21 7

Uranium Content of Thirty -Three Rock REferEnce Samples DEtErmined by thE Delayed Fission Neutron Counting Method

G.R. REDDY, D.R. PANT AND M. SANKAR DAS

A n a l y t i c a l Chemistry D i v i s i o n , Bhabha Atomic Resea rch C e n t r e , Trombay, Bombay 400 085, I n d i a

The delayed f i s s i o n neutron counting method has been used f o r the analysis of thir ty- three international geochemical reference samples f o r t h e i r uranium contents i n the range of 0.2 t o 1500 ppm. These include three from the Canada Centre f o r Mineral and Energy Technology, three from Atomic Energy Commission, USL, New Bmnswick Laboratory, two from Bhabha Atomic Research Centre. India, fourteen from Centre de Recherches Pe’trographiques e t Ggochimiques and eleven from US Geological Survey. The experimental set up has a detection limit of 0.08 r g and a determination l imit of 0 . 2 5 pg U. A t the lower l i m i t the precision of determination i s about 20 percent while a t concentrations higher than 1 p p m it i s bet ter than 1 0 percent.

The development and a p p l i c a t i o n of t h e well known de layed f i s s i o n n e u t r o n c o u n t i n g (DFNC) method f o r t h e a n a l y s i s of uranium i n v a r i o u s ma- te r ia l s of g e o l o g i c a l i n t e r e s t h a s been r e p o r t e d ear l ie r from t h i s l a b o r a t o r y (1). The t h e o r y and p r a c t i c e o f t h e DFNC method is a d e q u a t e l y d e a l t i n t h e l i t e r a t u r e and t h e y have been r e f e r r e d t o i n t h e ear l ie r s t u d y , hence w i l l n o t be c o n s i - de red h e r e . T h i s n o t e p r e s e n t s t h e r e s u l t s on t h e d e t e r m i n a t i o n o f uranium i n t h i r t y - t h r e e d i f f e - r e n t s t a n d a r d r o c k s .

Subsequent t o t h e ear l ie r s t u d y , t h e pneu- matic i r z -ad ia t ion f a c i l i t y of t h e CIRUS r e a c t o r h a s been s h i f t e d from a h igh f l u x ( - 6 x l o ” n cm-2S-’) t o a medium f l u x ( - 1 x l O ” n cm-’S-‘) p o s i t i o n . Hence i t was f e l t d e s i r a b l e t o r e - e v a l u a t e t h e a n a l y t i c a l para- meters such as s e n s i t i v i t y , p r e c i s i o n and accu- r a c y a t t a i n a b l e wi th t h e f a c i l i t y . These d a t a a r e a l so p r e s e n t e d i n t h i s n o t e .

RESULTS AND DISCUSSION

Exper imen ta l p r o c e d u r e s such as p r e p a r a t i o n of t h e samples and s t a n d a r d s , i r r a d i a t i o n and d e l a y p e r i o d s are e s s e n t i a l l y t h e same as r e p o r t e d earlier. The c o u n t i n g equipment i n t h e p r e s e n t work c o n s i s t e d of more s t a b l e s o l i d s ta te u n i t s ( E l e c t r o n i c s C o r p o r a t i o n of I n d i a L t d . , make) which r e p l a c e d t h e v a l v e u n i t s used i n t h e ear l ie r work. The p r e s e n t e x p e r i m e n t a l pa rame te r s unde r normal working c o n d i t i o n s of t h e system are g i v e n i n T a b l e 1. The d e t e c t i o n l i m i t of uranium w i t h t h e f a c i l i t y computed as t h e a b s o l u t e amount of uranium c o r r e s p o n d i n g t o t h e magnitude of f o u r times t h e s t a n d a r d d e v i a t i o n of a s i n g l e measu- rement of t h e sys t em background is 0.08 rg. The ‘ d e t e r m i n a t i o n l i m i t ‘ c o r r e s p o n d i n g t o a coun t r a t e equal t o t h e background on t h e use of a 0.5 g sample i s 0.15 ppm. C o n c e n t r a t i o n s o f uranium down t o abou t 0 .2 ppm c a n be de t e rmined , though w i t h a p o o r e r p r e c i s i o n (RSD 22%) is demons t r a t ed by t h e d a t a on MRG-1 g i v e n i n Tab le 2. A t h i g h l e v e l s o f uranium c o n c e n t r a t i o n s ( > 0.05%) t h e sample w e i g h t s a re s u i t a b l y chosen as t o avo id c o u n t i n g losses .

