n r a t e s of t he s o u r c e s c a n be m e a s u r e d wi th an a c c u r a c y of ±1% at t h e s e c o n d a r y s t a n d a r d l a b o r a t o r y wi th a n y i n s t r u m e n t s e n s i t i v e to t h e r a d i a t i o n e m i t t e d , and i t i s no t n e c e s s a r y to know t h e e x a c t a c t i v i t i e s o r e m i s s i o n r a t e s .

T h i s m e t h o d i s known a s t h e s i m i l a r i t y m e t h o d and c a n be a p p l i e d t o g a m m a and n e u t r o n m e a s u r i n g i n s t r u m e n t s a s w e l l a s t o b e t a - m e a s u r i n g i n s t r u m e n t s [ 7 6 ] , In t h e l a t t e r c a s e , g a m m a s o u r c e s a r e a l s o u s e d f o r c h e c k i n g s u b - r a n g e s wi th t h e s i m i l a r i t y m e t h o d . B e t a - m e a s u r i n g i n s t r u m e n t s

53

a r e s e n s i t i v e to t h e g a m m a - r a d i a t i o n and t h e s e c o n d a r y e l e c t r o n s a c t in t he s a m e way a s b e t a - p a r t i c l e s in t he d e t e c t o r . T h e s i m i l a r i t y m e t h o d i s e s p e c i a l l y u s e f u l f o r b e t a - m e a s u r i n g i n s t r u m e n t s b e c a u s e f o r " n o r m a l " c a l i b r a t i o n i t i s n e c e s s a r y to h a v e t h r e e s o u r c e s f o r e a c h s u b - r a n g e , i . e . a b o u t 15 s t a n d a r d b e t a - s o u r c e s in a l l f o r t he who le m e a s u r e m e n t r a n g e . U s i n g t h e s i m i l a r i t y m e t h o d m a y , in p r i n c i p l e , d e c r e a s e t h i s n u m b e r to one and one s e t of g a m m a - s o u r c e s c a n be u s e d f o r a l l t y p e s of b e t a - m e a s u r i n g i n s t r u m e n t s a s w e l l a s f o r g a m m a - m e a s u r i n g i n s t r u m e n t s .

If a b e t a - m e a s u r i n g i n s t r u m e n t h a s a d e v i c e f o r c o m p e n s a t i n g t h e b a c k g r o u n d , i t s h o u l d be s w i t c h e d off d u r i n g s u c h l i n e a r i t y c h e c k i n g . The b a c k g r o u n d c o m p e n s a t i o n m a y be s e p a r a t e l y c h e c k e d by m e a n s of any b e t a - s o u r c e .

E x a c t p o s i t i o n i n g of t he s o u r c e s d u r i n g the l i n e a r i t y c h e c k s d e p e n d s on t h e q u a l i t y of t h e d e v i c e u s e d . M a c h i n i n g the s o u r c e h o l d e r and a p -p r o p r i a t e s o c k e t p e r m i t s s o u r c e s to b e l o c a t e d w i t h an a c c u r a c y of 0 . 1 - 0 . 2 m m w h i c h l e a d s to an a d d i t i o n a l e r r o r of not m o r e t h a n 4% a t a d i s t a n c e of 1 c m f r o m t h e d e t e c t o r . H o w e v e r , d i s t a n c e s of 3 - 5 c m a r e u s e d n o r m a l l y . T h e r e f o r e , t h e a d d i t i o n a l e r r o r due to i n e x a c t p o s i t i o n i n g i s no t m o r e t h a n ±1%.

T h e a n a l y s i s of t h e e r r o r s a r i s i n g d u r i n g t h e l i n e a r i t y c h e c k s s h o w s t h a t t h e s i m i l a r i t y m e t h o d l e a d s to an a d d i t i o n a l e r r o r of about 1 - 2%. in c o m p a r i s o n wi th t h e c a l i b r a t i o n of a l l s u b - r a n g e s by m e a n s of s e c o n d a r y s t a n d a r d s o u r c e s c a l i b r a t e d a t a n a t i o n a l l a b o r a t o r y .

4 . E R R O R S AND Q U A L I T Y C O N T R O L

4 . 1 . U n c e r t a i n t y l i m i t s

T h e h a z a r d s a s s o c i a t e d wi th the u s e of r a d i a t i o n a r e s u b j e c t in m a n y c o u n t r i e s to l e g a l c o n t r o l wh ich s p e c i f y n u m e r i c a l l i m i t s . T h i s n e c e s s i t a t e s c o n f i d e n c e in any m e a s u r e m e n t s m a d e in c o n n e c t i o n wi th a m o n i t o r i n g p r o g r a m . T h e e r r o r s a s s o c i a t e d with any m e a s u r e m e n t and wi th i t s i n -t e r p r e t a t i o n shou ld t h e r e f o r e be known and a l l o w e d f o r . I C R P ( P u b l i c a t i o n 12, 1968) s t a t e s " T h e u n c e r t a i n t i e s a c c e p t a b l e in r o u t i n e i n d i v i d u a l m o n i -t o r i n g s h o u l d be s o m e w h a t l e s s t h a n t h e i n v e s t i g a t i o n l e v e l and c a n b e s t be e x p r e s s e d in r e l a t i o n to t h e a n n u a l d o s e . T h e u n c e r t a i n t y in a s s e s s i n g t h e u p p e r l i m i t s to t he a n n u a l d o s e e q u i v a l e n t to t h e whole body o r to t he o r g a n s of t h e body ( s e e p a r a . 92 of ICRU R e p . 12) s h o u l d no t e x c e e d 50%, W h e r e t h e s e d o s e s a r e l e s s t h a n 2 r e m an u n c e r t a i n t y of 1 r e m i s a c c e p t a b l e . T h i s u n c e r t a i n t y i n c l u d e s e r r o r s due to v a r i a t i o n s in t h e d o s i m e t e r s e n s i -t i v i t y wi th i n c i d e n t e n e r g y and d i r e c t i o n of i n c i d e n c e , a s we l l a s i n t r i n s i c e r r o r s in t h e d o s i m e t e r and i t s c a l i b r a t i o n " . T h i s d o e s no t m e a n t h a t e a c h m e a s u r e m e n t s h o u l d h a v e t h i s a c c u r a c y ; t h e r e a r e m a n y o c c a s i o n s when a g r e a t e r o r l e s s e r a c c u r a c y m a y s u f f i c e . When d e c i d i n g a p p r o x i m a t e w o r k i n g t i m e s in a n a r e a it m a y be r e q u i r e d to d i f f e r e n t i a t e b e t w e e n 10 m i n , 30 m i n , o r l o n g e r , t h e n g r e a t a c c u r a c y would not be r e q u i r e d . L i k e w i s e w h e r e t h e d o s e - r a t e s a r e s o s m a l l t h a t no m a t t e r h o w long a p e r s o n w a s e x p o s e d in t h e f i e l d h e would no t r e c e i v e a d o s e a p p r o a c h i n g the l e g a l l i m i t t h e n t h e a c c u r a c y n e e d no t be h i g h . H o w e v e r , w h e r e t h e d o s e - r a t e s a r e h igh o r w h e r e an i n d i v i d u a l ' s d o s e m a y a p p r o a c h h i s a n n u a l l i m i t t h e n a m o r e a c c u r a t e m e a s u r e m e n t s h o u l d be m a d e .

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4. 2. S o u r c e s of e r r o r

T h e a c c u r a c y a s s o c i a t e d wi th a m e a s u r e m e n t m a d e wi th a f i e l d i n s t r u -m e n t w i l l . d e p e n d upon a n u m b e r of f a c t o r s o t h e r t h a n t h e i n h e r e n t i n a c c u -r a c i e s of t h e i n s t r u m e n t . An i n s t r u m e n t a t b e s t c a n on ly be a s a c c u r a t e a s t h e m e a s u r e m e n t of t h e r a d i a t i o n f i e ld in which i t i s c a l i b r a t e d . T h e f o l l o w i n g d i a g r a m m a t i c e x a m p l e i l l u s t r a t e s t h e c o m p l e x i t y in e s t i m a t i n g t h e e r r o r s a s s o c i a t e d wi th f i e l d m e a s u r e m e n t s t h a t a r i s e f r o m the i n s t r u -m e n t c a l i b r a t i o n :

Primary Intercom- Secondary Secondary Intercom- Field Field measurement parison standard standard parison instrument > instrument Error A instrument instrument Error Error Error A instrument instrument Error Error

Error B Error A+B Error A+B+C Error D A+B+C+D A+B+C+D+E

At primary At secondary Field use laboratory laboratory

T h e t o t a l e r r o r in t he s e c o n d a r y s t a n d a r d c o r r e c t i o n v a l u e a t t he c o m p l e t i o n of t h e c a l i b r a t i o n at t h e p r i m a r y l a b o r a t o r y i s A+B, w h e r e A i s t h e e r r o r in t h e p r i m a r y s t a n d a r d and B t h e i n t e r c o m p a r i s o n e r r o r wh ich would i n c l u d e s u c h t h i n g s a s p o s i t i o n i n g and r e a d i n g e r r o r s . T h e s e c o n d a r y s t a n d a r d i s t h e n r e m o v e d to t h e s e c o n d a r y l a b o r a t o r y w h e r e i t i s p r o b a b l y u s e d in a d i f f e r e n t s p e c t r u m , g e o m e t r y and e x p o s u r e r a t e ; f u r t h e r m o r e , t h e i n s t r u m e n t cou ld h a v e s u f f e r e d a p h y s i c a l c h a n g e , s a y , to c o m p o n e n t s . T h e s e c a n a l l c o n t r i b u t e to a f u r t h e r e r r o r C. T h u s , a t t he s e c o n d a r y l a b o r a t o r y t h e s t a n d a r d h a s an e r r o r A+B+C. D i s t h e i n t e r c o m p a r i s o n e r r o r b e t w e e n t h e s e c o n d a r y s t a n d a r d and t h e f i e l d i n s t r u m e n t . T h e f i n a l e r r o r a s s o c i a t e d wi th t he c a l i b r a t i o n p r o c e d u r e i s t h e r e f o r e A+B+C+D and a f u r t h e r e r r o r E, s i m i l a r in o r i g i n to C, i s i n t r o d u c e d when u s i n g t h e c a l i b r a t e d i n s t r u m e n t f o r a c t u a l f i e ld m e a s u r e m e n t s . T h e e s t i m a t i o n of e r r o r E i s t he m o s t c o m p l i c a t e d p r o b l e m c o n n e c t e d wi th t he m e t h o d of m e a s u r e m e n t i n t e r p r e t a t i o n , e s p e c i a l l y f o r p e r s o n a l m o n i t o r i n g . T h e f a c t o r s c o n t r i b u t i n g to E and C a r e a s f o l l o w s :

(a) D i f f e r e n t s p e c t r a (b) D i f f e r e n t e x p o s u r e r a t e (c) In t h e c a s e of X - r a y and g a m m a c a l i b r a t i o n , d i f f e r e n t

s e c o n d a r y e l e c t r o n c o n t r i b u t i o n (d) D i f f e r e n t d i v e r g e n c e o r d i r e c t i o n of f i e l d (e) N o n - l i n e a r i t y of d i s p l a y c i r c u i t s (f) D i f f e r e n t e n v i r o n m e n t a l c o n d i t i o n s (g) I n c o n s i s t e n c y s i n c e c a l i b r a t i o n

At t h e m o m e n t v e r y l i t t l e d a t a e x i s t s on t he m a g n i t u d e of s o m e of t h e s e e r r o r s w h o s e s i z e wi l l d e p e n d on l o c a l c a l i b r a t i o n c o n d i t i o n s . A l s o t h e e x a c t s e q u e n c e of c a l i b r a t i o n wi l l not b e t h e s a m e in a l l c a s e s . A s a gu ide , h o w e v e r , s o m e f i g u r e s w i l l be g iven f o r t h e c a s e of X - r a y c a l i b r a -t i o n wi th e n e r g i e s b e t w e e n 20 k e V and 2 M e V . It s h o u l d be a p p r e c i a t e d t h a t t h e y r e p r e s e n t an i n s p i r e d g u e s s and a r e i l l u s t r a t i v e r a t h e r t h a n q u a n t i -t a t i v e , and m a y h a v e to be c o r r e c t e d when the r e l e v a n t d a t a b e c o m e s a v a i l a b l e .

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E r r o r s a s s o c i a t e d wi th t h e p r i m a r y s t a n d a r d w i l l be a p p r o x i m a t e l y 1% a s w i l l t h e i n t e r c o m p a r i s o n e r r o r B . A f t e r c a l i b r a t i o n a t t he p r i m a r y l a b o r a t o r y t h e e r r o r in t h e c o r r e c t i o n f a c t o r w i l l t h e r e f o r e p r o b a b l y be in t h e r e g i o n of 2%, a s s u m i n g t h a t we c a n add t h e e r r o r s , an a s s u m p t i o n m a d e on ly to s i m p l i f y t h e i l l u s t r a t i o n . A c o n s i d e r a t i o n of t he a b o v e - l i s t e d f a c t o r s a f f e c t i n g e r r o r C s h o w s tha t t h i s cou ld be b e t w e e n about 2 and 12%. T a k i n g t h e m e a n v a l u e of 7% would g ive a v a l u e of a b o u t ±9% f o r t h e m e a s u r e -m e n t of t h e e x p o s u r e l e v e l a t t h e s e c o n d a r y l a b o r a t o r y wi th t h e s e c o n d a r y s t a n d a r d . T h e i n t e r c o m p a r i s o n e r r o r D wi l l be a p p r o x i m a t e l y 1% and t h u s t h e f i e l d i n s t r u m e n t e r r o r (A+B+C+D) a f t e r c a l i b r a t i o n a t t he s e c o n d a r y l a b o r a t o r y wi l l b e abou t ±10%. It shou ld be e m p h a s i z e d t h a t f o r t h e p u r p o s e of t h i s i l l u s t r a t i o n we h a v e a s s u m e d t h a t t h e e r r o r s c a n b e added l i n e a r l y and f u t u r e w o r k m a y s h o w t h a t , f o r e x a m p l e , t h e s q u a r e r o o t of t h e s u m of t h e s q u a r e s of t h e e r r o r s shou ld be u s e d , i . e . t h e y a r e i n d e p e n d e n t v e c t o r q u a n t i t i e s :

Form of radiation Accuracy of instrument

after calibration at secondary laboratory

Parameter of calibration field

10-200 keV photons ±10 Exposure rate

200 keV-2 MeV photons ± 5 Exposure rate

Beta radiation ±15 Absorbed dose-rate

Neutron radiation ±25 Fluence rate

T h e a c c u r a c y of t h e f i e l d i n s t r u m e n t a s d i s t i n c t f r o m t h e o v e r a l l a c c u r a c y a s s o c i a t e d wi th t h e f i e l d m e a s u r e m e n t c a n be e x p r e s s e d , f o r e x a m p l e wi th an e x p o s u r e m e t e r , a s t h e r a t i o of t h e i n s t r u m e n t r e a d i n g to t h e c a l i b r a t e d e x p o s u r e r a t e .

T h e f o l l o w i n g t y p i c a l c a l i b r a t i o n f i g u r e s i l l u s t r a t e t h i s :

Calibrated (mR.li"1) Instrument reading CmR.h" ) Instrument range

15 14 X 1

30 30 x 1

45 47 x 1

50 45 x 10

100 95 X 10

300 310 x 10

450 460 x 10

1500 1600 x ioo

3000 3000 X100

4500 4800 x ioo

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T h i s i n s t r u m e n t w a s r a t e d a s h a v i n g an a c c u r a c y of ±10%, a s s u m i n g t h a t t h e r e w a s no e r r o r in t h e c a l i b r a t e d m R . h " 1 .

No t to b e c o n f u s e d w i t h a c c u r a c y i s t h e p r e c i s i o n of t h e i n s t r u m e n t [77], T h i s i s t h e a b i l i t y of t h e i n s t r u m e n t to r e p r o d u c e i t s r e a d i n g w h e n p l a c e d in e x a c t l y t h e s a m e r a d i a t i o n f i e l d . F o r e x a m p l e , an i n s t r u m e n t t h a t a l w a y s r e a d 150 R . h " 1 , w h e n p l a c e d in a r a d i a t i o n f i e l d w h e r e t h e " t r u e e x p o s u r e " r a t e w a s 100 R . h " 1 , would h a v e an a c c u r a c y of ±50% and a p r e c i s i o n of a 100%.

It i s m i s l e a d i n g to a s s u m e t h a t t h e p e r f o r m a n c e of a n i n s t r u m e n t u n d e r c a l i b r a t i o n c o n d i t i o n s i s r e f l e c t e d in t h e a c c u r a c y of a d o s e e s t i m a t i o n m a d e in a w o r k i n g e n v i r o n m e n t . W h e r e an i n s t r u m e n t c a n be a d j u s t e d s o t h a t i t s o p t i m u m r e s p o n s e u n d e r c a l i b r a t i o n c o n d i t i o n s i s in l i n e wi th t h e s e c o n d a r y s t a n d a r d t h e n i t f o l l o w s (by d e f i n i t i o n of " o p t i m u m " ) t h a t i t s e r r o r s w i l l be d i s t r i b u t e d bo th p o s i t i v e l y and n e g a t i v e l y o v e r t h e w o r k i n g r a n g e of e x p o s u r e r a t e s , e n e r g i e s , t e m p e r a t u r e s , b a t t e r y v o l t a g e s and s o o n . H o w e v e r , in p r a c t i c e t h e r a n g e o v e r wh ich t h e s e p a r a m e t e r s v a r y m a y be l i m i t e d , f o r e x a m p l e in o u t d o o r r a d i o g r a p h y t h e i n s t r u m e n t m a y be r e p e a t e d l y u s e d to m e a s u r e low e n e r g y s c a t t e r e d r a d i a t i o n , a t low e x -p o s u r e r a t e s , a t low a m b i e n t t e m p e r a t u r e s . It cou ld o c c u r t h a t t h e e r r o r s a r e c o n s i s t e n t l y a l l i n t h e s a m e d i r e c t i o n . An a p p e a l t o t h e " a v e r a g i n g of e r r o r s " shou ld on ly b e m a d e w h e r e t h e r e i s e v i d e n c e t h a t t h e p a r a m e t e r s a f f e c t i n g t h e i n s t r u m e n t ' s p e r f o r m a n c e v a r y r a n d o m l y o v e r t h e e n t i r e r a n g e f o r wh ich i t s " o p t i m u m s e t t i n g " w a s c h o s e n . T h e f a c t o r s p r e v i o u s l y d i s -c u s s e d w h i c h c a n c o n t r i b u t e to t h e e r r o r E wi l l c a u s e i n a c c u r a c i e s ; and a s m e n t i o n e d in t h e i n t r o d u c t i o n to t h i s h a n d b o o k , e v e n if t h e i n s t r u m e n t cou ld m e a s u r e t h e e x p o s u r e r a t e e x a c t l y v e r y l a r g e e r r o r s c a n s t i l l a r i s e in e s t i m a t i n g t h e d o s e t o a p a r t i c u l a r o r g a n . W h e r e a d e r i v e d w o r k i n g l i m i t (DWL) i s u s e d to c o n t r o l d o s e , t he e s t i m a t e d and p e r m i s s i b l e v a l u e s of d o s e c a n on ly b e c o m p a r e d a f t e r i d e n t i f y i n g wh ich I C R P r e c o m m e n d a t i o n i s t h e m o s t l i m i t i n g ( e . g . sk in , e y e , who le body) .

