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B ARC-1546
>aCJI
DEVELOPMENT OF SCINTILLATION AND LUMINESCENT DETECTORSAT BARC
Edited by
Dr. A. S. PradhanDivision of Radiological Protection
1991
B.A.R.C. -
GOVERNMENT OF INDIAATOMIC ENERGY COMMISSION
<CD
DEVELOPMENT OF SCINTILLATION AND LUMINESCENT
DETECTORS AT BARC
Edited byA.S. Pradhan
Division of Radiological Protection
BHABHA ATOMIC RESEARCH CENTREBOMBAY, INDIA
1991
B.A.R.C.-1546
Ol
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B.A.R.C.-1546
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10 Title and subtitle : Development of scintillation andluminescent detectors at BARC
11 Collation :
13 Project No. :
20 Personal author(s) :
110 p., figs.
A.S. Pradhan (ed.)
21 Affiliation of author(s) 1 Division of Radiological Protection,Bhabha Atomic Research Centre,Bombay — 400 085
22 Corporate author(s)
23 Originating unit :
24 Sponsor(s) Name :
Bhabha Atomic Research Centre,Bombay-400 085
Division of Radiological Protection,B.A.R.C., Bombay - 400 085
30 Date of submission :
31 Publication/Issue date :
Department of Atomic Energy
Government
February 1991
March 1991
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60 Abstract : Research and development work carried out at theBhabha Atomic Research Centre, Bombay, in the field qf radiationdetectors for various applications, particularly in the area ofscintillation and luminescent detectors.is reviewed. The reviewis presented in the form of 7 articles.
70 Keywords/Descriptors : BARC; LIQUID SCINTILLATORS; PLASTICSCINTILLATORS; INORGANIC PHOSPHORS; DOPED MATERIALS; RAREEARTHS; CRYSTAL GROWTH; THERMOLUMINESCENT DOSIMETRY;THERMOLUMINESCENT DOSEMETERS; LIQUID SCINTILLATION. DETECTORS;PLASTIC SCINTILLATION DETECTORS; PRODUCTION; CHEMICALPREPARATION; TRITIUM; RADIATION MONITORING; THIN FILMS
71 Class No. s INIS Subject Category : B24.10; E41.10; E41.40
99 Supplementary elements :
POUEVAKI)
With the ever increasing use of ionis ing rad ia t ions in
several f i e ld s such as indust ry , medicine, ag r i cu l tu r e and
pure and app l ied r e s e a r c h , a v a i l a b i l i t y of a p p r o p r i a t e
d e t e c t o r s fo r t h e measurement of m a n i f o l d a s p e c t s of
d i f f e r e n t i o n i s i n g r a d i a t i o n s has g a i n e d s i g n i f i c a n t
re levance and importance. In l i n e wi th i t s t r a d i t i o n , the
Bhabha Atomic Research Centre (BARC) has made p i o n e e r i n g
e f f o r t s in t h e deve lopmen t of d e t e c t o r s fo r v a r i o u s
a p p l i c a t i o n s . Since t h i s work has been in d i f f e ren t groups of
BARC, there has been a long- fe l t need to c o l l a t e the vast
information and e x p e r t i s e g e n e r a t e d a t t he C e n t r e . The
p r e s e n t r e p o r t i s i n t e n d e d t o meet t h i s r e q u i r e m e n t ,
p a r t i c u l a r l y in the area of S c i n t i l l t i t i o n and Luminiscent
d e t e c t o r s . The mater ia l covered in the repor t i s based on the
l ec tu re s given by the s t a f f members of BARC in a Workshop
organised by the Bombay Chapter of the Indian Society of
Had int. ion Physics and Western Regional I n s t r u m e n t a t i o n
C e n t r e . I t i s s i n c e r e l y hoped Lhut t h e r e p o r t would
con t r ibu te to the wide dissemination of valuable information
generated a t the Centre in th i s important area of Radiation
Physics .