The d a t a on tho r ium show t h a t i ts i n f l u e n c e i n t h e measurement of uranium, when p r e s e n t a t e q u a l c o n c e n t r a t i o n s is a b o u t 0.02%. T h i s r e p r e s e n t s a n improvement by a f a c t o r of 4 o v e r our ear l ie r va lue (1) of 0.08%. Thus t h e method p e r m i t s t h e a n a l y s i s o f uranium i n samples h igh i n thorium c o n t e n t . An example of t h i s is t h e a n a l y s i s o f sample No.AEC-NBL-79 ( T a b l e 2 ) which w a s p repa red o u t of monazi te and h a s a Th/U r a t i o of 25. The sample TKT-1 is a n in-house t r a c h y t e s t a n d a r d p repa red f o r r o u t i n e l y check ing t h e accu racy o f ou r measurements. The a n a l y t i c a l p r e c i s i o n o f t h e 54 r e s u l t s on t h i s sample ( T a b l e 2 ) o b t a i n e d o v e r a p e r i o d o f more than 2 y e a r s r e p r e s e n t t h e p r e c i s i o n of o u r uranium a n a l y s i s by delayed f i s s i o n neu t ron c o u n t i n g , which is g e n e r a l l y b e t t e r t han 10% a t lower l e v e l s and b e t t e r t h a n 5% a t h i g h e r l e v e l s o f uranium c o n c e n t r a t i o n .

218

Table 1 . Character is t ics of t h e DFNC system

Flux = - + I x l o i 3 n S-' ( 4 0 M W . o p e r a t i n g power of t h e r e a c t o r )

I r r a d i a t i o n time = 60 S

Delay time = 30 S

Count time = 50 S

Sample weight = 0.5 g

G a m m a d e s c r i m i n a t o r = 3 V b i a s

Counter background = 65 2 8 ( n = 12) with empty i r r a d i a t e d i.abbit i n count ing p o s i t ion

Coun t s p e r rg U n a t = 420 5 (Computed)*

Counts p e r mg Th = 90

* Measured v a l u e = 11990 120 for 28.2 pg U ( n = 6)

Table 2 . Uranium content o f reference samples

Sample Source No.of Uranium content (ppm)+ analyses This work Literature data

MRG-1 CCRMP

SY-2 CCRMP

SY-3 CCRMP

NBL-1 AEC-NBL

NBL-4 AEC-NBL

NBL-79 AEC-NBL

8-78 ACD-BARC

TKT-1 ACD-BARC

9

13

8

6

8

6

16

54

CCRMP = Canadian C e r t i f i e d Reference Materials Project

AEC-NBL = U.S. Atomic Energy Commission, N e w Bruns- wick Laboratory.

ACD-BARC = 'In-house' Standards of the Analytical Chemistry Div is ion, Bhabha Atomic R e - search Centre, INDIA,

+ Uranium Solution evaporated on polythene s h e e t s a1.e used as standards.

$ Personal communication t o the Editor (Dec. 1982) .

Our r e s u l t s on NBL s t a n d a r d s are w i t h i n 4% of t h e c e r t i f i e d v a l u e s (2). A va lue o f 280 ppm for SY-2 obta ined i n t h i s work agrees very w e l l w i t h t h e u s a b l e va lue of 290 ppm by Abbey (3) or 278 ppm by Ledger e t a1 (4 ) . However, a value of 730 ppm f o r SY-3 o b t a i n e d i n t h i s work or 695 ppm r e p o r t e d ear l ier by us (5) are high compared t o e i t h e r t h e u s a b l e va lue o f 650 ppm by Abbey (3) or 631 ppm r e p o r t e d by Ledger e t a1 (4).