4 . 3 . F r e q u e n c y of c a l i b r a t i o n

A p a r t f r o m w h e r e t h e r e i s a l e g a l r e q u i r e m e n t i t i s r e c o m m e n d e d t h a t an i n s t r u m e n t s h o u l d b e c a l i b r a t e d about o n c e e v e r y 12 m o n t h s . 1 A newly p u r c h a s e d i n s t r u m e n t s h o u l d a l s o be c a l i b r a t e d b e f o r e p u t t i n g i t in to s e r v i c e s i n c e v e r y f e w m a n u f a c t u r e r s p o s s e s s f a c i l i t i e s t ha t a r e c a p a b l e of c h e c k i n g t h e i n d i v i d u a l i n s t r u m e n t o v e r i t s c o m p l e t e e x p o s u r e r a t e o r d o s e - r a t e r a n g e : t h i s i s p a r t i c u l a r l y t r u e f o r h i g h - l e v e l i n s t r u m e n t s . T h i s i n i t i a l t e s t s h o u l d c h e c k t h e s c a l e l i n e a r i t y and r a n g e - c h a n g e e r r o r s . If t h e i n s t r u m e n t h a s s u f f e r e d any d a m a g e o r h a s b e e n r e p a i r e d , t h e n a d e q u a t e c h e c k s s h o u l d be m a d e to e n s u r e t h a t t h e o r i g i n a l c a l i b r a t i o n i s s t i l l v a l i d ; if no t , t h e n a r e - c a l i b r a t i o n s h o u l d b e u n d e r t a k e n . It i s o b v i o u s l y u n e c o n o m i c a l a s w e l l a s u n n e c e s s a r y t o m a k e a c o m p l e t e e v a l u a t i o n of e v e r y i n d i v i d u a l i n s t r u m e n t . W h e r e t h e i n s t r u m e n t i s of a n e w d e s i g n t h e n t h e t e s t s s h o u l d be c o m p r e -h e n s i v e and shou ld i n c l u d e a c o m p l e t e c h e c k of i t s e n e r g y r e s p o n s e , a c h e c k of t h e i n s t r u m e n t ' s s c a l e l i n e a r i t y , r a n g e - c h a n g e e r r o r s , f a l l - b a c k e f f e c t s a t r a d i a t i o n l e v e l s a b o v e t h e m a x i m u m l e v e l i n d i c a t e d on i t s s c a l e , a n d i t s r e s p o n s e t o f o r m s of r a d i a t i o n o t h e r t h a n t h o s e f o r w h i c h

1 In some laboratories it is done every three months.

57

i t w a s d e s i g n e d . E n v i r o n m e n t a l t e s t s s h o u l d a l s o b e m a d e s o t h a t if t h e i n s t r u m e n t i s l a t e r u s e d i n c o n d i t i o n s w h e r e t h e t e m p e r a t u r e o r p r e s s u r e d i f f e r s g r e a t l y f r o m a m b i e n t t h e n c o n f i d e n c e l i m i t s c a n be p l a c e d on the m e a s u r e m e n t s m a d e wi th i t . An a s s e s s m e n t s h o u l d a l s o b e m a d e of i t s m e c h a n i c a l and e l e c t r i c a l i n t e g r i t y , m e a s u r e m e n t s shou ld be m a d e to d e t e r m i n e if i t s a n g u l a r r e s p o n s e v a r i e s s i g n i f i c a n t l y and a l s o t he d e g r e e of v a r i a t i o n wi th r a d i a t i o n e n e r g y . I n s t r u m e n t s s e n t f o r c a l i b r a t i o n w h i c h a r e of t h e s a m e d e s i g n a s t h e one g iven t h e c o m p r e h e n s i v e t e s t s l i s t e d a b o v e , s h o u l d not r e q u i r e t e s t i n g a s t h o r o u g h l y . O c c a s i o n a l c h e c k s shou ld , h o w e v e r , be m a d e on t h i n g s l i k e t h e e n e r g y r e s p o n s e i n c a s e t h e m a n u f a c t u r e r h a s c h a n g e d t h e d e s i g n ; a known e x a m p l e of t h i s i s t he g r a p h i t e c o a t i n g of an i o n i z a t i o n c h a m b e r b e i n g c h a n g e d to a l u m i n i u m . T h e s e c o n d a r y s t a n d a r d s u s e d by t h e c a l i b r a t i o n f a c i l i t y shou ld a l s o b e p e r i o d i c a l l y i n t e r -c o m p a r e d to t h e m o s t s u i t a b l e p r i m a r y s t a n d a r d s . In s o m e c o u n t r i e s i n t e r v a l s of p e r i o d i c a l c h e c k i n g a r e d e t e r m i n e d by s p e c i a l r e g u l a t i o n s . "Where no su.ch r e g u l a t i o n s e x i s t t he f r e q u e n c y of t h i s c a l i b r a t i o n d e p e n d s on the n a t u r e of t h e s e c o n d a r y s t a n d a r d a s w e l l a s upon t h e p r o c e d u r e a d o p t e d to c h e c k i t s c o n s t a n c y . W h e r e t he s e c o n d a r y s t a n d a r d i s i n t he f o r m of a d e t e c t o r , e . g . an ion c h a m b e r , t h i s s h o u l d be c h e c k e d a g a i n s t p r i m a r y s t a n d a r d s a p p r o x i m a t e l y once e v e r y t w e l v e m o n t h s , f o r s e c o n d a r y s t a n d a r d s o u r c e s a v e r y m u c h l o n g e r p e r i o d i s a c c e p t a b l e .

4 . 4 . C o n s t a n c y c h e c k s

B e t w e e n t h e a n n u a l c a l i b r a t i o n s t h e u s e r s h o u l d be e n c o u r a g e d to c a r r y ou t c o n s t a n c y c h e c k s . W h e r e p o s s i b l e t h e s e s h o u l d be m a d e wi th t h e s a m e t y p e of r a d i a t i o n f o r wh ich t h e i n s t r u m e n t i s d e s i g n e d . A j ig i n c o r p o r a t i n g a b e t a s o u r c e m a y p r o v e u s e f u l in c h e c k i n g a g a m m a i n s t r u -m e n t , and f o r c e r t a i n t y p e s of i n s t r u m e n t s if t h e j ig i s p r o p e r l y d e s i g n e d i t c a n e v e n b e u s e d to c a l i b r a t e t h e i n s t r u m e n t [ 74] . S o m e i n s t r u m e n t s h a v e t h e i r own i n t e r n a l c h e c k s o u r c e s bu i l t in to t h e i n s t r u m e n t and t h e s e c a n b e u s e d to p r o v i d e c o n s t a n c y c h e c k s . C o n s t a n c y c h e c k s a r e doub ly i m p o r t a n t when t h e i n s t r u m e n t i s a h igh l e v e l one o r w h e n the i n s t r u m e n t i s u s e d on ly o c c a s i o n a l l y . T h e l e a k a g e r a d i a t i o n f r o m a r e a c t o r o r a c c e l e r -a t o r o p e r a t i n g u n d e r a f i x e d known cond i t i on c a n a l s o be u s e d a s a r o u g h c o n s t a n c y c h e c k . W h i l s t t he c o n s t a n c y c h e c k m a y be u s e d to s e e if t he i n s t r u m e n t s e n s i t i v i t y h a s c h a n g e d s i n c e i t w a s l a s t c a l i b r a t e d , shou ld i t c h a n g e t h e n i t i s g e n e r a l l y i n a d v i s a b l e to r e - a d j u s t t h e i n s t r u m e n t to g ive t h e o r i g i n a l c o n s t a n c y c h e c k r e a d i n g s i n c e e x t e r n a l f a c t o r s s u c h a s t h e a c t i v a t i o n and a c c u m u l a t i o n of f i s s i o n p r o d u c t s m a y h a v e c h a n g e d t h e r a d i a t i o n l e v e l . T h i s i s b e t t e r done a t t h e c a l i b r a t i o n l a b o r a t o r y w h e r e t h e w i d e r r a n g e of s t a n d a r d s o u r c e s m a y be u s e d to c h e c k t h e s e n s i t i v i t y o v e r t h e i n s t r u m e n t ' s c o m p l e t e e x p o s u r e o r d o s e r a n g e .

4 . 5 . E v a l u a t i o n of m e c h a n i c a l and e l e c t r i c a l p r o p e r t i e s

T h i s s e c t i o n h a s b e e n b a s e d upon a r e v i e w p a p e r [78] w h i c h d e s c r i b e s t h e e v a l u a t i o n t e c h n i q u e s u s e d a t B a t t e l l e - N o r t h w e s t , USA. T h i s e v a l u a t i o n , t o g e t h e r wi th t h e r a d i o l o g i c a l e v a l u a t i o n , shou l d p r o v i d e d a t a which wi l l a s s i s t in t h e p r o p e r s e l e c t i o n and u s e of r a d i a t i o n m o n i t o r i n g i n s t r u m e n t s . T h e l i m i t a t i o n s of a n i n s t r u m e n t shou ld be m a d e known to t he u s e r s , s i n c e t h e y m a y o f t e n be a s i m p o r t a n t , o r m o r e so , t h a n i t s good f e a t u r e s . F o r p o r t a b l e i n s t r u m e n t s , we igh t , s t r e n g t h and e a s e of h a n d l i n g and s e r v i c i n g

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T A B L E VIII . T Y P I C A L E V A L U A T I O N P R O G R A M F O R P O R T A B L E R A D I A T I O N S U R V E Y I N S T R U M E N T S ( E x c l u d i n g r a d i o l o g i c a l e v a l u a t i o n )

I. Mechanical

(a) Physical construction

(b) Shock and moisture resistance

(c) Human engineering factors

II. Electrical

(a) Power supplies

Cb) Input sensitivities

(c) Linearity

III. Environmental

(a) Electromagnetic interference

(1) Magnetic fields

(2) a . c . induced fields and transients

(3) Radio frequency

(4) Electrostatic fields

(b) Switching transients

(c) Capacitance effects

(d) Position dependence effects

(e) Temperature dependence

(f) Extracameral effects

(g) Sound and vibration effects

(h) Humidity dependence

a r e i m p o r t a n t p h y s i c a l c h a r a c t e r i s t i c s . H u m a n e n g i n e e r i n g f e a t u r e s s u c h a s e a s e of m e t e r r e a d i n g , a v a i l a b i l i t y of c o n t r o l s and p h y s i c a l s t a b i l i t y m u s t a l s o b e c o n s i d e r e d .

T h e e l e c t r i c a l e v a l u a t i o n s h o u l d i n c l u d e t e s t s of t he s e n s i t i v i t y , v o l t a g e d e p e n d e n c e , s t a b i l i t y and b a t t e r y l i f e t i m e . E n v i r o n m e n t a l t e s t s s h o u l d i n c l u d e t e m p e r a t u r e , m o i s t u r e and p r e s s u r e d e p e n d e n c e and r e s p o n s e t o e l e c t r o m a g n e t i c i n t e r f e r e n c e , n o i s e and v i b r a t i o n .

P r o p e r e v a l u a t i o n t e s t s a r e r e q u i r e d s i n c e a d e q u a t e and a c c u r a t e d a t a on wh ich to b a s e a m e a n i n g f u l e v a l u a t i o n i s u n a v a i l a b l e in m o s t i n s t a n c e s . T h e m o s t o b v i o u s s o u r c e of d a t a i s t h e p u b l i s h e d s p e c i f i c a t i o n of t h e m a n u -f a c t u r e r , bu t t h e s e a r e o f t e n i n c o m p l e t e o r c o u c h e d in l a n g u a g e t h a t i s not who l ly c o m p r e h e n s i b l e and m a y e v e n be m i s l e a d i n g . A t y p i c a l e v a l u a t i o n p r o g r a m f o r p o r t a b l e r a d i a t i o n s u r v e y m e t e r s i s o u t l i n e d in T a b l e VIII .

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W h i l s t t h i s p r o g r a m wi l l o b v i o u s l y no t be a p p r o p r i a t e to a l l i n s t r u m e n t s i t s h o u l d p r o v i d e e n o u g h i n f o r m a t i o n t o e n a b l e t h e h e a l t h p h y s i c i s t to m a k e a s o u n d s e l e c t i o n , c o n s i s t e n t w i th t h e n e e d s of h i s o p e r a t i o n , and the d i c t a t e s of h i s b u d g e t .

4 . 5 . 1 . M e c h a n i c a l e v a l u a t i o n

E v a l u a t i o n of t h e m e c h a n i c a l f e a t u r e s of an i n s t r u m e n t c a n be d iv ided in to two o v e r l a p p i n g a r e a s : p h y s i c a l c o n s t r u c t i o n and h u m a n e n g i n e e r i n g . To a r e l a t i v e l y l a r g e d e g r e e , t h i s p a r t of t h e e v a l u a t i o n i s s u b j e c t i v e , and r e q u i r e s t h e s o u n d j u d g m e n t t h a t i s o f t e n a c q u i r e d on ly t h r o u g h e x p e r i e n c e .

(a) P h y s i c a l _co_ns_trucWon

P h y s i c a l c o n s t r u c t i o n i s e v a l u a t e d by g e n e r a l a p p e a r a n c e of bo th t h e e x t e r i o r and t h e i n t e r i o r , p a y i n g p a r t i c u l a r a t t e n t i o n t o q u a l i t y c o n t r o l and w o r k m a n s h i p . S o l d e r j o i n t s a r e of p a r t i c u l a r v a l u e in t h i s e v a l u a t i o n ; t h e p r e s e n c e of co ld j o i n t s , e x c e s s i v e a m o u n t s of s o l d e r o r f lux , o r t h e l a c k of good m e c h a n i c a l c o u p l i n g a r e i n d i c a t i v e of p o o r q u a l i t y . C i r c u i t b o a r d s a r e s i m i l a r l y u s e f u l i n d i c a t o r s ; t h e q u a n t i t y of p a t h c o n d u c t o r and i t s p a t h l e n g t h , s h a r p n e s s of t he e t c h i n g , and l a b e l l i n g of c o m p o n e n t s a r e p o i n t s to be c o n s i d e r e d . T h e e x a m i n a t i o n of c o m p o n e n t l a y o u t shou ld i n c l u d e c o n s i d e r a t i o n of h e a t d i s s i p a t i o n and l e n g t h of l e a d s and c o n n e c t o r s .

(b) Shock and mo_istur_e _re_si£tajic_e

Whi l e e v a l u a t i o n of s t r u c t u r a l s t r e n g t h i s p r i m a r i l y s u b j e c t i v e , r e -s i s t a n c e t o s h o c k a n d m o i s t u r e c a n b e o b j e c t i v e l y e x a m i n e d . F i e l d i n s t r u -m e n t s g e n e r a l l y shou ld h a v e t h e ab i l i t y to w i t h s t a n d a d r o p of about one m e t r e on to a h a r d s u r f a c e and shou ld f u n c t i o n p r o p e r l y r e g a r d l e s s of a m b i e n t h u m i d i t y . M o r e o v e r , t h e y shou ld i d e a l l y be a b l e to s u r v i v e a s i n g l e , r a p i d t o t a l i m m e r s i o n in w a t e r w i thou t i l l e f f e c t s , an op in ion s h o u l d be f o r m e d a s to t he d e g r e e to wh ich t h e i n s t r u m e n t i s s h o w e r o r w a t e r - p r o o f . T h e s e s i m p l e c h e c k s c a n r e v e a l a g r e a t d e a l abou t t h e c o n -s t r u c t i o n of an i n s t r u m e n t ; t h e d e t e c t o r , of c o u r s e , m a y h a v e to be e x e m p t e d f r o m m e e t i n g s o m e of t h e s e r e q u i r e m e n t s . It s h o u l d be po in t ed out t h a t s i n c e s o m e of t h e s e t e s t s m a y d a m a g e t h e i n s t r u m e n t t h e y s h o u l d on ly b e m a d e a f t e r a l l o t h e r m e c h a n i c a l , e l e c t r i c a l and r a d i o l o g i c a l t e s t s h a v e b e e n c o m p l e t e d .

(c) H u m an_engin e_ering_

T h i s e v a l u a t i o n i s e s s e n t i a l l y s u b j e c t i v e , and s h o u l d i n c l u d e c o n s i -d e r a t i o n of s a f e t y h a z a r d s , a long wi th e a s e of h a n d l i n g , r e a d o u t and s e r v i c i n g . T y p i c a l s a f e t y h a z a r d s i n c l u d e s h a r p e d g e s , i n a d e q u a t e g r o u n d i n g , and o t h e r s h o c k h a z a r d s .

T h e s h a p e , s i z e and w e i g h t of t he i n s t r u m e n t a r e i m p o r t a n t and o b v i o u s f a c t o r s in e a s e of h a n d l i n g , a l t h o u g h e v e n t h e s e a r e s o m e t i m e s o v e r l o o k e d . H o w e v e r , o t h e r m o r e s u b t l e h u m a n e n g i n e e r i n g f e a t u r e s a l s o n e e d to be i n c l u d e d in t h e t o t a l e v a l u a t i o n — the l o c a t i o n of s w i t c h e s , s w i t c h i n g a r -r a n g e m e n t s , m e t e r s i z e , l o c a t i o n and r e a d a b i l i t y — t h e s e a r e a l l po in t s t o c o n s i d e r . E v e n t h e g e n e r a l e x t e r n a l a p p e a r a n c e of an i n s t r u m e n t c a n a f f e c t i t s a c c e p t a n c e by m o n i t o r i n g and o t h e r f i e l d p e r s o n n e l . S o m e c o n -s i d e r a t i o n s h o u l d a l s o be g i v e n to u s e of p o r t a b l e i n s t r u m e n t s by p e r s o n n e l w e a r i n g g l o v e s and o t h e r p e r s o n a l p r o t e c t i v e c l o t h i n g , and to e a s e of d e -c o n t a m i n a t i o n .