4s/" G4(D.V. Gopin.it h)
DirectorHealth, Safety & Environment. Group
C O N T E N T S
I J p e r Title and AuthorsNo.1 CRYSTAL GROWTH AND EVALUATION OF MATERIALS FOR SCINTI-
LLATION DETECTION AND THERMOLUMINESCENCE DOSIMETRY
M.P. Chougaonkar, V.H. GokhaJe, S.M.D. Rao and R.V. Srikantaiah
2 DEVELOPMENT OF THERMOLUMINESCENT MATERIALS FOR RADIATIONDOSIMETRIC APPLICATIONS AT DRP, BARC.
Bhuwan Chandra Bhatt
3 PRODUCTION OF CaSO. : Dy PHOSPHOR FOR DOSIMETRIC APPLI-CATIONS
R.K. Iyer
k PREPARATION AND CHARACTERISATION OF RARE EARTH BASEDPHOSPHORSG. Alexander
5 DEVELOPMENT OF LIQUID SCINTILLATION CHEMICALS IN BARC
K.A. Noras and CM. Paul
6 DEVELOPMENT OF PLASTIC SCINTILLATORS IN BARC
A.N. Rangarajan and CM. Paul
7 PLASTIC SCINTILLATOR SPONGE AND THIN FILM DETECTORS DEVE-LOPED FOR CONTINUOUS ON-LINE MONITORING OF TRITIUM IN WATERAND AIR
A.N. Singh, C.K.G. Nair and M. Rathnakaran
PAPER - 1
CRYSTAL GROWTH AND EVALUATION OF MATERIALS FOR SCINTILLATION
DETECTION AND THERMOLUMINESCENCE DOSIMETRY
M.P. Chougaonkar, V.H. Gokhale, S.M.D. Raoand R.V. Srikantaiah
CRYSTAL GROWTH AND EVALUATION OF MATERIALS FOR SCINTILLATION
DETECTION AND THERMOLUMINESCENCE DOSIMETRY
M.P. CHOUGAONKAR, V.H. GOKHALE*, S.M.D. RAO*and R.V. SRIKANTAIAH*
Environmntal Assessment Division^Technical Physics and Prototype Engg. Division
Bhabha Atomic Research CentreBombay 400 085
SYNOPSIS
Lithium Fluoride (TLD-100) and Calcium Fluoride (TLD-2OO)
are two well known TL phosphors marketed by Harshaw. TLD grade
LiF doped with 100 ppm of each Mg and Dy has been developed. The
commercially available LiF was used for the work after- repeated
distillation. The crystals grown were then crushed to 100-200
tayler mesh size. It was found that the phosphor thus developed
compares well with the TLD-100 phosphor. The performance as
regards the gamma ray sensitivity, linearity, etc. are
discussed.
Calcium fluoride doped with about 0.04% Dy was developed.
The starting material was CaF2 ultrapure obtained from the
Chemistry division. The Dy doped crystals were grown by the well
known Bridgmann technique under high vacuum. It was found that
the phosphor in the form of either cleaved plates or powder were
better than TLD 200. The performance is discussed.
Scintillator materials like CaF2 and BaFs also have been
grown. CaF2 (Eu) crystals were grown with about 0.6% of Eu3 as
dopant. The crystals thus grown were then cut and polished to get
the optical transparency. The performance studies like the alpha
and gamma ray spectra, linearity of response, have been carried
out. The details are discussed.
Barium Fluoride crystals also have been grown with the same
technique. The starting material '.as laboratory made BaF2.
Crystals upto 40 ram dia have been grown. Their properties have
been under study. Some of the experiments and their results are
discussed.
1. INTRODUCTION
Growth of good quality crystals is an important tool for
materials to be used in nuclear radiation detection. This is
mainly due to the optical transparency required for scintillation
detectors and also the uniform distribution of dopants. The
fluorides of alkali and alkaline earth metals pose special
problems due to the fact that they decompose when heated in air.
The paper discusses the growth technique of some of these
materials and also their performance as regards their use in
nuclear detection.
2. INSTRUMENTS
Two separate equipments will be described here. The first
is a distillation set up for lithium fluoride and the other is
vacuum furnace for growth of the crystals.