Table 3. Uranium content o f twenty-f ive new CRPG and USGS standards

Sample Uranium (ppml Mean Literature da ta*

CRPG (Cent re de Recherches PetrograDhisues e t Geochimiques )

BX-N

BR

DR-N

FK-N

GA

GH

GS-N

Mica-Fe

DT-N

BE-N

MA-N

Mica-Mg

UB-N

AN-G

9.2, 8.8, 9.3

2.5, 2.7, 2.8

1.7, 1.2, 1.7

0.32, 0.21, 0.22

4.6, 6.2, 5.7

18.7, 18.9. 18.1

8.7, 8.3, 8.3

86, 90, 90

3.2, 2.2, 2.5

2.5, 2.6, 2.6

See t a b l e 4

9.1

2.7

1.5

0.25

5.5

18.6

8.4

88.6

2.6

2.6

13.0

10.2

h0.2

40.2

U B (U.S. Geological Survep)

STM-1

MAG-1

BHVO-1

sco-1 SGR-1

QLO-1

RGM-1

SDC-1

w-2

BIR-1

DNC

9.3, 9.0, 8.7

3.0, 3.2, 3.0

0.5, 0.71. 0.65

3.2, 2.8, 2.7

5.5, 6.4, 5.8

2.0, 1.6, 2.2 6.3, 6.4, 5.3

3.9, 3.0, 3.0 0.70, 0.72, 0.50 0.60, 0.76

9.0

3.1

0.62

2.9

5.9

1.9 6.0

3 . 3

0.66

10.2 40.2

8.6

2.42

1.58

40.2

5.5

19.5

8.4

85.4

1.94

2.55 2.4(9)

13.2 12.1(9)

10.2 10.2 10.2

9.1

2.82

0.48

3.15

5.60

2.01 5.85

3.12

0.42

c0.1 10.1

A l l t h e L i t e r a t u r e va lues f o r the CRPG samples are those of Rowe (7) and those for t h e USGS samples a r e of Mi l la rd (8), u n l e s s otherwise i n d i c a t e d .

219

Table 4 . Homogeneity study of t h e g ran i t e MA-N

W t . of t h e 34.8 56.4 79.8 82.8 86.8 134.3 172.4 206.1 320.2 354.5 512 sample (rng)

u (ppm) 12.4 14.4 13.7 17.7 12.2 13.2 13.6 12.8 13.4 13.7 12.4

Mean = 13.0 2 0.7

n = 11

Twenty-five s t a n d a r d r o c k s from CRPG, F r a n c e and USGS have been a n a l y s e d i n t h e p r e s e n t work f o r t h e i r uranium c o n t e n t s . These r e s u l t s a r e g i v e n i n T a b l e 3 . R e c e n t l y Ledger e t a1 (6) have r e p o r t e d t h e uranium v a l u e s f o r t h e F rench s t a n d a r d s . Subsequen t ly t h e s e were c o r r e c t e d f o r a c o m p u t a t i o n a l e r r o r and r e p o r t e d by R o w e ( 7 ) . These v a l u e s are g i v e n i n T a b l e 3 f o r comparison. The d a t a by M i l l a r d (8) on t h e new USGS s t a n d a r d r o c k s are a lso g i v e n i n T a b l e 3 f o r comparison and t h e agreement between v a r i o u s v a l u e s i s f a i r l y good.

Of t h e new French s t a n d a r d s , t h e g r a n i t e MA-N w a s t r e a t e d d i f f e r e n t l y because o f two f a c t o r s v i z .

1. The sample i s r e p o r t e d (9) t o be inhomo- geneous w i t h r e s p e c t t o s t r o n t i u m i s o t o p i c compos i t ion . I n o r d e r t o see whe the r t h i s inhomogeneity is r e f l e c t e d i n t h e e l e m e n t a l compos i t ion as well. a t least i n r e s p e c t of uranium, r e p l i c a t e a n a l y s e s of t h e sample were made u s i n g sample w e i g h t s r a n g i n g from 35 - 512 mg and t h e r e s u l t s o b t a i n e d are g i v e n i n T a b l e 4. I t i s s e e n t h a t w i t h i n t h e a n a l y t i c a l c a p a b i l i t y of t h e t e c h n i q u e i t i s n o t p o s s i b l e t o conf i rm t h e inhomogenei ty o f t h e sample i n r e s p e c t of uranium.

2. The sample c o n t a i n s 1.06% L i , O and hence t h e r e is a p o s s i b i l i t y of el-rors due t o se l f s h i e l d i n g . T h i s w a s examined by d i l u t i n g t h e sample w i t h d u n i t e i n t h e r a t i o of 1:1, 1:1.6 and 1:3 and t h e n ana lysed . The r e s u l t s o b t a i n e d are r e s p e c t i v e l y , 13.9, 11.6 and 12.9 ppm of uranium. These r e s u l t s are n o t ve ry d i f f e r e n t f r o m t h e o n e s o b t a i n e d d i r e c t l y on t h e sample ( T a b l e 4) and hence e x c l u d e s t h e p o s s i b i l i t y o f se l f s h i e l d i n g errors .