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F i n a l l y , e a s e of s e r v i c i n g shou ld be e v a l u a t e d . B a t t e r i e s s h o u l d b e r e a d i l y a c c e s s i b l e w i thou t r e m o v a l of o t h e r c o m p o n e n t s o r a l a r g e n u m b e r of s c r e w s ; t h i s f a c t o r a l o n e c a n s a v e s e v e r a l m a i n t e n a n c e m a n - h o u r s p e r y e a r . P l u g - i n c i r c u i t b o a r d s s h o u l d be a c c e s s i b l e f r o m both s i d e s and i d e a l l y t h e c o m p o n e n t s s h o u l d b e l a b e l l e d . C i r c u i t b o a r d s s h o u l d a l s o be k e y e d to p r e v e n t i n a d v e r t e n t e r r o n e o u s p o s i t i o n i n g in t h e s o c k e t . C a l i b -r a t i o n a d j u s t m e n t s s h o u l d be a b l e to b e m a d e w i t h o u t d i s m a n t l i n g t h e i n -s t r u m e n t a l t h o u g h t h e y s h o u l d , n e v e r t h e l e s s , b e h i d d e n b e n e a t h a r e m o v a b l e c o v e r t o m i n i m i z e t h e c h a n c e s of u n a u t h o r i z e d a d j u s t m e n t s a l t e r i n g t h e c a l i b r a t i o n u n b e k n o w n to t h e H e a l t h P h y s i c i s t .

4 . 5 . 2. E l e c t r i c a l e v a l u a t i o n

(a) J S l e c t r i c a l p o w e r ^supp l i e s

S t a b i l i t y : S o m e p o w e r s u p p l i e s h a v e v a r i a b l e ou tpu t , e s p e c i a l l y f o r t h e h i g h v o l t a g e . Wi th c o n s t a n t i npu t v o l t a g e t h e ou tpu t r a n g e i s c h e c k e d a t t h e end p o i n t s , and t h e s t a b i l i t y n o t e d . T h e m e a s u r e m e n t s s h o u l d i n c l u d e bo th v o l t a g e and c u r r e n t o u t p u t s . G e n e r a l l y , t h e c i r c u i t r y of t h e i n s t r u m e n t i s u s e d a s t h e l o a d , bu t f i x e d r e s i s t o r s c a n b e u s e d to p r o v i d e l o a d s of a n y d e s i r e d v a l u e . T h e s t a b i l i t y of t h e v o l t a g e and c u r r e n t ou tpu t i s o b -s e r v e d a s a f u n c t i o n of l o a d , ou tpu t v o l t a g e and inpu t v o l t a g e . M o d e r n p o w e r s u p p l i e s s h o u l d g e n e r a l l y be c a p a b l e of p r o v i d i n g s t a b l e h i g h v o l t a g e s to w i t h i n a f e w p e r c e n t o r b e t t e r .

B a t t e r y l i f e : F o r b a t t e r y - o p e r a t e d s u p p l i e s , t h e ou tpu t v o l t a g e a n d c u r r e n t a r e m e a s u r e d a s a f u n c t i o n of b a t t e r y v o l t a g e . T h i s t e s t i s b e s t p e r f o r m e d u s i n g t h e s p e c i f i c b a t t e r i e s d e s i g n a t e d f o r u s e i n t h e i n s t r u m e n t , s i n c e t h e p r o p e r o p e r a t i o n of m a n y p o w e r s u p p l i e s i s h i g h l y d e p e n d e n t on bo th b a t t e r y v o l t a g e and r e s i s t a n c e .

An i n s t r u m e n t m a y b e d e s i g n e d to o p e r a t e w i t h s e v e r a l t y p e s of p r i m a r y o r s e c o n d a r y b a t t e r i e s . T h u s , o r d i n a r y c a r b o n - z i n c c e l l s m a y be u s e d f o r t e m p e r a t u r e s down to - 1 8 t o - 1 2 ° C . B e l o w t h e s e t e m p e r a t u r e s z i n c a l k a l i n e p r i m a r y c e l l s s h o u l d b e u s e d . B a t t e r y l i f e t e s t s shou ld c o v e r t h o s e t y p e s of b a t t e r i e s wh ich wi l l b e i n s t a l l e d f o r t h o s e e n v i r o n m e n t s in w h i c h t h e i n s t r u m e n t w i l l be u s e d .

F r e s h , t e s t e d b a t t e r i e s s h o u l d be i n s t a l l e d in t h e i n s t r u m e n t , a n d p r o v i s i o n s m a d e f o r m e a s u r i n g b a t t e r y v o l t a g e , b a t t e r y c u r r e n t d r a i n , p o w e r - s u p p l y v o l t a g e s , p o w e r - s u p p l y c u r r e n t s , and o v e r a l l i n s t r u m e n t r e s p o n s e . T h e d e t e c t o r e l e m e n t of t h e m o n i t o r i n g i n s t r u m e n t i s p l a c e d in a r a d i a t i o n f i e l d w h i c h s h o u l d p r o v i d e a m i d - s c a l e r e a d i n g on the m o s t f r e q u e n t l y u s e d r a n g e . T h e i n s t r u m e n t i s t u r n e d on and t h e a b o v e v a r i a b l e s r e c o r d e d o v e r t h e l i f e of t h e b a t t e r y . T h e end po in t of t he b a t t e r y m a y be t a k e n a s t h a t v o l t a g e at w h i c h t h e i n s t r u m e n t r e s p o n s e h a s v a r i e d by 10% f r o m t h e r e s p o n s e o b t a i n e d wi th a f r e s h b a t t e r y .

A b a t t e r y t e s t p o s i t i o n i s a h igh ly d e s i r a b l e f e a t u r e of any p o r t a b l e s u r v e y i n s t r u m e n t . T h e b a t t e r y v o l t a g e shou ld be c h e c k e d a g a i n s t t h e i n -d i c a t e d p o s i t i o n to e n s u r e t h a t i t i s p r o p e r l y l o c a t e d .

(b) Input s e n s i t i v i t i e s

M a n y p o r t a b l e i n s t r u m e n t s a r e of t h e p u l s e t y p e m e t e r and a u r a l o u t p u t . T h e inpu t s e n s i t i v i t y of v o l t a g e s e n s i t i v e i n s t r u m e n t s ( e . g . G e i g e r - M u l l e r c o u n t e r s ) i s d e t e r m i n e d wi th a s i g n a l g e n e r a t o r p r o v i d i n g a v o l t a g e p u l s e

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s i m i l a r to t h a t g i v e n by t h e d e t e c t o r . T h e input s e n s i t i v i t y i s e x p r e s s e d a s v o l t s o r m i l l i v o l t s n e c e s s a r y to g ive s t e a d y m e t e r r e a d i n g o r a u r a l o u t p u t f o r a g i v e n p u l s e inpu t f r e q u e n c y r a t e .

F o r c u r r e n t m e a s u r i n g i n s t r u m e n t s , s u c h a s ion c h a m b e r s and p r o -p o r t i o n a l c o u n t e r s , a c u r r e n t s o u r c e i s u s e d in p l a c e of t he d e t e c t o r . T h e c u r r e n t n e c e s s a r y f o r f u l l - s c a l e d e f l e c t i o n i s d e t e r m i n e d , and the input s e n s i t i v i t y e x p r e s s e d a s t h e c u r r e n t f o r f u l l - s c a l e d e f l e c t i o n . If t h e a c t i v e v o l u m e of t he d e t e c t o r i s known, the input s e n s i t i v i t y m a y be c a l c u l a t e d in t e r m s of R / h r e q u i r e d f o r f u l l - s c a l e d e f l e c t i o n . T h i s c a l c u l a t e d v a l u e c a n be c h e c k e d by p l a c i n g t h e i n s t r u m e n t in a r a d i a t i o n f i e l d of t h e a p -p r o p r i a t e s t r e n g t h and m e a s u r i n g t h e c u r r e n t ou tpu t of t he d e t e c t o r . T h i s t e c h n i q u e c a n a l s o be u s e d to d e t e r m i n e t he l i n e a r i t y of t he d e t e c t o r a l o n e . R e s p o n s e in p u l s e d f i e l d s , wh ich is a s s u m i n g g r e a t e r i m p o r t a n c e , c a n a l s o be d e t e r m i n e d f r o m t h i s t e c h n i q u e . H o w e v e r , it i s n e c e s s a r y to b e a r in m i n d t h a t b e c a u s e of t he e f f e c t s of s a t u r a t i o n , e l e c t r o n a t t a c h m e n t , ion and h o l e r e c o m b i n a t i o n and o t h e r e f f e c t s in t h e d e t e c t o r v o l u m e , t h e r e s p o n s e d o e s s t r o n g l y d e p e n d on t h e f r e q u e n c y of t h e r a d i a t i o n p u l s e s and t h e i r d u r a t i o n . T h e r e f o r e , t h e c h a r a c t e r i s t i c s of t h e c a l i b r a t i o n p u l s e d f i e l d h a v e to be s i m i l a r t o t h o s e of t h e f i e ld to b e m e a s u r e d .

(c) _ L i n e a r i t y

T h e l i n e a r i t y of t h e r a t e m e t e r s h o u l d not b e c o n f u s e d wi th t h e l i n e a r i t y of t h e i n s t r u m e n t a s a w h o l e . T h e i n s t r u m e n t l i n e a r i t y can on ly be c h e c k e d by t e s t s in r a d i a t i o n f i e l d s w h o s e l e v e l s c a n b e v a r i e d o v e r t h e w o r k i n g r a n g e of t h e i n s t r u m e n t . T h e r a t e - m e t e r l i n e a r i t y i s e a s i l y and q u i c k l y c h e c k e d by pu t t i ng a s i g n a l s i m i l a r to t h a t p r o v i d e d by t h e d e t e c t o r in to t h e d e t e c t o r inpu t of t h e c i r c u i t . The s i g n a l g e n e r a t o r s h o u l d b e i n i t i a l l y s e t to p r o v i d e a p u l s e r a t e (in t he c a s e of p u l s e d e t e c t o r s ) o r c u r r e n t (in t h e c a s e of c u r r e n t m e a s u r i n g c i r c u i t s ) e q u a l t o a b o u t h a l f - s c a l e d e f l e c t i o n . In t he c a s e of c h a r g e s e n s i t i v e input s y s t e m , a v o l t a g e to c h a r g e c o n v e r t o r i s put on t h e ou tpu t of t h e v o l t a g e s i g n a l g e n e r a t o r .

L i n e a r i t y shou ld b e c h e c k e d a t s e v e r a l p o i n t s o v e r t h e r a n g e of 10 to 100% of f u l l s c a l e . It i s i m p o r t a n t to w a t c h t h e e r r o r s n e a r f u l l - s c a l e m e t e r r e a d i n g s i n c e s o m e s y s t e m s t e n d to s a t u r a t e in t h i s r e g i o n . L i n e a -r i t y shou ld b e e x p r e s s e d in t e r m s of t h e p e r c e n t a g e d e v i a t i o n of m e t e r r e a d i n g f r o m t h e c o r r e c t v a l u e a t any po in t on t h e s c a l e r a t h e r t h a n t h e f u l l - s c a l e r e a d i n g . T h u s , an i n s t r u m e n t t h a t r e a d 11% of f u l l s c a l e when t h e inpu t s i g n a l w a s s u c h a s to g ive a r e a d i n g of 10% of f u l l s c a l e , h a s a 10% d e v i a t i o n . A s t a t e m e n t of ± a f ew p e r c e n t i s m o s t u s e f u l to b r a c k e t l i n e a r i t y .

4 . 5 . 3 . E n v i r o n m e n t a l e v a l u a t i o n

(a) R e s p o n s e t o _ e l e c t r o m a g n e t i c i n t e r f e r e n c e

E l e c t r o m a g n e t i c r a d i a t i o n of m a n y k i n d s can c a u s e u n u s u a l e f f e c t s and s p u r i o u s r e s p o n s e . T h e c i r c u i t r y , r a t h e r t h a n t h e d e t e c t o r s , i s m o r e c o m m o n l y a f f e c t e d . In m o s t c a s e s , w e l l - d e s i g n e d c i r c u i t s wi th good e l e c t -r i c a l s h i e l d i n g wi l l no t be a f f e c t e d by e l e c t r o m a g n e t i c f i e l d s .

M a g n e t i c f i e l d s : T h e m e t e r i s u s u a l l y a f f e c t e d m o r e f r e q u e n t l y t h a n o t h e r c o m p o n e n t s by m a g n e t i c f i e l d s . T h e e f f e c t of m a g n e t i c f i e l d s c a n u s u a l l y b e s t b e d e t e r m i n e d by a c t u a l o b s e r v a t i o n u n d e r t he s p e c i f i c c o n d i t i o n s

62

e n c o u n t e r e d in t he f i e l d , o r i e n t i n g t h e i n s t r u m e n t a l o n g an x, y , and z a x i s wi th r e s p e c t to t h e d i r e c t i o n of t h e m a g n e t i c l i n e s of f o r c e . D i f f e r e n c e s in i n s t r u m e n t r e s p o n s e wi l l u s u a l l y be a t t r i b u t a b l e to t h e m a g n e t i c f i e l d . T h e e f f e c t s of m a g n e t i c f i e l d s c a n s o m e t i m e s be r e d u c e d by w r a p p i n g the i n s t r u m e n t c a s e o r s o m e s e n s i t i v e c i r c u i t s wi th e l e c t r o m a g n e t i c s h i e l d i n g .

A. C . - i n d u c e d f i e l d s and t r a n s i e n t s : T r a n s i e n t s c a u s e d b y i n t e r r u p t i n g an a . c . c i r c u i t u n d e r l oad a r e s o m e of the w o r s t o f f e n d e r s . T h i s i s e s -p e c i a l l y t r u e of a . c . l i n e o p e r a t e d i n s t r u m e n t s , but a l s o h o l d s f o r b a t t e r y -p o w e r e d i n s t r u m e n t s o p e r a t e d c l o s e to u n s h i e l d e d a . c . l i n e s . Of c o u r s e , in t he c a s e of b a t t e r y p o w e r e d i n s t r u m e n t s , r a d i a t e d r a d i o f r e q u e n c y (RF) i s t h e c a u s e of e r r o n e o u s r e a d i n g s . E l e c t r i c t y p e w r i t e r s and e l e c t r o m e c h a n i -c a l c a l c u l a t o r s a r e f r e q u e n t l y found in a r e a s w h e r e m o n i t o r s a r e u s e d .

T h e e f f e c t of a . c . t r a n s i e n t s and induced f i e l d s can b e c r u d e l y c h e c k e d by u s i n g an e l e c t r i c d r i l l o r s i m i l a r d e v i c e . T r a n s i e n t s and a . c . - i n d u c e d f i e l d s s h o u l d no t c a u s e d e f l e c t i o n s in t h e m e t e r n e e d l e g r e a t e r t h a n 10% of t h e m e t e r r e a d i n g .

R e s p o n s e to r a d i a t e d e n e r g y ( r a d i o f r e q u e n c y ) : T h e m o s t c o m m o n s o u r c e s of r a d i a t e d e n e r g y t h a t i n t e r f e r e wi th m o n i t o r i n g i n s t r u m e n t s a r e :

(i) I n d u c t i o n t y p e h e a t e r s (ii) R a d i o f r e q u e n c y w e l d e r s (ii i) R a d i o f r e q u e n c y g e n e r a t e d by a. c . l i ne t r a n s i e n t s (iv) R a d a r b e a m s (v) Ign i t i on s y s t e m s

W e l d i n g and r a d a r a p p a r a t u s m a y h a v e X - r a d i a t i o n a s s o c i a t e d wi th t h e m , and c a r e m u s t be t a k e n to e n s u r e t h a t the r e s p o n s e of t h e i n s t r u m e n t w a s no t e l i c i t e d by s t r a y X - r a y s , I n t e r f e r e n c e f r o m i g n i t i o n s y s t e m s c a n be p a r t i c u l a r l y v e x i n g . E r r o r s g r e a t e r than ±10% of a g iven m e a s u r e m e n t h a v e b e e n n o t e d at d i s t a n c e s of 60 m f r o m the s o u r c e of i n t e r f e r e n c e .

In g e n e r a l , i n s t r u m e n t s s h o u l d be f i e ld t e s t e d f o r r a d i o f r e q u e n c y r e s p o n s e u n d e r t h e c o n d i t i o n s i n wh ich they wi l l b e u s e d . T h e u n t o w a r d e f f e c t s of R F c a n s o m e t i m e s b e a l l e v i a t e d by a m e t a l l i c c a s e , p r o p e r l y g r o u n d e d .

E l e c t r o s t a t i c c h a r g e s : A s a r u l e , ion c h a m b e r i n s t r u m e n t s a r e t h e o n e s m o s t s e n s i t i v e to s t a t i c c h a r g e s . T h e e f f e c t of e l e c t r o s t a t i c c h a r g e i s e a s i l y c h e c k e d q u a l i t a t i v e l y by m o v i n g a c h a r g e d c o m b o r p i e c e of p l a s t i c a r o u n d the i n s t r u m e n t and d e t e c t o r , and n o t i n g any d e v i a t i o n in m e t e r r e a d i n g . T h e d e v i a t i o n shou ld not e x c e e d ±10% of t h e r e a d i n g wi th s t a t i c c h a r g e ; m e t e r d e f l e c t i o n f r o m z e r o shou ld be m i n i m a l — not m o r e t h a n 1% of f u l l s c a l e .

(b) Swdtching_tr_an_sients

S w i t c h i n g t r a n s i e n t s r e f e r to qu ick , v i o l e n t , l a r g e e x c u r s i o n s of t h e m e t e r when t h e r a n g e s w i t c h i s c h a n g e d f r o m one r a n g e to t he n e x t . S w i t c h i n g t r a n s i e n t s s h o u l d not d r i v e t h e m e t e r p o i n t e r off s c a l e .

A s a g u i d e l i n e , t h e p o i n t e r shou ld r e t u r n t o t h e o r i g i n a l z e r o r e a d i n g in two s e c o n d s o r l e s s , w i thou t t he a id of a m e t e r r e s e t s w i t c h .

When c h e c k i n g s w i t c h i n g t r a n s i e n t s , t h e i n s t r u m e n t s h o u l d be a l l o w e d to w a r m u p c o m p l e t e l y , and t h e r a n g e s w i t c h c h a n g e d s t e p w i s e in both d i r e c t i o n s .