2.1 Lithium fluoride distillation set up:- It has been found
that the commercially available LiF is not at all good for ob-
taining transparent crystals. It is essential that the LiF thus
obtained is purified. A simple set up was designed by SMD Rao< i >
and was used for the work. Figure 1 shows a schematic of the set
up. It essentially consists of a vacuum chamber in which a
nickel cylinder partly filled with LiF is placed. The open end
of nickel tube faces a water cooled cold finger. The chamber un-
der a high vacuum of the order of 10-5 Torr is heated externally
by resistance heater furnace to a temperature slightly above the
melting point of LiF. After a prolonged heating for about 7-8
hours the chamber is slowly cooled and the cold finger removed.
The distillate collected on it and also on the upper wall of Ni
cylinder is collected. Lithium fluoride thus distilled is then
subjected to next cycle of distillation. Three or four such dis-
tillations are needed to obtain a sufficiently good starting
material. Table-1 shows a typical impurity analysis of LiF by
ICP method after repeated distillations.
Table -1 : Impurity Analysis of LiF
(Impurity content given in r>v>m)
LiF Fe tin Pb Mg Al Ti Ni Ca Na K
Starting
material 100 10 50 50 200 20 10 200 50 50
After 2nd
distillation <10 <10 <10 10 <10 <10 <10 10 10 <10
2.2 Vacuum furnace for the crystal growth*2> :- Figure-2 shows
a schematic of the set up. This is a high temperature high vacuum
furnace where these crystals are grown by Bridgmann technique.
It consists of the following parts : The design details are dis-
cussed earlier*2) .
The high temperature high vacuum furnace consists of a
double wall jacketed vacuum chamber(l). A side port (2) is
provided to it for inserting R.F. heater. Another side part (3)
connects the chamber to pumping system through L bend (4). The
dome shaped top (5) is for covering as well as making the furnace
suitable for Czochralski technique crystal growth through the
port(6). The viewing port (7) is used for viewing the sample as
well as monitoring the temperature by optical pyrometer.
Two water cooled electrodes (9), crucible support (10),
thermocouple seals etc. all pass through base plate (8).
A squirrel cage type graphite heater (11) is fixed on the
graphite plates (12) which are fastened to the electrodes. This
heater is surrounded by graphite felt cylinders which act as
radiation shields. Typical temperature profile of a heater is as
shown in a figure-3 and the heater in figure-4. The graphite
crucible (13) sits inside the heater, on the crucible support
which can be lowered through a vacuum tight wilson seal by lead
screw arrangement.
3. CRYSTAL GROWTH :-
The sample with its dopants is accurately weighed and is
throughly mixed. It is then transferred to a graphite crucible
of required dimensions. The crucible is then placed inside the
heater and the furnace is evacuated. After the desired vacuum
has been achieved, the heating is started. Initially the sample
is heated to about 100 -150»C at a very slow rate so that all the
moisture present is evaporated and the chances of hydrolysis are
reduced. In addition, except in the case of LiF about 0.5% of
PbF2 is also added to charge for the same purpose. The charge is
then further heated to about 15-20°C above the melting point and
is kept at that temperature for a time ranging from 1/2 hour to 1
1/2 hours depending on amount of charge and the size of crystal.
The temperature is then slightly reduced to the level of about 3-
5<>C above the melting point and the growth is then started at the
predetermined rate. After the desired length has been grown, the
crystal growth is stopped and the furnace is then cooled down to
room temperature in 24-96 hours depending on the size of
crystals. Figure-5 shows some of the crystals grown in the
furnace.
3.1 Growth of scintillator crystals:- Crystals like CaF2(Eu)
and BaF2 are grown typically in 25 mm, 40 mm or 50 mm dia
crucibles. About 0.6% Eu+3 is added to CaF2 along with about .5%
PbF2 as a. scavenger. As for Barium fluoride, since it is an
intrinsic scintillator, only adequate amount of PbFa is added as
a scavenger. From our experience, commercially available BaFs
has been found to be unsatisfactory. Hence it is being prepared
in the laboratory using following reaction :
Ba(N03)2 + NH4HF2 = BaF2 + NH4NO3 + HNO3 O>
Table-2 shows a typical impurity analysis of BaF2
prepared.carried out by ICP method.