RESUME

Trente t r o i s echantil lons ont ete analyses pour leur teneur en U u t i l i s a n t l ' a c t i v a t i o n neutronique avec des neutrons re ta rdes . La game des valeurs o s c i l l e d e 0.2 8 1500 ppm. Les echant i l lons analyses sont: t r o i s provenant du Centre Canadien de l a Technoloqie des Mineraux e t

de l 'Energie, t r o i s de l a Commission Atomique des Etats-Unis, d e u x d e "Bhabha Atomic Research Centre" ( Inde ) , quatorze du CRPG e t onze de 1'USGS. La procedure appliquee permet une limite de detection de 0.08 g e t une l imi t e de dosage de 0.15 g U . Au niveau de f a i b l e s teneurs, l a r ep roduc t ib i l i t e e s t de l ' o rd re de 20% t andis qu'au niveau plus 6leves que 1 ppm, e l l e e s t de 10% ou m i l l e u r e .

REFERENCES

(1 ) G . R . Reddy and M. S a n k a r D a s ( 1 9 7 4 ) A n a l y s i s o f u ran ium by d e l a y e d f i s s i o n n e u t r o n c o u n t i n g method. I n d . J. Tech . 12: 405-409.

( 2 ) C . J . Rodden (L950) U n i t e d S t a t e s Atomic Energy Commission. N e w BurnswLck L a b o r a t o r y . P r o v i s i o n a l C e r t i f i c a t e s o f A n a l y s i s .

(3) S . Abbey (1980) S t u d i e s i n " s t a n d a r d samples ' ' f o r u s e i n t h e g e n e r a l a n a l y s i s of s i l i c a t e r o c k s a n d m i n e r a l s . P a r t 6 : 1979 E d i t i o n o f ' U s a b l e ' v a l u e s , P a p e r 80-14.

( 4 ) E . B . Ledge r , T . T . T i e h and M.W. R o w e (19801 De layed n e u t r o n a c t i v a t i o n d e t e r m i n a t i o n of u ran ium i n t w e l v e r o c k reference s t a n d a r d s , C e o s t a n d a r d s News- l e t t e r , 4 : 15.3-155.

(51 C . R . Reddy, D.R. P a n t , B . L . Rao and M . Sankai' Das ( 1 9 7 6 ) Neu t ron a c t i v a t i o n a n a l y s i s of 13 minor and trace e l e m e n t s i n g e o l o g i c a l s a m p l e s , J o u m a l R a d i o a n a l y t i c a l C h e m i s t r y , 33: 39-51.

( 6 ) E . B . Ledgel'. T.T. T i e h and M . W . Rowe (1980) I l e l ayed n e u t r o n a c t i v a t i o n d e t e r m i n a t i o n o f uranium i n t h i r t e e n F r e n c h P O C ~ !*efer-ence s a m p l e s , C e o s t a n d a r d s Newsletter, 4 : 5-8.

( 7 ) M . W . Rowe ( 1 9 8 1 ) De layed n e u t r o n a c t i v a t i o n d e t e r m i n a t i o n o f u ran ium i n t h i i ' t e e n F r e n c h r o c k r e f e r e n c e s a m p l e s , G e o s t a n d a r d s Newslettel.. 5: 220.

(8) H . T . M i l l a r d , J r . (1976) O e t e r m i n a t i o n of u ran ium and tho r ium i n USGS s t a n d a r d t w c k s by t h e d e l a y e d n e u t r o n t e c h n i q u e i n d e s c r i p t i o n s and a n a l y s i s o f e i g h t new 1ISGS r o c k s t a n d a r d s compi l ed by F . J . F l a n a g a n . G e o l o g i c a l S u r v e y F r o f e s s i o n a l P a p e r 8411: I;l-Bi..

(9) K . G o v i n d a r a j u ( lW<l ) Repui-1 (198~11 on t h r e e 2 I T - I W Z r o c k r e f e r e n c e s a m p l e s : A n o r t h o s i t e fl'oni G r e e n l a n d , AN-G; B a s a l t d ' E s s e y - l a - c 8 t e , RE-N; W a n l t e d e Beauvo i r . M A - N , G e o s t a n d a r d s N e w s l e t t e r . 4 : 49-139.