63

(c) C a p a c i t a n c e e f f e c t s

C a p a c i t a n c e e f f e c t s a r e c h a n g e s in m e t e r r e a d i n g s o r i n s t r u m e n t r e s p o n s e f r o m p h y s i c a l m o t i o n of p a r t s of t he i n s t r u m e n t . H i s t o r i c a l l y , t h e w o r d c a p a c i t a n c e i s u s e d , f o r in t h e e a r l y d a y s of r a d i a t i o n m o n i t o r i n g i n s t r u m e n t s , c a s e m o v e m e n t s would c a u s e c a p a c i t y c h a n g e s in t he c i r c u i t , wh ich would l e a d t o c h a n g e s in m e t e r r e a d i n g s . A s e c o n d t y p e of c a p a c i t a n c e e f f e c t — h a n d c a p a c i t a n c e — i s no t ed when t h e h a n d i s b r o u g h t n e a r t h e c a s e o r d e t e c t o r .

In g e n e r a l , c a p a c i t a n c e e f f e c t s s h a l l no t c a u s e m e t e r d e f l e c t i o n s in e x c e s s of p l u s o r m i n u s o n e m i n o r m e t e r s c a l e d i v i s i o n . T o c h e c k f o r c a p a c i t a n c e e f f e c t s , t h e i n s t r u m e n t i s t u r n e d on and a l l o w e d to s t a b i l i z e on t h e m o s t s e n s i t i v e r a n g e . P r e s s i n g on any p a r t of t h e c a s e , c o n t r o l knobs , l i f t i n g t h e c o n t r o l knobs l i g h t l y o r any o t h e r m a n i p u l a t i o n , w h i c h i s a s s o c i a t e d wi th n o r m a l p r o c e d u r e , s h a l l no t c a u s e m e t e r d e f l e c t i o n in e x c e s s of t h a t l i s t e d a b o v e . T h e t e s t shou ld be r e p e a t e d on a l l r a n g e s .

(d) Positio_n d e p e n d e n c e _ e f f e c t s _

P o s i t i o n d e p e n d e n c e e f f e c t s a r e r e l a t e d to c a p a c i t a n c e e f f e c t s . In an i d e a l i n s t r u m e n t t h e m e t e r r e a d i n g w i l l not c h a n g e r e g a r d l e s s of t h e p o s i t i o n in w h i c h t h e i n s t r u m e n t i s o p e r a t e d . G e n e r a l l y t h e r e i s s o m e p o s i t i o n d e p e n d e n c e due to g r a v i t y e f f e c t s on t h e m e t e r .

P o s i t i o n d e p e n d e n c e c a n be q u i c k l y and e a s i l y d e t e r m i n e d by r o t a t i n g a s t a b i l i z e d i n s t r u m e n t t h r o u g h a f u l l 360° in two p l a n e s n o r m a l to e a c h o t h e r and p a r a l l e l to t h e g r o u n d . T h e i n s t r u m e n t s h o u l d b e p l a c e d in a r a d i a t i o n f i e l d s u c h t h a t t he s c a l e r e a d i n g i s a p p r o x i m a t e l y hal f f u l l s c a l e , and t h e s o u r c e - d e t e c t o r d i s t a n c e shou ld a l s o be l a r g e so t h a t on r o t a t i n g t h e i n s t r u m e n t e r r o r s c a n n o t be i n t r o d u c e d owing to p o s i t i o n a l c h a n g e s of t h e c e n t r e of t h e d e t e c t o r . O r d i n a r i l y , d e v i a t i o n s of l e s s t h a n 1 o r 2% of t h e f u l l - s c a l e r e a d i n g a r e a c c e p t a b l e .

(e) T e m p e r a t u r e d e p e n d e n c y

T h e e f f e c t s of t e m p e r a t u r e on the i n s t r u m e n t a s a who le i s of p r i m e i m p o r t a n c e , s i n c e i n s t r u m e n t s m a y be u s e d in t h e r m a l e n v i r o n m e n t s r a n g i n g f r o m s u b - f r e e z i n g to m o r e t h a n 50° C. It i s d e s i r a b l e t h a t t h e t e m p e r a t u r e c o e f f i c i e n t of t h e i n s t r u m e n t be l e s s t h a n ±0 . 2 % / d e g . C ; i f , h o w e v e r , i t i s g r e a t e r t h a n t h i s , t h e n t h e v a r i a t i o n shou ld b e known s o t h a t a p p r o p r i a t e c o r r e c t i o n f a c t o r s c a n b e a p p l i e d w h e r e t h e i n s t r u m e n t i s u s e d a t e x t r e m e t e m p e r a t u r e s . G e n e r a l l y , t h e d e t e c t o r i s t he m a j o r c o n t r i b u t o r t o t e m p e r -a t u r e d e p e n d e n c e , bu t o t h e r c o m p o n e n t s m a y a l s o c o n t r i b u t e s i g n i f i c a n t l y . O c c a s i o n a l l y a s i n g l e c o m p o n e n t s u c h a s a r e s i s t o r m a y r e q u i r e r e p l a c e m e n t in o r d e r to m i n i m i z e t e m p e r a t u r e e f f e c t s .

E v a l u a t i o n of t e m p e r a t u r e d e p e n d e n c e r e q u i r e s an e n v i r o n m e n t a l o r t e m p e r a t u r e t e s t c h a m b e r . T h e i n s t r u m e n t i s p l a c e d in t h e t e s t c h a m b e r and a s o u r c e i s p o s i t i o n e d o u t s i d e t h e c h a m b e r t o g i v e a c o n s t a n t m e t e r r e a d i n g a t r o o m t e m p e r a t u r e . T h e t e m p e r a t u r e i s r a i s e d a t a r a t e of a b o u t 1 0 ° C / h u n t i l a m a x i m u m t e m p e r a t u r e of 60° C i s r e a c h e d . M e t e r r e a d i n g s a r e t a k e n a b o u t e v e r y 5 ° C , and a p lo t m a d e of s c a l e r e a d i n g s v s t e m p e r a t u r e m a d e . T h e t e m p e r a t u r e i s t h e n l o w e r e d a t t he r a t e of 1 0 ° C / h u n t i l t he l o w e r l i m i t of - 2 0 ° C i s r e a c h e d . M e t e r r e a d i n g s a r e t a k e n e v e r y 5 ° C . When t h e l o w e s t t e m p e r a t u r e h a s b e e n r e a c h e d , t h e t e m p e r a t u r e i s a g a i n r a i s e d a t t h e r a t e of about 1 0 ° C / h u n t i l r o o m t e m p e r a t u r e i s r e a c h e d .

64

A s p r e v i o u s l y , r e a d i n g s a r e t a k e n abou t e v e r y 5 ° C . T h e e x a c t t e m p e r a t u r e r a n g e o v e r wha t t h e i n s t r u m e n t i s t e s t e d wi l l d e p e n d u p o n t h e c l i m a t i c c o n d i t i o n s t h e i n s t r u m e n t w i l l m e e t in p r a c t i c e .

T h e t e s t a t l o w e r t e m p e r a t u r e s i s a l s o a t e s t f o r h igh h u m i d i t y o p e r a t i o n . At t h e s t a r t of t h e t e s t , t h e a i r i s at r o o m t e m p e r a t u r e i n t h e t e s t c h a m b e r , and n o r m a l l y h a s a r e l a t i v e h u m i d i t y of 30 t o 50%. A s t h e t e m p e r a t u r e i s l o w e r e d t h e a i r in t h e c h a m b e r wi l l b e c o m e s a t u r a t e d and m o i s t u r e m a y f o r m i n s i d e t h e i n s t r u m e n t c a s e . In m o s t i n s t a n c e s t r o u b l e i s not e n c o u n t e r e d wi th m o i s t u r e u n t i l t h e t e m p e r a t u r e i s i n c r e a s e d f r o m t h e l o w e s t po in t b a c k t o w a r d s r o o m t e m p e r a t u r e .

In t h e a b o v e t e m p e r a t u r e t e s t s t h e r e m a y be t i m e s w h e n l a r g e c a l i b r a -t i o n d e v i a t i o n s o c c u r . It w i l l t h e n be n e c e s s a r y to c h e c k s u b - u n i t s of t h e s y s t e m o n e o r two a t a t i m e , u n t i l t h e t e m p e r a t u r e s e n s i t i v e p a r t s a r e d e t e r m i n e d .

A s t h e s e t e s t s m a y l a s t o v e r a p e r i o d l o n g e r t h a n t h e l i f e of t h e i n s t r u -m e n t ' s b a t t e r y i t m a y p r o v e n e c e s s a r y to c a r r y out t h e s e t e s t s u s i n g an e x t e r n a l c o n s t a n t v o l t a g e s o u r c e . F u r t h e r t e s t s m a y t h e r e f o r e b e r e q u i r e d t o e v a l u a t e t h e v a r i a t i o n of b a t t e r y p e r f o r m a n c e wi th t e m p e r a t u r e s .

(f) E x t r a c a m e r a l _ e f _ f e c t £

E x t r a c a m e r a l e f f e c t s a r e m e t e r d e f l e c t i o n s c a u s e d by i n t e r a c t i o n of t h e i o n i z i n g r a d i a t i o n wi th p a r t of t h e i n s t r u m e n t o t h e r t h a n the t r u e d e t e c t o r o r i o n i z a t i o n c h a m b e r c o m p o n e n t . T h e e f f e c t i s m o s t p r e v a l e n t in i o n i z a t i o n c h a m b e r and s c i n t i l l a t i o n t y p e i n s t r u m e n t s . O b v i o u s l y , i t i s n e c e s s a r y to c h e c k e x t r a c a m e r a l e f f e c t s by s h i e l d i n g t h e d e t e c t o r . T h e r e s t of t h e i n s t r u m e n t c a n t h e n be i r r a d i a t e d wi th d i f f e r e n t e n e r g y r a d i a t i o n to s e e if any e f f e c t s a r e o b s e r v e d . If s i g n i f i c a n t e f f e c t s a r e o b s e r v e d t h e n t h e s o u r c e of t r o u b l e c a n b e l o c a t e d by s c a n n i n g t h e i n s t r u m e n t wi th a w e l l c o l l i m a t e d b e a m , 2 m m w i d e . T h e m e t e r r e a d i n g c a n t h e n b e p l o t t e d a s a f u n c t i o n of l o c a t i o n . In g e n e r a l , e x t r a c a m e r a l r e s p o n s e s h o u l d be l e s s t h a n a f ew p e r c e n t of t h e a v e r a g e r e a d i n g o b t a i n e d wi th t h e b e a m d i r e c t e d t h r o u g h t h e d e t e c t o r , r e g a r d l e s s of r a n g e . It i s d e s i r a b l e to c h e c k t h i s e f f e c t a t d i f f e r e n t e x p o s u r e l e v e l s .

(g) Sound and_ v i b r a t i o n e f f e c t s _

R e s p o n s e to s o u n d o c c u r s m o s t l y in p u l s e - t y p e i n s t r u m e n t s . At t i m e s t h e s p e a k e r r e s p o n s e m a y c a u s e u n w a n t e d f e e d b a c k , n e g a t i n g t h e u s e of t h e i n s t r u m e n t . I dea l l y , n o i s e e f f e c t s s h o u l d be c h e c k e d in a sound c h a m b e r wi th v a r i a b l e i n t e n s i t y p u r e t o n e s . T h e t e s t i s m a d e by e x p o s i n g t h e i n s t r u -m e n t to n o i s e s of v a r i o u s f r e q u e n c i e s and i n t e n s i t y , and any a b n o r m a l r e s p o n s e i s n o t e d . T h i s i s o b v i o u s l y i m p r a c t i c a l , and s o r a n d o m n o i s e g e n e r a t o r s o r g e n e r a l i n d u s t r i a l n o i s e s o u r c e s c a n be u s e d . R e s p o n s e to v i b r a t i o n o r i m p a c t n o i s e m a y r e s u l t in t e m p o r a r y o r p e r m a n e n t c h a n g e in c a l i b r a t i o n . In a d d i t i o n v i b r a t i o n m a y c a u s e p h y s i c a l d a m a g e to t h e i n s t r u m e n t . A s h a k e r t a b l e i s a good d e v i c e f o r p r o v i d i n g v i b r a t i o n f o r t e s t p u r p o s e s . One of t h e m o s t s a t i s f a c t o r y and p r a c t i c a l v i b r a t i o n t e s t s i s t o f i e l d t e s t t h e i n s t r u m e n t f o r a p e r i o d of t i m e . O r d i n a r i l y , n o i s e and v i b r a t i o n shou ld not n o t i c a b l y a f f e c t i n s t r u m e n t p e r f o r m a n c e .

65

5. I N S T R U M E N T M A I N T E N A N C E AND R E P A I R

I n s t r u m e n t s t h a t h a v e g r o s s p h y s i c a l d e f e c t s , e l e c t r o n i c m a l f u n c t i o n s , o r c a n n o t be c a l i b r a t e d e v e n a f t e r s u c h s i m p l e r e m e d i e s a s b a t t e r y c h a n g e s , s h o u l d b e s e n t to t he r e p a i r f a c i l i t y . An i n s t r u m e n t r e p a i r f a c i l i t y shou ld be e q u i p p e d wi th t y p i c a l e l e c t r o n i c t e s t e q u i p m e n t , s u c h a s o s c i l l o s c o p e s and e l e c t r o s t a t i c v o l t m e t e r s , f o r u s e in d e t e r m i n i n g t h e n a t u r e of t h e p r o b l e m . T h e r e p a i r s h o p shou ld be equ ipped to r e p l a c e and r e p a i r d e f e c t i v e p r i n t e d c i r c u i t b o a r d s , and to a s c e r t a i n and r e p l a c e f a u l t y v a c u u m t u b e s , t r a n s i s t o r s , r e s i s t o r s , c o n d e n s e r s , and r e l a t e d e l e c t r o n i c c o m p o n e n t s .

If t h i s r e p a i r i s on an i n s t r u m e n t s e n t f o r c a l i b r a t i o n by an o u t s i d e o r g a n i z a t i o n t h e n i t w i l l be n e c e s s a r y to m a k e an a p p r o p r i a t e c h a r g e f o r t h e w o r k .

A w e l l - e q u i p p e d r e p a i r s h o p shou ld a l s o b e a b l e to m a k e m i n o r r e p a i r s of d e t e c t o r s , i n c l u d i n g r e p l a c e m e n t of t h e l i gh t t i gh t s c r e e n s o r c o v e r s on s c i n t i l l a t o r s and p r o p o r t i o n a l c o u n t e r s . R e p l a c e m e n t and c h e c k o u t of m e t e r s , n e w i n s t r u m e n t s , and m e c h a n i c a l c o m p o n e n t s a r e a l s o wi th in t he s c o p e of t h e r e p a i r f a c i l i t y .

66

APPENDIX I

LIST O F P R I M A R Y STANDARD L A B O R A T O R I E S ""

C o u n t r y

A u s t r a l i a

C a n a d a

C z e c h o s l o v a k S o c i a l i s t R e p u b l i c

G e r m a n y , F e d e r a l R e p u b l i c of

F r a n c e

F r a n c e

I t a l y

Ind ia

J a p a n

N e t h e r l a n d s

New Z e a l a n d

I n s t i t u t e

C o m m o n w e a l t h X - r a y and R a d i u m L a b o r a t o r y ,

M e l b o u r n e

N a t i o n a l R e s e a r c h C o u n c i l ,

O t t a w a

D o s i m e t r y D e p a r t m e n t of t h e N u c l e a r R e s e a r c h I n s t i t u t e ,

P r a g u e

I n s t i t u t e S u p e r i o r e de S a n i t a ,

R o m e

B h a b h a A t o m i c R e s e a r c h C e n t r e ,

B o m b a y

E l e c t r o t e c h n i c a l L a b o r a t o r y ,

T o k y o

R o t t e r d a m s c h R a d i o -t h e r a p e u t i s c h I n s t . ,

R o t t e r d a m

N a t i o n a l R a d i a t i o n L a b o r a t o r y ,

C h r i s t c h u r c h

C u s t o d i a n

D . J . S t e v e n s

C . G a r r e t t

F . B e h o u n e k

B . L . Gup ta P . N . K r i s h n a m o o r t h y

Y . I n o u e

A . S o m e r w i l

G . E . Roth

P h y s i k a l i s c h - T e c h n i s c h e - W. H i i b n e r B u n d e s a n s t a l t ,

B r a u n s c h w e i g ( B r u n s w i c k )

L a b o r a t o i r e de M e t r o l o g i e B . G r i n b e r g de l a R a d i o a c t i v i t e

C E N S a c l a y , 91 G i f - s u r - Y v e t t e

B u r e a u I n t e r n a t i o n a l d e s A . A l l i s y P o i d s e t M e s u r e s (BIPM),

P a r i s

67

Norway Sta tens Ins t i tu t t f o r S t ra lehygiene ,

Os lo , 3, Montebel lo

S. Maudal

Poland Glowny U r z a d M i a r , Warsaw

Z . R e f e r o w s k i

South A f r i c a Nat ional P h y s i c a l R e s e a r c h Lab . ,

P r e t o r i a M.L . du P r e e z

Sweden Sta tens S t r å l skydd ins t i t u t , B. Lindel i Stockholm

Swi tzer land E i d g e n ö s s i s c h e s Amt fiir M. M a i e r M a s s und Gewicht ,

Wabe rn bei B e r n

Union of Soviet Socia l i s t Ins t i tu te of D . I .Mende l eev M . F . Yudin Republ ics Len ingrad

United Kingdom Nat ional P h y s i c a l L a b o r a t o r y ,

Teddington, Middlesex

United Sta tes of A m e r i c a Nat ional B u r e a u of S tanda rds ,

Washington

W.A. Jenn ings

R. Loev inge r

# This list is taken from Ref. [65] and a few additional laboratories are included from a report on Secondary Standard Laboratories for Radiation Dosimetry, a consultation of experts held by WHO from 4 - 8 November 1968 in Geneva and from comments on the draft handbook. Some details regarding primary standards may be found in Ref. [65]. More remote laboratories in the same country or from other countries can get their secondary standards calibrated in these laboratories by posting their instruments or by asking the laboratories to send their primary standards for temporary use.

68

APPENDIX II

USSR S T A T E STANDARD

Monitoring instruments for the measurement of exposures and exposure rates of gamma radiation with photon energy ranging from 30 to 250 fj ( 0 . 2 - 1 . 5 MeV).