Table - 2 : Impurity Analysis of BaF2
Raw Material
Sample
Ba(NOs )2
Pb(N03)2
Prepared Material
BaF2-TB1
BaF2-TB2
Ca
<5
- BaF2
«.°
Cu
<5
<5
<5
<5
Fe
<25
<25
6
6
Ni
<50
<50
a
Pb
100
<5
7
3.2 Growth of TLD crystals:- These crystals are grown typically
in 6 mm or 10 mm dia 7 hole crucibles. Each of the seven holes
acts as a separate crucible and seven crystals are grown simul-
taneously under identical conditions. For LiF (Mg, Dy), 100 ppm
of each of Mg+2 and Dy+3 are used as dopents while for CaF2 (Dy),
about .04% of Dy+3 is used as a dopant.
4. EVALDTION :-
Fig.6 shows a typical glow curve of LiF (Mg, Dy) grown by above
technique. It is seen here that the performance is comparable to
that with commercially available TLD 100 from M/s Harshaw. The
other studies like fading with time, response vs annealing cycles
etc. are discussed earlier < *) .
Fig. 7 shows a typical glow curve of CaF2 (Dy) thus grown.
The fading studies were carried out with these samples. The
crystals were powdered and sieved between 100 - 200 taylor mesh
and 30 mg packets were used for the work. The samples were all
irradiated simultaneously and the TL output was recorded after
various intervals. Fig. 8 shows the fading characteristics of
CaF2 (Dy) prepared.
Fig. 9 shows a x ray spectra of CaFz(Eu) for various
energies. Fig.10 shows the Am24* alpha spectrum*5> .While the idea
was only to optimise the Eu+8 concentration, after the Eu+3 con-
centration has been standardised, the work for making these scin-
tillator crystals for tritium detection etc. will be taken up.
Various studies have been carried out for BaF2 crystals
grown. Fig. 11 shows an optical transmission spectra of a 40 mm
dia x 50 mm crystal. It is seen that in the UV region the trans-
mission is around 30%. While the good crystal should show about
30% of transmission, the best resolution at 660 KeV gamma (Cs137)
has been found to use about 25% whereas the best reported value
is about 13%. Efforts are being made to improve the quality of
crystals.
Fig. 12 shows a typical gamma spectra of Cs*37 . It is
felt that the resolution is affected by Pb present in the
crystal. However, it is very difficult to grow a good quality
large dia BaF2 crystal without PbF2 as a scavenger. Optimisation
of PbF2 content is under investigation.BaF2 scintillates in UV
range. Its two components being at 220 nm (decay time .6ns) and
310 nm (decay time 600 ns). This requires special quartz window
PMTs, suitable optical couplings, UV reflectors etc. We are plan-
ning to work in the direction of shifting the light to near
visible range where common PMTs can safely be used. This could be
acheived by doping BaF2 with Ce in which case the wavelength is
shifted to 365nm with a decay time of 50 ns.(6)It could also be
achieved by vacuum coating of p-terphenyl on the BaF2 crystal.*7)
After the satisfactory performance of the small crystals,
growth of large crystals will be initiated.
REFERENCES:-
1. S.M.D. Rao, Ph.D. Thesis.
2. A versatile high temperature high vacuumfurnace for crystal
growing, S.M.D.Rao and M.P. Chougaonksr, 1st National Sem.
on Crystal Growth, Madras, 1982.
3. S.M.D. Rao, M.P. Chougaonkar et al, Proc. of National Symp.
on Nuclear Electronics and Instrumentation, Feb.15-17, 1989
PP 72-77.
4. Development of Lithium fluoride (Mg, Dy) phosphor for Ther-
moluminescent dosimetry, P. Ayappan etal. INd. Jl. of Pure
and Appl. Physics, Vol.19 (1981), P P 323-325.
5. Preparation of CaF2(Eu) single crystals and study of their
scintillation properties, S.M.D. Rao, L.H. Peshori, Proc.
of National Symp. on Rad. Physics, Vol. XXVI, JUne 1976,
PP 389-393.
6.- R.C.Tailor et.al. IEEE Trans.Nucl Sci.NS 33.
7. E.Dafni Nucl ins.and methods.A254 (1987) 54-60.
( 3 ) TO VACUUM *SYSTEM
NICKEL FOIL
(L)NICKEL CONTAINER
WATER COOLER COILS@
LID®
STAINLESS STEEL VESSEL(T
COLO FINGER ( ? )
STAiNLESS STEEL TUBE
FIG. 1 LiF DISTILLATION SET UP
FIG. 2 SCHEMATIC DIAGRAM OF VACUUM FURNACE
oen
O(Xu.IDU2
28 r-
20
16
8
0600
| l__~700 X800 900 1000
TEMPERATURE °C
1100
FIG. 3 TEMPERATURE PROFILE OFA GRAPHITE HEATER
FIG. 4 GRAPHITE HEATER FOR CRYSTAL GROWTH
BaF2
AS GROWN
g 1 ' 7 1 ' ? ' ' 9 ! ' 110 ' 111 ' 112
FIG. 5 PHOTOGRAPHS OF BaF2 CRYSTALS
zo_ Jr—1
o1
oo
IAR
SH
/
*~*
PCD
—1
]0o1—
oO
oURV
ES
o
m"0m>
mo°
o
oo
>JOn
LIGHT EMITTED (ARB. UNITS)
5> o ro
<0
CD
oo
300*
QUJ
UJ
100 200
TEMPERATURE °C300
FIG. 7 GLOW CURVE OF CaF2 : Dy GROWN
BY BRIDGMANN TECHNIQUE
O
o
CM
(O
CMO»
2 </>
sga:a.
CM
IDO•
O
o
auu.o
UJ
a
z
i00
S2
(D N O «r- «- ©
(O <4 (M• • •o o o
aav) AIISNSINI 11
90
80
70
60
CD
a. 50<
30
20
10 / \ f\
. i
50 100 150
CHANNEL NUMBER
30
20o
zUJ
200
FIG. 9 GAMMA RAY SPECTRA OF CaF2 (Eu)
50 100
CHANNEL NUMBER
150
FIG. 10 Am241 ALPHA SPECTRUM
80
190
BaF2 CRYSTAL F8
4Omm0 x A8mm HEIGHT
GROWN ON 23-10-1990
474 616
WAVELENGTH A (nm)
758 900
FIG. 11 OPTICAL TRANSMISSION OF BaF2
V)»-zz>oou.octUJ00
s
137SOURCE : Cs (660keV)P. M. TUBE : EMI 9556 QBBIAS : U00 VAMPLIFIER : ORTEC 572TIME CONSTANT 2/ULS
BaF2 CRYSTAL F8
40 mm 0 x 48 mm HEIGHT
GROWN ON 23-10-1990
CHANNEL NUMBER
FIG. 12 GAMMA RAY SPECTRA OF BaF2 CRYSTAL
PAPER - 2
DEVELOPMENT OF THERMOLUMINESCENT MATERIALS FOR RADIATION
DOSIMETRIC APPLICATIONS AT DRP, BARC.
Bhuwan Chandra Bhatt
DEVELOPMENT OF THERMOLUMINESCENT MATERIALS FOR RADTATTOIfDOSIMETRIC APPLICATIONS AT DRP, BARC.