Checking methods.*

T h e p r e s e n t s t a n d a r d c o v e r s s e c o n d a r y - s t a n d a r d and w o r k i n g m o n i t o r i n g i n s t r u m e n t s u s e d f o r t h e m e a s u r e m e n t of e x p o s u r e s of g a m m a r a d i a t i o n w i t h p h o t o n e n e r g y r a n g i n g f r o m 30 to 250 f j (2 X 10"1 t o 1.5 MeV) a n d e x p o s u r e r a t e s b e t w e e n 3X 10"12 and 3X 10 "3 a m p / k g ( I X 10" 8 t o 10 R / s e c ) , and l a y s down w a y s a n d m e a n s by w h i c h t h e s e i n s t r u m e n t s c a n b e c h e c k e d .

1. PREPARATION OF MONITORING INSTRUMENTS FOR CHECKING AND CHECKING MEANS

1 .1 . I n s t r u m e n t s due to be c h e c k e d shou ld h a v e a n a c c o m p a n y i n g log ( t e c h n i c a l d a t a s h e e t ) of the m a n u f a c t u r e r , o r e l s e a c e r t i f i c a t e .

1 .2 . When r e c e i v e d f o r c h e c k i n g the i n s t r u m e n t s a r e i n s p e c t e d e x t e r n a l l y to e n s u r e t h a t t h e i r m a r k i n g s a r e c o r r e c t and t h a t t h e y a r e c o m p l e t e , in a c c o r d a n c e wi th t he l o g , t o t he e x t e n t r e q u i r e d f o r t h e p u r p o s e .

1 .3 . T h e i n s t r u m e n t s s h o u l d h a v e s c a l e s c a l i b r a t e d in u n i t s of e x p o s u r e o r e x p o s u r e r a t e , o r in u n i t s t h a t a r e m u l t i p l e s o r f r a c t i o n s of s u c h . It i s a l s o p e r m i s s i b l e t o h a v e a n a r b i t r a r y s c a l e , in w h i c h c a s e t he i n s t r u m e n t s h o u l d be a c c o m p a n i e d by a c a l i b r a t i o n c h a r t o r by a n i n d i c a -t i o n of t he v a l u e s of t he i n s t r u m e n t ' s c o n s t a n t K.

T h e i n s t r u m e n t s shou ld no t s h o w a n y k ind of d e f e c t s u c h a s r u s t , d i f f e r e n t t y p e s of c o n t a m i n a t i o n , o r d a m a g e to the g l a s s and s c a l e . It s h o u l d be m a d e c e r t a i n t h a t t h e k n o b s and s w i t c h e s f ix p r o p e r l y in p o s i t i o n , t h a t t h e z e r o a d j u s t e r and s e n s i t i v i t y c o r r e c t o r c a n be s m o o t h l y s e t , t ha t t h e m o v e m e n t of t he p o i n t e r o r v i e w e r i s s m o o t h and f r e e f r o m j e r k i n e s s , t h a t t h e r e i s a r a t e d p o w e r s u p p l y , and if c h a r g i n g d e v i c e s a c c o m p a n y the d o s i m e t e r s , t h a t t h e y f u n c t i o n p r o p e r l y .

1 .4 . P r e p a r a t i o n of t h e i n s t r u m e n t s f o r c h e c k i n g s h o u l d c o n f o r m wi th t he i n s t r u c t i o n m a n u a l ( s p e c i f i c a t i o n ) f o r t h o s e i n s t r u m e n t s .

1 .5 . F o r t h e c h e c k i n g of t h e i n s t r u m e n t s a w o r k i n g a r e a s h o u l d be s e t u p e q u i p p e d w i t h t h e i n s t a l l a t i o n s , m e a s u r e s and i n s t r u m e n t s l i s t e d b e l o w .

1 .6 . If t h e i n s t r u m e n t s a r e c h e c k e d a g a i n s t a s t a n d a r d s o u r c e , t h e w o r k i n g a r e a s h o u l d be f i t t e d out wi th :

(a) P r i m a r y and s e c o n d a r y s t a n d a r d s o u r c e s of g a m m a r a d i a t i o n (depend ing on the i n s t r u m e n t s b e i n g c h e c k e d ) p r o v i d i n g e x p o s u r e r a t e s

* GOST 12518-67 Committee for Standards, Measures and Measuring Instruments of the USSR Council of Ministers, 1 July 1967.

69

r a n g i n g f r o m 2 . 6 X 10"1 2 t o 2 . 6X 1CT6 a m p / k g ( I X 10" 8 t o I X 10~2 R / s e c ) a t a d i s t a n c e of 1 m o v e r t he pho ton e n e r g y r a n g e 30 - 250 f j (0.2 t o 1.5 MeV);

(b) A c a l i b r a t i o n j i g p e r m i t t i n g t he i n s t r u m e n t s to be c h e c k e d in a c o l l i m a t e d g a m m a r a d i a t i o n b e a m .

1 .7 . T h e c a l i b r a t i o n j ig shou ld m e e t t he f o l l o w i n g r e q u i r e m e n t s :

(a) It s h o u l d h a v e a s t a n d a r d c o l l i m a t o r of t he fo l l owing d i m e n s i o n s : l e n g t h of c y l i n d r i c a l c o l l i m a t o r c h a n n e l 195 - 210 m m , d i a m e t e r of c y l i n d r i c a l c h a n n e l 30 , 60 and 90 m m , d i s t a n c e b e t w e e n c e n t r e of g a m m a s o u r c e and e d g e of c o l l i m a t o r (at the po in t w h e r e t he b e a m e m e r g e s ) 150 ± 1 m m ; it s h o u l d h a v e a s t a n d and a g r a d u a t e d r u l e wi th d i v i s i o n s of no t m o r e t h a n 1 m m ; it s hou ld h a v e a s y s t e m f o r v i e w i n g the s c a l e f r o m a d i s t a n c e , and a n o p t i c a l s y s t e m f o r a d j u s t i n g t he j i g and i n s t r u -m e n t s b e i n g c a l i b r a t e d in t h e g a m m a r a d i a t i o n b e a m ;

(b) T h e c e n t r a l r a y of t he b e a m shou ld be p a r a l l e l t o t he a x i s of the c a l i b r a t i o n s t a n d , t he d e v i a t i o n f r o m p a r a l l e l not e x c e e d i n g 5 m m / m ;

(c) It s h o u l d be p o s s i b l e to m o v e the d e t e c t o r s a l o n g the s t a n d and lock t h e m in p o s i t i o n , w i t h i n t he r a d i a t i o n f i e l d , w i t h a n e r r o r not e x c e e d i n g ± 0 . 2 % of t he d i s t a n c e b e t w e e n r a d i a t i o n s o u r c e and d e t e c t o r ;

(d) T h e u n i f o r m i t y of the g a m m a r a d i a t i o n f i e l d wi th r e s p e c t to t he e x p o s u r e r a t e s h o u l d be s u c h tha t t he l a t t e r d o e s not v a r y by m o r e t h a n 5% a t the po in t w h e r e t he d e t e c t o r s a r e p o s i t i o n e d in the p l a n e of the b e a m s e c t i o n .

N O T E : T h e U P G D - 1 M s e t c a n be u s e d a s a m o d e l c a l i b r a t i o n j i g . F i g u r e 20 s h o w s the d e s i g n of t he c o l l i m a t o r in t h i s s e t .

SOURCE POSITION

SOURCE HOLDER POSITION

FIG.20. Design of collimator.

70

1 .8 . When c h e c k i n g i n s t r u m e n t s a g a i n s t a s t a n d a r d i n s t r u m e n t t he w o r k i n g a r e a s h o u l d be f i t t e d out w i th t he fo l l owing :

(a) A c a l i b r a t i o n j i g wi th a d i r e c t e d b e a m of g a m m a r a d i a t i o n p r o d u c i n g a n e x p o s u r e r a t e r a n g i n g f r o m 2.6 X 10"1 1 t o 2.6 X 10~3 a m p / k g ( I X 10~7 t o 10 R / s e c ) and wi th a s y s t e m f o r v i e w i n g the s c a l e f r o m a d i s t a n c e ; it i s p e r m i s s i b l e to u s e c a l i b r a t i o n j i g s e q u i p p e d wi th a s e t of s o u r c e s a s w e l l a s j i g s p r o d u c i n g d i f f u s e g a m m a r a d i a t i o n f i e l d s ;

(b) A p r i m a r y o r s e c o n d a r y s t a n d a r d m o n i t o r i n g i n s t r u m e n t p e r -m i t t i n g m e a s u r e m e n t of t he p h y s i c a l v a l u e s i n d i c a t e d in 1 .8 (a ) .

N O T E : 1. M o d e l s D I M - 6 0 and R P - 1 c a n b e u s e d a s s t a n d a r d d o s i m e t e r s .

2. It i s p e r m i t t e d t o u s e one s t a n d a r d i n s t r u m e n t f o r s e v e r a l c a l i b r a t i o n j i g s .

1 .9 . T h e c a l i b r a t i o n j i g w i t h d i r e c t e d g a m m a r a d i a t i o n b e a m s h o u l d m e e t the r e q u i r e m e n t s s e t f o r t h in 1 .6 , e x c e p t f o r t he p r e s e n c e of t he m o d e l c o l l i m a t o r ; c o l l i m a t i n g c o n d i t i o n s a r e no t s t i p u l a t e d .

1 .9 .1 . A c a l i b r a t i o n j i g wi th s o u r c e s c r e a t i n g a d i f f u s e f i e l d s h o u l d s a t i s f y t he f o l l o w i n g r e q u i r e m e n t s :

(a) The u n i f o r m i t y of t he g a m m a r a d i a t i o n f i e l d wi th r e s p e c t t o e x p o s u r e r a t e s h o u l d be s u c h tha t t he l a t t e r d o e s no t v a r y by m o r e t h a n 5% o v e r t he who le of the d e t e c t o r s of t he i n s t r u m e n t b e i n g c h e c k e d a n d of the s t a n d a r d i n s t r u m e n t ;

(b) It s h o u l d p r o v i d e f o r p r o p e r p o s i t i o n i n g of t he d e t e c t o r in t he g a m m a r a d i a t i o n f i e l d .

1 .10 . In a d d i t i o n to t h e m e a s u r e s , i n s t r u m e n t s and i n s t a l l a t i o n s l i s t e d in 1.6 and 1 .8 , t h e w o r k i n g a r e a s h o u l d a l s o be e q u i p p e d wi th the f o l l o w i n g m e a s u r i n g d e v i c e s :

(a) A b a r o m e t e r c o m p l y i n g w i t h G O S T 4 8 6 3 - 5 5 ; (b) A t h e r m o m e t e r wi th g r a d u a t i o n s of 1°C c o m p l y i n g wi th

GOST 2 0 4 5 - 4 3 ; (c) A t i m i n g d e v i c e c o m p l y i n g w i t h GOST 5 0 7 2 - 6 2 .

1 .11 . A l l m e a s u r e s and i n s t r u m e n t s ( i n s t a l l a t i o n s ) u s e d t o c h e c k m o n i t o r i n g i n s t r u m e n t s s h o u l d h a v e a c e r t i f i c a t e s h o w i n g t h a t t h e y h a v e b e e n c h e c k e d .

C e r t i f i c a t i o n of p r i m a r y s t a n d a r d m e a s u r e s and i n s t r u m e n t s and of c a l i b r a t i o n j i g s s h o u l d be c a r r i e d out by t he C o m m i t t e e f o r S t a n d a r d s , M e a s u r e s and M e a s u r i n g I n s t r u m e n t s of the USSR C o u n c i l of M i n i s t e r s .

1 .12 . T h e p r e p a r a t i o n of t he c a l i b r a t i o n j i g a s d e f i n e d by the r e q u i r e -m e n t s s e t f o r t h in 1.6 i s c a r r i e d out in a c c o r d a n c e w i t h t he s p e c i f i c a t i o n f o r t h a t j i g .

1 .12 .1 . T h e g a m m a r a d i a t i o n b e a m i s c l o s e d off by a d i a p h r a g m in s u c h a w a y t h a t the d i a m e t e r of t he w o r k i n g b e a m a t the s i t e of t he i n s t r u m e n t d e t e c t o r i s a t l e a s t t w i c e t he l i n e a r d i m e n s i o n of the l a t t e r .

N O T E : It i s p e r m i s s i b l e t o u s e s m a l l e r f i e l d s p r o v i d e d the i n v e r s e s q u a r e law i s f u l f i l l e d f o r t h e g iven type of i n s t r u m e n t .

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1 .12 .2 . A s t a n d a r d s o u r c e i s s e l e c t e d wi th a n e x p o s u r e r a t e c o r r e s p o n d i n g t o t h e e x p o s u r e r a t e r a n g e of the i n s t r u m e n t t o b e c h e c k e d , and t h e r a d i a -t i on e x p o s u r e r a t e s f r o m the s o u r c e a r e c a l c u l a t e d f o r v a r i o u s p o i n t s a l o n g t h e s t a n d c o r r e s p o n d i n g to t h r e e p o i n t s in e a c h of t he r a n g e s c h e c k e d ( s e e 2 .4 ) .

C a l c u l a t i o n of the p o i n t s c h e c k e d i s m a d e by the e q u a t i o n :

P = P x o o ( ^ ) ! . e - ^ . K d ( 1 )

w h e r e P1 (

M R

K ,

i s t he e x p o s u r e r a t e p r o d u c e d by the s t a n d a r d g a m m a s o u r c e a t a d i s t a n c e of 1 m (as i n d i c a t e d in the s o u r c e c e r t i f i c a t e ) ; i s t he l i n e a r a t t e n u a t i o n f a c t o r f o r t he r a d i a t i o n in a i r ; i s t he d i s t a n c e in c m b e t w e e n the c e n t r e of t he s o u r c e and the po in t a t w h i c h the e x p o s u r e r a t e i s d e t e r m i n e d ; i s a c o e f f i c i e n t t h a t t a k e s i n t o a c c o u n t d e c a y of t he i s o t o p e o v e r t he t i m e e l a p s i n g b e t w e e n the d a t e w h e n t h e s o u r c e w a s c a l i b r a t e d and the da t e w h e n it w a s u s e d f o r c a l i b r a t i o n p u r p o s e s ;

K d = - 0 . 6 9 3 -

T

w h e r e e i s t h e n a t u r a l l o g a r i t h m b a s e ; t i s t he t i m e e l a p s i n g b e t w e e n the d a t e of c e r t i f i c a t i o n of

the s o u r c e and the da t e of i t s u s e f o r c h e c k i n g p u r p o s e s ; T i s t h e h a l f - l i f e of the i s o t o p e .

( 2 )

T h e v a l u e s of t and T m u s t be e x p r e s s e d in t he s a m e u n i t s of t i m e , f o r e x a m p l e , d a y s o r m o n t h s .

T h e c o e f f i c i e n t K a i s d e t e r m i n e d f r o m the c o r r e s p o n d i n g t a b l e s f o r the r e l a t i o n

K d = e" X 0 .693 ^

v a l u e s f o r /i a r e g i v e n in A n n e x 2, and f o r e~* f o r x f r o m 0 . 0 0 0 0 1 to 1 .48 .

When c h e c k i n g s e c o n d a r y - s t a n d a r d m o n i t o r i n g i n s t r u m e n t s u s e s h o u l d b e m a d e of p r i m a r y - s t a n d a r d i n s t r u m e n t s o r g a m m a r a d i a t i o n s o u r c e s , and w h e n c h e c k i n g w o r k i n g i n s t r u m e n t s s e c o n d a r y - s t a n d a r d i n s t r u m e n t s o r s o u r c e s s h o u l d b e e m p l o y e d .

1 .12 .3 . S u p e r f l u o u s o b j e c t s a r e r e m o v e d f r o m the g a m m a r a d i a t i o n b e a m .

1 .13 . P r e p a r a t i o n of the c a l i b r a t i o n j ig and s t a n d a r d i n s t r u m e n t a s d e f i n e d by the r e q u i r e m e n t s in 1.8 i s c a r r i e d out in a c c o r d a n c e w i t h t he s p e c i f i c a t i o n s f o r t he c a l i b r a t i o n j ig and s t a n d a r d i n s t r u m e n t .

2. CHECKING PROCEDURE

2 .1 . M o n i t o r i n g i n s t r u m e n t s shou ld be c h e c k e d by one of t he two f o l l o w i n g m e t h o d s - t he s t a n d a r d s o u r c e m e t h o d o r s t a n d a r d - i n s t r u m e n t m e t h o d .

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2 . 1 . 1 . U n d e r t he s t a n d a r d s o u r c e m e t h o d the d e t e c t o r of t he i n s t r u m e n t t o b e c h e c k e d i s p l a c e d in t he c o l l i m a t e d g a m m a , r a d i a t i o n b e a m in s u c h a w a y t h a t t he c e n t r a l r a y of t he b e a m p a s s e s t h r o u g h the c e n t r e of t he d e t e c t o r . T h e e x p o s u r e r a t e p r o d u c e d by the s t a n d a r d g a m m a s o u r c e a t t he po in t w h e r e t h e c e n t r e of the d e t e c t o r i s p l a c e d i s t h e n c o m p a r e d w i t h t h e r e a d i n g of t he i n s t r u m e n t .

2 . 1 . 2 . U n d e r t he s t a n d a r d - i n s t r u m e n t m e t h o d the r e a d i n g s of t he i n s t r u m e n t b e i n g c h e c k e d and the s t a n d a r d i n s t r u m e n t in t he g a m m a r a d i a t i o n f i e l d a r e c o m p a r e d .

2 .2 . T h e c h e c k i n g of m o n i t o r i n g i n s t r u m e n t s i s c a r r i e d out u n d e r the f o l l o w i n g c o n d i t i o n s :

(a) T e m p e r a t u r e of the e n v i r o n m e n t , 2 0 ± 5 ° C (b) R e l a t i v e h u m i d i t y of the a i r not m o r e t h a n 80%

It i s p e r m i t t e d t o m a k e the c h e c k a t o t h e r t e m p e r a t u r e s , in w h i c h c a s e t he r e s u l t s s h o u l d be r e f e r r e d t o n o r m a l c o n d i t i o n s : t = 20°C and N = 1 .01X 1 0 s N / m 2 (760 m m H g ) .

2 .3 . T h e f o l l o w i n g s h o u l d be e s t a b l i s h e d d u r i n g t h e c h e c k i n g p r o c e d u r e :

(a) C o r r e s p o n d e n c e b e t w e e n the r e a d i n g s of the i n s t r u m e n t b e i n g c h e c k e d and the t r u e v a l u e s of t he m e a s u r e d q u a n t i t y , and t h e e r r o r in m e a s u r e m e n t ;

(b) T h e i n s t r u m e n t c o n s t a n t K, shou ld an a r b i t r a r y s c a l e be u s e d in t he i n s t r u m e n t .

2 .4 . T h e m e a s u r e m e n t s a r e m a d e a t no t l e s s t h a n t h r e e p o i n t s in e a c h s u b - r a n g e ; it i s a d v i s a b l e t o u s e f r a c t i o n s of 0 . 3 , 0 .6 a n d 0 .9 of t he n o m i n a l v a l u e of the s c a l e of the i n s t r u m e n t to be c h e c k e d a s t he p o i n t s in q u e s t i o n . E a c h m e a s u r e m e n t i s m a d e a t l e a s t t h r e e t i m e s .

2 .5 . M o n i t o r i n g i n s t r u m e n t s a r e c h e c k e d a g a i n s t a s t a n d a r d s o u r c e in the f o l l o w i n g s e q u e n c e :

(a) T h e t e m p e r a t u r e , h u m i d i t y and p r e s s u r e of t he a i r a r e m e a s u r e d ; (b) The i n s t r u m e n t r e a d i n g s f r o m the s e n s i t i v i t y c h e c k a r e v e r i f i e d ; (c) The d e t e c t o r of t he i n s t r u m e n t b e i n g c h e c k e d i s p l a c e d a t a s u i t a b l e

d i s t a n c e f r o m t h e s o u r c e f o r t h e c h e c k to be m a d e in a c c o r d a n c e wi th p a r a g r a p h s 1 .12 .1 and 1 .12 .2 ;

(d) T h e e x p o s u r e r a t e s a r e m e a s u r e d wi th t he i n s t r u m e n t a t t he s e l e c t e d d i s t a n c e s in a c c o r d a n c e w i t h 2.4; t he i n s t r u m e n t r e a d i n g s shou ld c o i n c i d e , w i t h i n t he l i m i t s of the i n s t r u m e n t e r r o r , w i th e x p o s u r e r a t e s f r o m the s t a n d a r d s o u r c e a s c a l c u l a t e d f r o m e x p r e s s i o n (1);

(e) The r e a d i n g s f r o m the i n s t r u m e n t ' s s e n s i t i v i t y a r e v e r i f i e d a g a i n .

2 .6 . M o n i t o r i n g i n s t r u m e n t s a r e c h e c k e d by the s u b s t i t u t i o n m e t h o d in the fo l l owing s e q u e n c e :

(a) The t e m p e r a t u r e , h u m i d i t y and p r e s s u r e of t h e a i r a r e m e a s u r e d ;

73

(b) T h e s e n s i t i v i t y of the s t a n d a r d i n s t r u m e n t and i n s t r u m e n t b e i n g c a l i b r a t e d i s c h e c k e d f r o m the r e a d i n g f r o m a c o n t r o l s o u r c e o r s o m e o t h e r t ype of c o n t r o l , t h e s e n s i t i v i t y t h e n b e i n g c h e c k e d p e r i o d i c a l l y t h r o u g h o u t t he w h o l e of t he c a l i b r a t i o n c y c l e ;

(c) T h e e x p o s u r e r a t e ( e x p o s u r e ) i s m e a s u r e d in t u r n by the s t a n d a r d i n s t r u m e n t and i n s t r u m e n t b e i n g c h e c k e d a t t he s a m e p o i n t , the m e a s u r e -m e n t s b e i n g m a d e in a c c o r d a n c e wi th 2.4; t he r e a d i n g s of t he two i n -s t r u m e n t s s h o u l d c o i n c i d e w i t h i n t he l i m i t s of m e a s u r e m e n t a c c u r a c y .

2 .7 . In t he c a s e of i n s t r u m e n t s wi th a s c a l e g r a d u a t e d in r e l a t i v e u n i t s , the i n s t r u m e n t ' s c o n s t a n t K i s d e t e r m i n e d f r o m the fo l l owing e q u a t i o n :

(3)

w h e r e P 0 i s t he t r u e v a l u e of the e x p o s u r e r a t e ; N i s t he a r i t h m e t i c m e a n of t he i n s t r u m e n t r e a d i n g s e x p r e s s e d

in t e r m s of t he s c a l e d i v i s i o n s ; t he m e a s u r e m e n t i s m a d e a t l e a s t t h r e e t i m e s .

T h e c o n s t a n t K i s c a l c u l a t e d in s i m i l a r f a s h i o n when the e x p o s u r e i s m e a s u r e d .

2 .8 . T h e i n s t r u m e n t ' s p r i n c i p a l e r r o r 6 in % i s d e t e r m i n e d by t h e e q u a t i o n :

6 = V 6 0 + 6 r W

w h e r e 6 0 i s t he e r r o r of the s t a n d a r d i n s t r u m e n t o r s t a n d a r d s o u r c e in % (as t a k e n f r o m the c e r t i f i c a t e f o r the i n s t r u m e n t o r s o u r c e ) ;

6 r i s t he r a n d o m e r r o r of the i n s t r u m e n t b e i n g c h e c k e d in %.

I E (6 Pj ) 2

6 r =V ^ T i — ' 1 0 0 ( 5 )

w h e r e 6 P , = (Pc - PQ ; 0

n i s t he n u m b e r of m e a s u r e m e n t s ; P c and P0 a r e r e a d i n g s in t e r m s of t he e x p o s u r e r a t e of t he

i n s t r u m e n t b e i n g c h e c k e d and the s t a n d a r d i n s t r u m e n t (if a s t a n d a r d s o u r c e is u s e d P0 i s t he c a l c u l a t e d v a l u e of the e x p o s u r e r a t e ) .

T h e i n s t r u m e n t e r r o r shou ld not e x c e e d ± 7% f o r s e c o n d a r y s t a n d a r d i n s t r u m e n t s , and in t he c a s e of w o r k i n g i n s t r u m e n t s s h o u l d not e x c e e d the v a l u e s i n d i c a t e d in t h e i r s p e c i f i c a t i o n s .

3. RECORDING THE RESULTS OF THE CHECKING

3 .1 . T h e r e s u l t s of t he m e a s u r e m e n t s a r e e n t e r e d in a r e p o r t book o r r e c o r d w i t h an i n d i c a t i o n of t he fo l lowing :

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(a) The n a m e and type of i n s t r u m e n t c h e c k e d and the m a n u f a c t u r e r (b) T h e n a m e and type of s t a n d a r d i n s t r u m e n t ( s o u r c e ) (c) The n a m e of t he o r g a n i z a t i o n s u b m i t t i n g t he i n s t r u m e n t (d) T h e a i r p r e s s u r e and t e m p e r a t u r e d u r i n g the m e a s u r e m e n t (e) T h e c o n d i t i o n s u n d e r w h i c h the m e a s u r e m e n t s w e r e m a d e

( c o l l i m a t o r , d i s t a n c e b e t w e e n s o u r c e and i n s t r u m e n t d e t e c t o r , e t c . ) (f) T h e i n s t r u m e n t r e a d i n g s f r o m the s e n s i t i v i t y c h e c k (g) T h e r e s u l t s of the m e a s u r e m e n t s (h) T h e d a t a o b t a i n e d f r o m p r o c e s s i n g the r e s u l t s (i) T h e d a t e of t he m e a s u r e m e n t s

3 .2 . M o n i t o r i n g i n s t r u m e n t s m e e t i n g t h e r e q u i r e m e n t s of t he p r e s e n t s t a n d a r d a r e i s s u e d w i t h a c e r t i f i c a t e to show t h e y h a v e b e e n c h e c k e d , the r e c o m m e n d e d f o r m a t i s s h o w n in Annex 1.

3 .3 . M o n i t o r i n g i n s t r u m e n t s t h a t do not m e e t t he r e q u i r e m e n t s s e t f o r the s t a n d a r d a r e k e p t f r o m c i r c u l a t i o n .

ANNEX 1

CERTIFICATE

T y p e of d o s i m e t e r r e g i s t r a t i o n n u m b e r

m a n u f a c t u r e d b y . • — — ( n a m e of m a n u f a c t u r e r )

a n d s u b m i t t e d by — ( n a m e of o r g a n i z a t i o n s u b m i t t i n g i n s t r u m e n t f o r c h e c k i n g )

h a s b e e n c h e c k e d wi th c a l i b r a t i o n j ig — (type and s e r i a l n u m b e r )

u s i n g — ( c h e c k i n g m e t h o d , t y p e of s t a n d a r d i n s t r u m e n t and

i t s n u m b e r of s t a n d a r d s o u r c e )

1 . R e s u l t s of c h e c k i n g

I n s t r u m e n t c o n s t a n t K =

I n s t r u m e n t e r r o r 6 =

2. T h e r e a d i n g s f r o m the s e n s i t i v i t y c h e c k d u r i n g the c a l i b r a t i o n

(N = m m H g ; t = °C) c o n s t i t u t e d —

d i v i s i o n s on t h e s c a l e —

3 . P e r i o d of v a l i d i t y of c e r t i f i c a t e

H e a d of l a b o r a t o r y

C h e c k i n g c a r r i e d out by

(Date) 19

ANNEX 2

LINEAR ATTENUATION FACTOR FOR GAMMA RADIATION IN AIR AT 20°C AND 760 mmHg AND HALF"LIFE

F o r c o b a l t - 6 0 r a d i a t i o n n = 6 .8 X 10"5 c m " i ; T = 5 .265 y r . F o r c a e s i u m - 1 3 7 r a d i a t i o n u = 9.3 X 10"5 c m " i ; T = 26.6 ,yr .

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APPENDIX III

A T Y P I C A L L A B O R A T O R Y BUILDING AND S T A F F

The building space, methodology and staff necessary to properly operate a calibration laboratory depend on the volume and type of work undertaken. Calibration facil i t ies exist that employ from a fraction of one employee ' s t ime up to tens of people. Appendix III gives data on a number of calibration laboratories that are operating in different parts of the world. These are included as examples of the range of calibration work undertaken and the associated equipment and staff required to deal with the varying volume of calibration work.

As a guide to people setting up a calibration laboratory this section gives a more detailed descrip-tion of a fictional calibration laboratory. It is assumed that the laboratory is moderately sized and calibrates a thousand personnel dosimeters and several hundred portable instruments per month, and util izes most of the assemblies discussed in Sect ion 3.

1. BUILDING S P A C E

F a c i l i t i e s r e q u i r e d f o r t h e c a l i b r a t i o n l a b o r a t o r y c o n s i s t of t he l and on w h i c h t h e l a b o r a t o r y b u i l d i n g i s p l a c e d and the b u i l d i n g i t s e l f . T h e l and and b u i l d i n g a s p e c t s of t he l a b o r a t o r y n e e d to be c l o s e l y i n -t e g r a t e d . T h e r e i s a n e e d to h a v e a low s c a t t e r i r r a d i a t i o n b u i l d i n g s o t h a t c a l i b r a t i o n w o r k m a y b e c o m p l e t e d wi th the r e q u i r e d a c c u r a c y and t h e r e is a l s o a n e e d f o r low r a d i a t i o n l e v e l a t e x t e r n a l b u i l d i n g w a l l s . T h e s e c o n d i t i o n s c a n be m e t by a r e l a t i v e l y l a r g e b u i l d i n g s o t h a t t he i n v e r s e s q u a r e l aw and m o d e r a t e s h i e l d i n g in t h e o u t e r w a l l s w i l l r e d u c e t he d o s e - r a t e f r o m e x p o s e d r a d i a t i o n s o u r c e s and r a d i a t i o n g e n e r a t i n g m a c h i n e s to a c c e p t a b l e l e v e l s a t t h e e x t e r i o r of t he b u i l d i n g w a l l s . T h e c o n s t r u c t i o n of a m o d e r a t e - s i z e b u i l d i n g on a l a r g e l o t , wh ich c a n h a v e r i g i d l y c o n t r o l l e d a c c e s s s o t h a t t he e x t e r n a l e n v i r o n m e n t a t t he b u i l d i n g w a l l n e e d not b e r e d u c e d to n o n - c o n t r o l l e v e l s , i s p r e f e r r e d on a c o s t b a s i s . In a r e a s of h igh l and c o s t , h o w e v e r , t h i s a l t e r n a t i v e m a y not be p r e f e r r e d and a m o d e r a t e l y l a r g e b u i l d i n g wi th a d e q u a t e e x t e r n a l w a l l s h i e l d i n g m a y be r e q u i r e d . In a n y c a s e , the v a r i a t i o n in t he s c a t t e r a t the p o s i t i o n s of i n s t r u m e n t c a l i b r a t i o n s s h o u l d not e x c e e d the v a r i a t i o n s due t o i n s t r u m e n t p o s i t i o n i n g . o n the t e s t i r r a d i a t i o n a s s e m b l i e s .

F o r c e r t a i n t y p e s of w o r k s h i e l d i n g i s v e r y n e c e s s a r y f o r r e a s o n s o t h e r t h a n p r o t e c t i o n . T o c a l i b r a t e i n s t r u m e n t s t h a t m e a s u r e b a c k g r o u n d l e v e l s of r a d i a t i o n it i s n e c e s s a r y t o p r o v i d e a v e r y low b a c k g r o u n d c a l i b r a t i o n c e l l . W h e r e t he n o r m a l b a c k g r o u n d l e v e l s a r e h igh s h i e l d i n g s h o u l d be u s e d to o b t a i n a low b a c k g r o u n d wi th in t he c e l l . W h e r e t h e r e a r e o t h e r e x t e r n a l r a d i a t i o n s o u r c e s n e a r b y , f o r e x a m p l e a c c e l e r a t o r s o r r e a c t o r s , t h e s h i e l d i n g w i l l a l s o s e r v e to k e e p the b a c k g r o u n d l e v e l s w i t h i n the c a l i b r a t i o n c e l l c o n s t a n t .

C o n t r o l of t he s c a t t e r v a r i a t i o n s c a n be a c c o m p l i s h e d by two m e t h o d s if i t i s not f e a s i b l e t o e l i m i n a t e t h e m by t h e f a c i l i t y d e s i g n . F i r s t , if t he t o t a l r a d i a t i o n d o s e - r a t e i n c l u d i n g s c a t t e r e d r a d i a t i o n c o n t r i b u t i o n s i s m e a s u r e d a t e a c h d o s i m e t e r o r i n s t r u m e n t c a l i b r a t i o n l o c a t i o n , t h e n the a c t u a l d o s e - r a t e to e a c h i n s t r u m e n t i s known . S e c o n d , a r o t a t i n g e x p o s u r e a s s e m b l y c a n be d e s i g n e d s o tha t e a c h i n s t r u m e n t s p e n d s t he s a m e p o r t i o n of i t s t o t a l c a l i b r a t i o n t i m e a t e a c h p o s i t i o n a b o u t t h e s o u r c e . In s p i t e of t h e s e m e t h o d s t o m i n i m i z e t he e f f e c t s of t he s c a t t e r e d r a d i a t i o n s , e r r o r s m a y ye t a r i s e b e c a u s e of t he d i f f e r e n c e in e n e r g y

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b e t w e e n the p r i m a r y r a d i a t i o n and the s c a t t e r e d r a d i a t i o n . T h e l a r g e s c a t t e r - f r e e i r r a d i a t i o n r o o m i s p r e f e r r e d f o r c o n d u c t i n g an a c c u r a t e i r r a d i a t i o n p r o g r a m .

T h e c a l i b r a t i o n l a b o r a t o r y w i l l r e q u i r e a b u i l d i n g d e s i g n e d f o r r a d i a t i o n s o u r c e h a n d l i n g and s t o r a g e and f o r p r o v i s i o n of low s c a t t e r i r r a d i a t i o n r o o m s in wh ich i n s t r u m e n t s w i l l be e x p o s e d . In add i t i on an o f f i c e a r e a i s r e q u i r e d f o r the s t a f f . S p e c i a l f e a t u r e s of t he bu i ld ing shou ld i n c l u d e s h i e l d e d f i l m and o t h e r p e r s o n n e l d o s i m e t e r d e t e c t o r s t o r a g e a r e a of c o n s t a n t low b a c k g r o u n d r a d i a t i o n and c o n t r o l l e d t e m p e r a -t u r e and h u m i d i t y s o tha t d o s i m e t e r s c a n b e s t o r e d b e f o r e and a f t e r e x p o s u r e . In the d e s i g n of the l a b o r a t o r y bu i l d ing , f l e x i b i l i t y in the u s e of a l l of i t s f a c i l i t i e s shou ld be p r o v i d e d . T h e r e q u i r e d l a b o r a t o r y f a c i l i t i e s a r e :

(i) I r r a d i a t i o n r o o m s (ii) R a d i a t i o n s o u r c e s t o r a g e v a u l t

(iii) P e r s o n n e l d o s i m e t e r d e t e c t o r s t o r a g e vau l t (iv) A d m i n i s t r a t i o n o f f i c e s

1 .1 . I r r a d i a t i o n r o o m s

F o r o p t i m u m u t i l i t y , two i r r a d i a t i o n r o o m s a r e n e c e s s a r y f o r s i m u l t a n e o u s e x p o s u r e s . Bo th r o o m s shou ld be a t l e a s t 10 m on a s i d e wi th t h e h e i g h t of a b o u t 3 m . One of the r o o m s would be u s e d f o r X and g a m m a i r r a d i a t i o n s and the o t h e r f o r be t a and n e u t r o n i r r a d i a t i o n s . T h e two r o o m s would be s e p a r a t e d by a d e c o n t a m i n a t i o n r o o m tha t would be u s e d to e n s u r e t h a t i n s t r u m e n t s and d o s i m e t e r s a r e i n i t i a l l y f r e e of c o n t a m i n a t i o n . The c o n c r e t e w a l l s of t he d e c o n t a m i n a t i o n r o o m shou ld be 30 c m t h i c k . T h e wid th of t he r o o m and the c o n c r e t e s h i e l d i n g p r o v i d e d i s t a n c e and s h i e l d i n g f o r p e r s o n n e l and c a l i b r a t i o n e x p o s u r e s b e i n g c o n d u c t e d in t he t w o i r r a d i a t i o n r o o m s . Bo th i r r a d i a t i o n r o o m s shou ld be p r o v i d e d wi th a s u i t a b l e i n t e r l o c k to p r e v e n t i n a d v e r t e n t e n t r y . T h e c e l l s and d e c o n t a m i n a t i o n r o o m shou l d be v e n t i l a t e d , and the f l o o r , w a l l , c e i l i n g and o t h e r w o r k i n g s u r f a c e s shou ld h a v e s u r f a c e s c h o s e n f o r e a s y d e c o n t a m i n a t i o n .

1 .2 . R a d i a t i o n s o u r c e s t o r a g e v a u l t

When not in a c t u a l u s e o r h o u s e d in w e l l - s h i e l d e d s t o r a g e c a s k s d e s i g n e d a s p a r t of a r a d i a t i o n e x p o s u r e a s s e m b l y , a l l r a d i a t i o n s o u r c e s shou ld be s t o r e d in a f i r e p r o o f , v e n t i l a t e d s h i e l d e d s t o r a g e v a u l t . T h e s t o r a g e v a u l t shou ld p r o v i d e a l a b y r i n t h e n t r a n c e and s h i e l d i n g e q u i -v a l e n t to a b o u t 60 c m of c o n c r e t e on a l l s i d e s and the c e i l i n g . A s u i t a b l e v a u l t e n t r y i n t e r l o c k shou ld b e p r o v i d e d . I n t e r n a l d i m e n s i o n s of abou t 2.5 m 3 a r e d e s i r a b l e . C o n t i n u o u s a i r s a m p l e s shou ld be d r a w n f r o m the v a u l t . T h e a c t i v i t y of the a i r s a m p l e i s r e q u i r e d to p r o v i d e a c o n s t a n t k n o w l e d g e of t he c o n t a m i n a t i o n s t a t u s of the v a u l t and to p r o v i d e p r o m p t d e t e c t i o n of a n y r a d i a t i o n s o u r c e l e a k a g e . E x p e r i e n c e h a s shown tha t a n a i r - s a m p l e s y s t e m s u c h a s t h i s is e f f e c t i v e in r a d i a t i o n s o u r c e l e a k a g e d e t e c t i o n .

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1.3. D o s i m e t e r d e t e c t o r s t o r a g e v a u l t

A d o s i m e t e r d e t e c t o r s t o r a g e v a u l t w i th a c o n s t a n t low r a d i a t i o n b a c k g r o u n d s h o u l d be p r o v i d e d . T h i s vau l t i s n e c e s s a r y to p r o v i d e a s t o r a g e a r e a f o r a l l d o s i m e t e r s w h i l e not a c t u a l l y b e i n g e x p o s e d a t the l a b o r a t o r y . T h e t e m p e r a t u r e and h u m i d i t y in t he v a u l t s h o u l d be c o n -t r o l l e d to p r o v i d e s t o r a g e a t t h e c o n d i t i o n s r e c o m m e n d e d by the d o s i -m e t e r m a n u f a c t u r i n g c o m p a n i e s . An i n t e r n a l v a u l t v o l u m e of a b o u t 3 m 3 s hou ld be p r o v i d e d . Sh ie ld ing of l e a d , s t e e l o r c o n c r e t e m a y be u s e d . T h e v a u l t s h o u l d be l o c a t e d n e a r the a r e a of p a c k a g e d e l i v e r y t o t he b u i l d i n g . T h e c o n d u c t of the l a b o r a t o r y p r o g r a m s h o u l d not i n c r e a s e t he d o s e - r a t e w i t h i n t he vau l t by m o r e than a f a c t o r of 2 to 5.

1 .4 . A d m i n i s t r a t i o n o f f i c e s

F o u r a d m i n i s t r a t i v e o f f i c e s of a b o u t 15 m 2 e a c h a r e n e c e s s a r y . T h e s e o f f i c e s p r o v i d e s p a c e f o r t he d i r e c t o r , e n g i n e e r , t e c h n i c i a n s and s e c r e t a r y . T h e d o s e - r a t e s in the o f f i c e s shou l d not e x c e e d 0.2 m r e m / h d u r i n g the n o r m a l o p e r a t i o n of the l a b o r a t o r y . T h e r e a r e no o.ther s p e c i a l r e q u i r e m e n t s f o r t he o f f i c e d e s i g n .

2. T Y P I C A L L A B O R A T O R Y M E T H O D O L O G Y

A b r i e f s u m m a r y of t he t y p i c a l l a b o r a t o r y m e t h o d o l o g y i s p r e s e n t e d h e r e to c h a r a c t e r i z e t he l a b o r a t o r y o p e r a t i o n . S ince t h e s e a r e to be c o n s i d e r e d a s t y p i c a l m e t h o d s of o p e r a t i o n , t he l a b o r a t o r y d i r e c t o r s h o u l d be a u t h o r i z e d to r e v i s e t h e s e m e t h o d s a s a p p r o p r i a t e to m e e t t he n e e d s of t he l a b o r a t o r y and i t s c u s t o m e r s .

T h e t y p i c a l l a b o r a t o r y o p e r a t i o n a l m e t h o d s m a y be d i v i d e d in to t he s e v e n m a j o r a r e a s :

(i) R e c e i p t and h a n d l i n g d o s i m e t e r s and i n s t r u m e n t s (ii) C a l i b r a t i o n of t e s t i r r a d i a t i o n s y s t e m s

(ii i) C a l i b r a t i o n of i n s t r u m e n t s and d o s i m e t e r s (iv) R e t u r n of c a l i b r a t e d d o s i m e t e r s and i n s t r u m e n t s (v) R e s u l t s

(vi) K e e p i n g of r e c o r d s of l a b o r a t o r y a c t i v i t i e s (vii) M a i n t e n a n c e and d e v e l o p m e n t of t h e i r own r a d i a t i o n s t a n d a r d s

and t h e i r own c a l i b r a t i o n p r o c e d u r e s .

2 .1 . R e c e i v i n g and h a n d l i n g

W h e n the m o n i t o r i n g i n s t r u m e n t s f o r c a l i b r a t i o n a r e r e c e i v e d by the l a b o r a t o r y , t h e y s h o u l d be p r o m p t l y s u r v e y e d f o r c o n t a m i n a t i o n and the d o s i m e t e r s p l a c e d in the low b a c k g r o u n d , c o n t r o l l e d e n v i r o n m e n t v a u l t u n t i l t h e t e s t i r r a d i a t i o n s a r e m a d e . A R e c e i v i n g and Shipping R e p o r t s h o u l d be p r e p a r e d f o r e a c h s h i p m e n t of i n s t r u m e n t s a r r i v i n g a t t he l a b o r a t o r y . A t y p i c a l R e c e i v i n g and Shipp ing R e p o r t i s s h o w n in F i g . 21. T h e r e c e i v i n g n u m b e r shou ld be i s s u e d and r e c o r d e d on a l l f o r m s g e n e r a t e d in t he c a l i b r a t i o n and s h i p m e n t of t he i n s t r u m e n t s .

R E C E I V I N G AND S H I P P I N G R E P O R T (Shipped with Instruments)

I. Request for calibration from ( f i l l ed in by customer):

Establishment Address Principal Contact Telephone Date Shipped Dosimeter

Number Type

Portable Instruments Number Make _ _ _ _ _ _ _ _ _ _ _ Type Serial Numbers

Instructions

II. Received ( f i l l ed in by Calibration Laboratory):

Date Contamination Status _ Dosimeters

Number Type

Portable Instruments Number Make Type Serial Numbers

III. Shipped ( f i l led in by Calibration Laboratory):

Date Dosimeters

Number Type

Portable Instruments Number Make _ Type Serial Numbers

FIG. 21. Receiving and shipping report.

80

D O S I M E T E R T E S T I R R A D I A T I O N E X P O S U R E

(X and Gamma Radiation)

I. Owner Receiving Date

CALIBRATION SYSTEM:

"R" Meter Used "R" Meter Date: Chamber Number Bureau of Standards T i m e On

Correction Factor T ime Off _ Bureau of Standards Total T ime

Certification Date

Pressure:

CALCULATIONS: Start _ Finish

273+ t f = 2. 576 ( ) X F Ave .

P

f = Total Correction Factor „ Temperature: t = Temperature, °C p = Pressure in mm Hg s t a r t

F = Chamber Correction Factor . , ~ Finish from Bureau of Standards . Ave .

Chamber Reading Total Correction Factor Corrected Exposure

III. TEST IRRADIATION DATA T i m e Exposure

Dose Delivered Dosimeter No. On Off T i m e

Tested by ' Checked by

FIG. 22. Dosimeter test irradiation exposure.

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P a r t I of t he R e c e i v i n g and Shipping R e p o r t i d e n t i f i e s t he c u s t o m e r , g i v e s h i s a d d r e s s and t h e p r i n c i p l e c o n t a c t a t t he c u s t o m e r 1 s f a c i l i t y f o r a n y a d d i t i o n a l i n f o r m a t i o n r e q u i r e d . I m m e d i a t e l y upon r e c e i p t of the i n s t r u m e n t s , t h e p a c k a g e shou ld be opened u n d e r c o n t r o l l e d c o n d i t i o n s and a c o n t a m i n a t i o n s u r v e y p e r f o r m e d to e n s u r e t h a t a l l i n s t r u m e n t s a r e f r e e of r a d i o a c t i v e c o n t a m i n a t i o n . T h e c o n t a m i n a t i o n s u r v e y s h o u l d be c o n d u c t e d w i t h r a d i a t i o n d e t e c t i o n i n s t r u m e n t a t i o n c a p a b l e of d e t e c t i n g t h e p r e s e n c e of e i t h e r b e t a - g a m m a o r a l p h a r a d i a t i o n s in q u a n t i t i e s g r e a t e r t h a n n o r m a l b a c k g r o u n d . W h e r e t he i n t e r n a l c h e c k s o u r c e of a n i n s t r u m e n t r e n d e r s t h i s t e s t i m p r a c t i c a l s w a b s s h o u l d b e t a k e n f o r t e s t i n g f o r c o n t a m i n a t i o n . I m m e d i a t e l y b e f o r e c a l i b r a t i o n t h e i n s t r u m e n t i s g i v e n a p r e l i m i n a r y c h e c k , e l e c t r i c a l and wi th a s u i t a b l e t e s t s o u r c e , t o e n s u r e t h a t it i s in good w o r k i n g o r d e r . In p a r t s II and III of t he Sh ipp ing and R e c e i v i n g R e p o r t , c a l i b r a t i o n l a b o r a t o r y p e r s o n n e l would v e r i f y t he r e c e i p t and s h i p m e n t of d o s i m e t e r s and i n s t r u m e n t s .

2 .2 . C a l i b r a t i o n of i n s t r u m e n t s and d o s i m e t e r s

T h e i r r a d i a t i o n e x p o s u r e would be m a d e on the c a l i b r a t e d e x p o s u r e a s s e m b l i e s d e s i g n e d f o r t h i s p u r p o s e . T h e d o s i m e t e r s o r i n s t r u m e n t s s h o u l d be p l a c e d in t he e x p o s u r e a s s e m b l i e s and e a c h l o c a t i o n s h o u l d be m e a s u r e d to e n s u r e t h a t t h e d o s i m e t e r o r i n s t r u m e n t i s a t the c o r r e c t d i s t a n c e and a n g l e f o r t he p r o p e r e x p o s u r e . A t y p i c a l f o r m a t f o r r e -c o r d i n g the i r r a d i a t i o n and e x p o s u r e a s s e m b l y c a l i b r a t i o n d a t a i s s h o w n in F i g . 2 2 . T h i s i s p r o v i d e d to s u g g e s t the a p p r o a c h n e c e s s a r y to r e c o r d t he i n f o r m a t i o n a b o u t e a c h c a l i b r a t i o n . It w a s d e s i g n e d to p r o v i d e f o r :

(i) I d e n t i f i c a t i o n of t he i n s t r u m e n t o w n e r , and the i d e n t i f i c a t i o n of t he i n s t r u m e n t t e s t e d

(ii) R e c o r d i n g c a l i b r a t e d s o u r c e s ( r a d i a t i o n s t a n d a r d s ) o r s t a n d a r d i n s t r u m e n t s u s e d f o r c a l i b r a t i o n and n e c e s s a r y c a l c u l a t i o n s

(ii i) I r r a d i a t i o n d a t a (iv) R e c o r d of who p e r f o r m e d and who v e r i f i e d t he w o r k

2 .3 . R e t u r n of c a l i b r a t e d d o s i m e t e r s and p o r t a b l e i n s t r u m e n t s

A f t e r s u r v e y i n g to e n s u r e tha t t h e y a r e f r e e f r o m r a d i o a c t i v e c o n -t a m i n a t i o n a l l c a l i b r a t e d d o s i m e t e r s and i n s t r u m e n t s s h o u l d be p a c k e d and r e t u r n e d to t he c u s t o m e r . T h i s shou ld b e c o m p l e t e d a s s o o n a s p o s s i b l e a f t e r c a l i b r a t i o n of the d o s i m e t e r s and i n s t r u m e n t s . F i l m d o s i m e t e r s s h o u l d be r e t u r n e d wi th in 10 d a y s of r e c e i p t .

2 .4 . R e c o r d s of l a b o r a t o r y a c t i v i t i e s

A c o m p l e t e r e c o r d of t he t e c h n i c a l a c t i v i t i e s of t he l a b o r a t o r y s h o u l d b e m a i n t a i n e d f o r s e v e r a l y e a r s s o t h a t t he i n s t r u m e n t s ' h i s t o r y i s a v a i l a b l e f o r f u t u r e r e f e r e n c e . T h e m o r e s i g n i f i c a n t t e c h n i c a l r e c o r d s t o be m a i n t a i n e d a r e :

(i) R e c e i p t and s h i p m e n t of c a l i b r a t e d d o s i m e t e r s and i n s t r u m e n t s (ii) D o s e r e c e i v e d (if any) whi le s t o r e d a t t he l a b o r a t o r y . T h i s

r e c o r d s h o u l d v e r i f y t h a t the d o s i m e t e r s r e c e i v e d on ly t he c a l i b r a t i o n d o s e w h i l e a t the l a b o r a t o r y

82

(iii) C a l i b r a t i o n d a t a i n c l u d i n g e x p o s u r e a s s e m b l y and s t a n d a r d i n s t r u m e n t s u s e d p l u s a n y o t h e r r e l e v a n t d a t a on t h e d o s i m e t e r o r i n s t r u m e n t , i . e . r e c e i v e d , c o n t a m i n a t e d e t c .

(iv) R a d i a t i o n s o u r c e c a l i b r a t i o n c e r t i f i c a t e s and r e c a l i b r a t i o n s c h e d u l e s

2 .5 . M a i n t e n a n c e and d e v e l o p m e n t of t h e i r r a d i a t i o n s t a n d a r d s and t h e i r c a l i b r a t i o n p r o c e d u r e s

T h e l a b o r a t o r y s h o u l d s e t up a c o m p l e t e s e t of q u a l i t y c o n t r o l s to m a i n t a i n a c o n t i n u o u s r e c o r d of t he p e r f o r m a n c e of t he l a b o r a t o r y . T h e s e c h a r t s shou ld r e l a t e t he p r i m a r y r a d i a t i o n s t a n d a r d c a l i b r a t i o n s and s e c o n d a r y r a d i a t i o n s t a n d a r d c a l i b r a t i o n s wi th t i m e . T h e e x a c t f o r m and n u m b e r of c h a r t s to be m a i n t a i n e d shou ld be e s t a b l i s h e d by the l a b o r a t o r y d i r e c t o r .

T h e n e e d f o r a t h i r d - p a r t y a u d i t of the l a b o r a t o r y to e n s u r e i t s a d e q u a t e p e r f o r m a n c e w a s c o n s i d e r e d . A l though t h i s m a y be d e s i r e d f r o m t i m e to t i m e , it i s not n e c e s s a r y to e s t a b l i s h a f i r m p r o c e d u r e o r s c h e d u l e f o r s u c h an a u d i t . T h e d o u b l e - c h e c k s y s t e m of l a b o r a t o r y o p e r a t i o n and the f r e q u e n t c o n t a c t s w i th a m a j o r s t a n d a r d s l a b o r a t o r y s h o u l d e n s u r e a d e q u a t e l a b o r a t o r y p e r f o r m a n c e .

T h e l a b o r a t o r y s h o u l d p u b l i s h a l l i t s d e t a i l e d s t a n d a r d o p e r a t i n g p r o c e d u r e s . T h i s p u b l i c a t i o n g i v e s i t s c u s t o m e r s and o t h e r s a n o p p o r t u n i t y t o c o m m e n t on t h e m e t h o d s and e n a b l e s o t h e r s to s e e t h a t t h e i r p r a c t i s e s fo l low a n y a p p l i c a b l e s t a t e s t a n d a r d . T h e l a b o r a t o r y s h o u l d c o n s i d e r a l l c o m m e n t s r e c e i v e d and adop t t h o s e t h a t m a y i m p r o v e i t s o v e r a l l q u a l i t y c o n t r o l and p e r f o r m a n c e . A c o n s i d e r a b l e a m o u n t of e f f o r t of t h e l a b o r a t o r y shou ld be d e v o t e d to d e v e l o p i n g new r a d i a t i o n s t a n d a r d s a s t h e y a r e r e q u i r e d , and m a i n t a i n i n g t h e i n t e g r i t y of t h e i r e x i s t i n g s t a n d a r d s , and in u p - d a t i n g t h e i r c a l i b r a t i o n t e c h n i q u e s . T h e s t a f f s h o u l d a l s o be s u i t a b l y t r a i n e d in h e a l t h p h y s i c s p r o t e c t i o n and be e q u i p p e d wi th s u i t a b l e m o n i t o r i n g e q u i p m e n t t o e n a b l e t h e m t o o p e r a t e in a s a f e m a n n e r .

3. P R O P O S E D L A B O R A T O R Y S T A F F

T h e l a b o r a t o r y s t a f f , of c o u r s e , would d e p e n d on t h e v o l u m e of b u s i n e s s . F o r o u r h y p o t h e t i c a l l a b o r a t o r y , a t y p i c a l s t a f f would c o n s i s t of a d i r e c t o r , a p h y s i c i s t , s e v e r a l t e c h n i c i a n s and a s e c r e t a r y . T h i s p r o v i d e s a s t a f f of two p r o f e s s i o n a l l y q u a l i f i e d p e r s o n s t o look a f t e r t he s c i e n t i f i c a s p e c t s of the l a b o r a t o r y w o r k and s e v e r a l t e c h n i c i a n s to a s s i s t in t he p e r f o r m a n c e of t he r o u t i n e a s p e c t s of t he l a b o r a t o r y c a l i b r a t i o n p r o g r a m . T h e t e c h n i c a l i n t e g r i t y of t he l a b o r a t o r y s h o u l d b e a b o v e r e p r o a c h , and a c o m p e t e n t s t a f f of r e c o g n i z e d c a p a b i l i t y i s r e q u i r e d t o e n s u r e t h i s s t a t u s .

T h e q u a l i f i c a t i o n s of e a c h m e m b e r of t he l a b o r a t o r y w i l l r e q u i r e c a r e f u l c o n s i d e r a t i o n to e n s u r e t h a t an e x p e r i e n c e d and c a p a b l e s t a f f a r e o b t a i n e d . It i s no t a f e a s i b l e p l a n t o s t a f f t h i s t y p e of l a b o r a t o r y wi th i n e x p e r i e n c e d p e r s o n n e l , no m a t t e r wha t d e g r e e of t e c h n i c a l p r o f i c i e n c y t h e y m a y h a v e a t t a i n e d . T h e q u a l i f i c a t i o n s of t he s t a f f a r e s u m m a r i z e d in t he f o l l o w i n g s e c t i o n s .

83

L a b o r a t o r y d i r e c t o r

T h e d i r e c t o r shou ld have a n h o n o u r s d e g r e e in one of the s c i e n c e s -p h y s i c s , c h e m i s t r y , m a t h e m a t i c s o r e n g i n e e r i n g . He shou ld h a v e a m i n i m u m of f ive y e a r s ' p r a c t i c a l e x p e r i e n c e in p e r s o n n e l d o s i m e t r y and r a d i a t i o n m o n i t o r i n g p r o g r a m s . He shou ld u n d e r s t a n d the b a s i c p r i n c i p l e s invo lved and be c o m p e t e n t in the u s e of s e c o n d a r y o r p r i m a r y r a d i a t i o n s t a n d a r d s and e q u i p m e n t . F o r e x a m p l e , h e shou ld b e c o m p e t e n t in t he u s e of f r e e - a i r i o n i z a t i o n c h a m b e r s , e x t r a p o l a t i o n c h a m b e r s , R - m e t e r s , p r e c i s i o n long c o u n t e r s and s i m i l a r r a d i a t i o n m e a s u r i n g e q u i p m e n t s o t h a t f u n d a m e n t a l c a l i b r a t i o n s c a n b e p r o v i d e d . He shou ld p o s s e s s a good u n d e r s t a n d i n g of s t a t i s t i c s and the a p p l i c a t i o n of s t a t i s t i c s and m a t h e m a t i c a l a n a l y s i s to the qua l i t y c o n t r o l p r o g r a m s . He shou ld h a v e d e m o n s t r a t e d good a d m i n i s t r a t i v e c a p a b i l i t i e s , ab i l i t y to i s s u e c l e a r and c o n c i s e s c i e n t i f i c r e p o r t s , and w o r k we l l with p e o p l e .

P h y s i c i s t

T h e p h y s i c i s t shou ld h a v e e a r n e d a d e g r e e o r a n e q u i v a l e n t q u a l i f i c a -t ion in one of the s c i e n c e s - p h y s i c s , c h e m i s t r y , m a t h e m a t i c s o r e n g i n e e r i n g , and m i n i m u m of two y e a r s ' e x p e r i e n c e with p e r s o n n e l d o s i m e t r y and r a d i a t i o n m o n i t o r i n g . In g e n e r a l , to about t he s a m e e x t e n t , the p h y s i c i s t shou ld h a v e knowledge , e x p e r i e n c e and c a p a b i l i t y in a l l a r e a s p r e s c r i b e d f o r the d i r e c t o r .

T e c h n i c i a n s

T h e t e c h n i c i a n s a r e r e q u i r e d to a s s i s t in m a k i n g the e x p o s u r e s . The c a p a b i l i t i e s and e x p e r i e n c e of the t e c h n i c i a n shou ld l ie in the f ie ld of p h y s i c a l s c i e n c e s . E x p e r i e n c e in c a l i b r a t i o n s and r a d i a t i o n p r o t e c t i o n o r h e a l t h p h y s i c s a c t i v i t i e s i s d e s i r a b l e .

S e c r e t a r y

A s e c r e t a r y i s r e q u i r e d f o r the o r g a n i z a t i o n . She shou ld h a v e the q u a l i f i c a t i o n s e s t a b l i s h e d f o r a d v a n c e d s e c r e t a r i a l p o s i t i o n s . H e r d u t i e s wi l l i nc lude the p r e p a r a t i o n , f i l i ng and m a i n t e n a n c e of t he r e c o r d c o p i e s of the e x p o s u r e s a s we l l a s the c o n v e n t i o n a l s e c r e t a r i a l du t i e s a s d i r e c t e d by the l a b o r a t o r y d i r e c t o r and the p h y s i c i s t .

4 . F A C I L I T Y ACQUISITION

T h e m e t h o d of f i n a n c i n g the c o n s t r u c t i o n , equ ipp ing and o p e r a t i n g t he c a l i b r a t i o n l a b o r a t o r y is m a r k e d l y d e p e n d e n t on the c o u r s e of a c t i o n t a k e n to e s t a b l i s h the l a b o r a t o r y and on the a c t u a l c u r r e n t c a p a b i l i t i e s of the g r o u p s e l e c t e d to s t a r t up and o p e r a t e the l a b o r a t o r y .

84

4 . 1 . E s t i m a t e d L a b o r a t o r y C o n s t r u c t i o n C o s t

Land

T h e c o s t of p u r c h a s e o r l e a s e of land f o r t he c o n s t r u c t i o n of a l a b o r a t o r y d e p e n d s on t h e p a r t of t he w o r l d w h e r e t he l a b o r a t o r y i s to be c o n s t r u c t e d a n d t h e l o c a t i o n of t he l a b o r a t o r y wi th r e s p e c t to t he s u r r o u n d i n g u r b a n a r e a . F o r t he h y p o t h e t i c a l l a b o r a t o r y d i s c u s s e d e a r l i e r i n t h i s s e c t i o n , a lot of 1000 to 5000 m 2 would be r e q u i r e d t o p r o v i d e s o m e s u r r o u n d i n g a r e a f o r p a r k i n g and b u i l d i n g a c c e s s c o n t r o l . To i l l u s t r a t e t h i s e x a m p l e t h e c o s t f i g u r e s a r e in d o l l a r s and a r e b a s e d a p p r o x i m a t e l y on c u r r e n t c o s t s in t he Uni ted S t a t e s of A m e r i c a . T h e a c t u a l c o s t s i nvo lved w i l l o b v i o u s l y d e p e n d on the l o c a t i o n and c o u n t r y w h e r e t he c a l i b r a t i o n l a b o r a t o r y i s b e i n g bu i l t . C o s t co u l d v a r y f r o m a s low a s a few t h o u s a n d s of d o l l a r s t o s e v e r a l t e n s of t h o u s a n d s of d o l l a r s f o r t h e l a n d , d e p e n d i n g on the l o c a t i o n f a c t o r s . C o s t f o r u t i l i t i e s a c c e s s cou ld be c o m p a r a b l e to t he l and a c q u i s i t i o n c o s t .

T A B L E IX. L A B O R A T O R Y BUILDING S P A C E

Room designation Size each

Number , , , ( m 2 )

Total area ( r f )

Irradiation rooms 2 ~ 9 0 ~ 180

Offices 4 ~ 1 5 ~ 60

Decontamination room 1 ~ 3 0 ~ 30

Vault 1 ~ 1 0 ~ 10

Sub-total: ~ 2 8 0

Support space (halls, heat and venti lation, wash rooms, e t c . ) at 40°lo of useful space

Total floor space:

~ 1 0 0

- 3 8 0

T A B L E X . L A B O R A T O R Y C O N S T R U C T I O N COST

Item Rate Total cost

Item Rate ($)

Laboratory 380 m 2 a $ 4 0 0 / m 2 152 000

Design 10% of laboratory cost 15 200

Sub-total: 167200

Contingency 12.5"7o of total cost 20 900

Total: 188100

85

T A B L E XI . E Q U I P M E N T COST E S T I M A T E S

Total cost Item Number required

($)

(1) Exposure assemblies

(a) Beta-gamma assembly (Fig. 9) 1 1500 (b) Beta-gamma assembly (F ig .4c ) 1 500 (c) Intermediate X-ray assembly (Fig. 2) 1 1750 (d) Low energy X-ray assembly (Fig. 3) 1 2 500

(e) Beta radiation exposure assembly 1 1750 (point source - Fig. 11)

( 0 Beta radiation exposure assembly 1 500 (surface - Fig. 12)

(g) Fast neutron assembly (Fig. 13) 1 2 500 (h) Thermal neutron assembly 1 1500

(polyethylene moderator - Fig. 14)

Total 12 500

(2) Calibrated radiation sources (sealed)

(a) Radium (0. 5 Ci) 2 8 000

(b) Radium ( 0 , 0 1 Ci) 2 160 (c) Cobalt-60 (5 Ci) 1 325 (d) Cobalt-60 (1 Ci) 1 250

(e) Cobalt-60 ( 0 . 2 Ci) 1 180

(f) Caesium-137 (1 Ci) 1 400

(g) Iridium-192 (1 Ci) 1 200

(h) Strontium-90 ( 0 . 0 0 3 Ci) 2 250

(i) Strontium-90 ( 0 . 0 0 0 1 Ci) 2 200

(i) Uranium (200 X 25 X 6 . 3 mm) 20 400

(k) PuBe (80 g Pu) 2 3140

Total 13 505

(3) X-ray machines 2 18200

Total 18200

(4) Secondary standard instruments

(a) "R" meter sets 3 4 500

(b) Extrapolation chambers 3 4 200 (c) Free-air ion chamber 1 5 000

(d) Precision long counter 1 8 000

(e ) Activation foil sets 5 500

( 0 Electrometer standard 1 1000

(g) T i m e standard 1 800

Total 24000

86

T A B L E X I . (Cont . )

Total cost Item Number required

($)

(5) Radiation monitoring instrumentation

(a) Beta, gamma dose-rate meters 4 800 (b) Neutron dose-rate meters 2 1200 (c) Beta-gamma monitors 4 1200 (d) Alpha monitors 4 1200 (e) Area monitors 2 1500

Total 5 900

(6) Office furniture and equipment Total 9 300

m Miscellaneous

(a) Source storage casks 8 5 000 (b) Vault air sampler 1 500 (c) Source system air samplers 2 1000 (d) Air filter counter 1 500 (e) Monitoring instrument calibration jigs 3 1500 ( f ) Constant voltage transformers 5 2 500

Total 11000

L a b o r a t o r y b u i l d i n g

T h e f l o o r a r e a of t h e b u i l d i n g i s d e t e r m i n e d by the r e q u i r e m e n t s f o r i r r a d i a t i o n r o o m s , o f f i c e s p a c e , s h i p p i n g and r e c e i v i n g a r e a , and v a u l t s . S p a c e a l l o t m e n t a s s h o w n in T a b l e IX p r o v i d e s a n e s t i m a t e of t he t o t a l b u i l d i n g s i z e .

In t he Un i t ed S t a t e s of A m e r i c a t he c o n s t r u c t i o n c o s t f o r t he l a b o r a t o r y , i n c l u d i n g p o u r i n g the c o n c r e t e v a u l t s , w e l l s and s h i e l d i n g w a l l s b e t w e e n the i r r a d i a t i o n r o o m s , i s e s t i m a t e d a t $ 4 0 0 / m 2 . T h e t o t a l e s t i m a t e d c o n s t r u c t i o n c o s t , i n c l u d i n g a d e s i g n c o s t of 10% and a c o n t i n g e n c y of 12.5%, i s s h o w n in T a b l e X.

T h e t o t a l e s t i m a t e d c o n s t r u c t i o n c o s t of t he l a b o r a t o r y would be a b o u t $190 000 .

4 . 2 . E s t i m a t e d e q u i p m e n t c o s t

T h e c o s t in t he USA of t h e e q u i p m e n t w a s d e t e r m i n e d f r o m c o n s t r u c -t i o n c o s t e s t i m a t e s f o r f a b r i c a t i n g t he t y p i c a l e x p o s u r e a s s e m b l i e s s h o w n e a r l i e r , t he c o s t of p u r c h a s i n g t h e r a d i a t i o n s o u r c e s and X - r a y m a c h i n e s , and d o s e m e a s u r e m e n t and c o n t r o l e q u i p m e n t . T h e s e e s t i m a t e s a r e s u m m a r i z e d in T a b l e XI .

T a b l e XII s u m m a r i z e s t h e c o s t of p r o v i d i n g a s u i t a b l e c a l i b r a t i o n l a b o r a t o r y .

87

T A B L E XII . L A B O R A T O R Y COST ( U N I T E D S T A T E S O F A M E R I C A )

Item Cost estimates

($)

Land 10 000

Laboratory building 190 000

Exposure assemblies 12 500

Calibrated radiation sources 13 500

X-ray machines 18 200

Secondary standard instruments 24000

Radiation monitoring instrumentation 5 900

Off ice furniture and equipment 9 3 0 0

Miscellaneous 11000

294400

T A B L E XIII . D I R E C T S A L A R I E S

Staff Salary range

Staff ($)

Director 14 000 - 18000

Physicist 10 000 - 15 000

Technician 6 000 - 10 000

Secretary 5 000 - 7 000

Totals: 35 000 - 50 000

4 . 3 . E s t i m a t e d o p e r a t i o n a l c o s t

T h e c o s t f o r t he r o u t i n e o p e r a t i o n of t he l a b o r a t o r y w a s a l s o e s t i m a t e d . T h e e s t i m a t e s a r e m a d e by c o n s i d e r i n g the c o s t of m a i n -t a i n i n g t h e n e c e s s a r y s t a f f , f a c i l i t y and e q u i p m e n t .

T o s e c u r e p e r s o n n e l w i t h t he c a p a b i l i t i e s and e x p e r i e n c e r e q u i r e d f o r t he p r o p e r o p e r a t i o n of t he l a b o r a t o r y and to e n s u r e t he q u a l i t y of t he l a b o r a t o r y , t he s a l a r y s c h e d u l e s h o w n in T a b l e XIII i s g i v e n a s a g u i d e . T h i s s c h e d u l e i s c o n s i s t e n t w i th s a l a r y r e v i e w s of i n d u s t r y f o r the q u a l i t y of s t a f f m e m b e r s r e q u i r e d f o r e a c h p o s i t i o n . It i s i m p o r t a n t t h a t t he p e r s o n n e l s e l e c t e d f o r o p e r a t i n g a s e c o n d a r y s t a n d a r d l a b o r a t o r y s h a l l b e a d e q u a t e l y q u a l i f i e d and s h a l l be g i v e n a d e q u a t e s t a t u s and r e -m u n e r a t i o n . T h i s i s p a r t i c u l a r l y i m p o r t a n t f o r t he m o r e h i g h l y t r a i n e d m e m b e r s of t he s t a f f to e n s u r e con t inu i ty of t he w o r k and to avo id t h e i r e x p e r t i s e b e i n g l o s t t o t h i s f i e l d .

88

T A B L E XIV. L A B O R A T O R Y A N N U A L O P E R A T I N G COST

Item Cost est imate.

($)

Salaries 40 000

Benefits at l i t y of salaries 6000

Building:

Maintenance 10 000

Util it ies 10 000

Depreciation:

Building, $180 000 over 40 yr 4 500

Equipment, $ 9 7 4 0 0 o v e r 10yr 9740

Supplies:

Laboratory 5000

Office 2 000

Support services 10 000

Travel 1200

Equipment maintenance 5000

US Bureau of Standards 5 000

Shipping and receiving 2 000

Sub-total: 110440

Profit at 8°lo of operation cost plus 6% of investment 26080

Capital equipment additions 10 000

Total : 146 520

T h e o p e r a t i n g c o s t of t he l a b o r a t o r y p r e s e n t e d in T a b l e XIV w a s e s t i m a t e d by u s i n g t h e e x p e r i e n c e a t t a i n e d in o p e r a t i n g s i m i l a r f a c i l i t i e s in t he USA. T h e t o t a l c o s t e s t i m a t e i s a b o u t $145 000 .

89

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95

HOW TO ORDER IAEA PUBLICATIONS

Exclusive sales agents for IAEA publications, to whom all orders and inquiries should be addressed, have been appointed in the following countries:

UNITED KINGDOM Her Majesty's Stationery Office, P.O. Box 569, London S.E.I

UNITED STATES OF AMERICA UNIPUB, Inc., P.O. Box 433, New York, N.Y. 10016

In the following countries IAEA publications may be purchased from the sales agents or booksellers listed or through your major beai booksellers. Payment can be made in local currency or with UNESCO coupons.

ARGENTINA Comisión Nacional de Energia Atòmica, Avenida del Libertador 8250, Buenos Aires

AUSTRALIA Hunter Publications, 23 McKillop Street, Melbourne, C.l BELGIUM Office International de Librairie, 30, avenue Marnix, Brussels 5 CANADA Information Canada, Ottawa

C.S.S.R. S.N.T.L., Spálená 51, Prague 1 Alfa, Publishers, Hurbanovo námestie 6, Bratislava

FRANCE Office International de Documentation et Librairie, 48, rue Gay-Lussac, F-75 Paris 5 e

HUNGARY Kultura, Hungarian Trading Company for Books and Newspapers, P.O.Box 149, Budapest 62

INDIA Oxford Book and Stationery Comp., 17, Park Street, Calcutta 16 Prakash Publishers, Film Colony, Chaura Rasta, Jaipur-3 (Raj.)

ISRAEL Heiliger and Co., 3, Nathan Strauss Str., Jerusalem ITALY Agenzia Editoriale Commissionaria, A.E.I.O.U., Via Meravigli 16,

1-20123 Milan JAPAN Maruzen Company, Ltd., P.O.Box 5050, 100-31 Tokyo International

NETHERLANDS Martinus Nijhoff N.V., Lange Voorhout 9—11, P.O.Box 269, The Hague PAKISTAN Mirza Book Agency, 65, The Mall, P.O.Box 729, Lahore-3

POLAND Ars Polona, Centrala Handlu Zagranicznego, Krakowskie Przedmiescie 7, Warsaw

ROMANIA Cartimex, 3-5 13 Decembrie Street, P.O.Box 134-135, Bucarest SOUTH AFRICA Van Schaik's Bookstore, P.O.Box 724, Pretoria

Universitas Books (Pty) Ltd., P.O.Box 1557, Pretoria SWEDEN C.E.Fritzes Kungl. Hovbokhandel, Fredsgatan 2, Stockholm 16

U.S.S.R. Mezhdunarodnaya Kniga, Smolenskaya-Sennaya 32-34, Moscow G-200 YUGOSLAVIA Jugoslovenska Knjiga, Terazije 27, Belgrade

Orders from countries where sales agents have not yet been appointed and requests for information should be addressed directly to:

Publishing Section, ViyP ê International Atomic Energy Agency, ^ ^ Kärntner Ring 11, P.O.Box 590, A-1011 Vienna, Austria

INTERNATIONAL ATOMIC ENERGY AGENCY V IENNA, 1971

PRICE: US $3 .00 Austr ian Schil l ings 72 , -(£1.21; F.Fr.16,60; DM 10,-)

SUBJECT GROUP: II Hea l th , Safety and W a s t e M a n a g e m e n t / Radiation Protection