Bhuwan Chandra BhattDivision of Radiological Protection
Bhabha Atomic Research Centt.frTrombay, Bombay 400 085
1. Introduction
The programme of development of TL materials for various
radiation dosimetrie applications was initiated in DPP in the
year 1969.The materials initially tried were Lii.-B4f7.-Mn and
CaS0->:Dy. L12B.1O- :Mn was being developed d,j«.' to i •• • tissue
equivalence (zy f i =7 . 4 j and possible applications in tht,- field of
Medical Physics. CaSCU:Dy (Ze 1 1 = 15.3) was being tried du.-i t<
its reported^'•2' high TL sensitivity (CaSO4:Dy is about 100
times more sensitive131 than LizB-iOTiMn and 33 ti:;u-s niori;
sensitive than LiF(TLD-lOO). Other materials that were tried
[4,5] subsequently were Li;>B.iO-:Cu and MgB-i O- : Dy (7,-ti = 8.4)
having their respective TL sensitivities 1.5 and 2 times than
that of LiF TLD-100. However, out of all these materials tried,
CaSO.i : Dy was found by -is to be most suitable material f<u various
radiation dosimetric applications. This is bt-i.aus*- of two main
reasons : (i) Its high TL sensitivity and (ii) the simple method
of preparation of this phosphor as compared to other TL materials
which involve firing of the mixture of the host and the
appropriate dopant at high temperatures in a muffle furnace.
Thus CaSO4 iDy/CaSOi:Tm TLD phosphors in various physical fot^s
such as loose powder (in the grain size range 74-210 urn) ,
phosphor encapsulated glass capillaries, phosphor embedded teflon
discs etc., find various applications in thw fi«ld of radiation
dosi^etry.
The aim of this review is to highlight the properties and
applications of these CaSO-i :Dy/Tm based TL dosimeters and iheir
various physical forms in the field of radiation dosimetry. The
review concentrates mainly on the developments and R « D efforts
that took place in the Division of Radiological Protection for
the use of this system in various dosimetric applications during
last twenty one years.
2. T L D Materials Investigated
2.1 LizB4O7 :Mn (0.1% by wt) was prepared by following the method
of Kirk et al* 6 ] and Christensen et alt7' by mixing Li* CO3 and
H3BO3 in the ratio of 1:2 respectively alongwith appropriate
amount of the activator and drying the wet mixture at 80°C for 16
hrs. The dried and finely powdered product is fused in a
platinum crucible at 950°C for 30 min and then quenching the
molten mass to room temperature. The phosphor is used for
dosimetric applications in the grain size range 74-210 urn. Its
usable dose range is 5 mGy to 103 Gy. The main factor in favour
of lithium borate has been its tissue equivalence. Due to its
tissue equivalence it finds applications in patient dosimetry in
the field of medical physics.
2.2 L12B4O7 :Cu
Subsequently LiaB-tOrrCu TL phosphor was prepared! i ] by
sintering at 900°C for lhr the raw L12 B4 O7 along with an
appropriate amount {0.05% by wt) of Cu2* activator. The TL
sensitivity of this phosphor was 0.64 of the reported value by
Takenaga et al f 8 ]. Its TL sensitivity was twice the sensitivity
of LiF TLD-700 (or TLD-100) and five times that of LizB-jOvrMn.
The advantages of this phosphor are: (i) its TL emission is at
360 nra as compared to red emission (600 rim) given by Li2B«07:Mn
and (ii) a linear dose response from 0.3 mGy to 3 x 102 Gy
beyond which its TL response saturates.
This phosphor has advantage over Li2B.iO7:Mn for medical
physics applications because of its higher TL sensitivity and
linear dose vs TL response. If stored for long duration (nor<-
than one year) the phosphor has a tendency to absorb moisture and
form solid lumps. In such a situation the phosphor needs to be
crushed lightly and annealed at 300°C in order to remove
moisture.
2 . 3 MgB*O7:Dy TLD Phosphor
The phosphor MgBiO-:Dy (0.5 mol%) was prepared^9' by making
a mixture of MgCOa and Ha BO3 in the weight ratio of 1 : 2.5 -=ilontj
with an appropriate amount of DyzOa (dissolved in dilute nitric
acid) was dried in a Teflon beaker at 100°C for 16 hr. The dry
residue was finely ground and then kept in a platinum crucible
and sintered in a muffle furnace at 950°C for 2 h and then it was
quickly cooled to room temperature {argon gas was flushed over
the sample during cooling). The TL sensitivity of this phosphor
was 1.5 times that of TLD-100. Its TL response was linear in the
dose range 1 mGy to 40 Gy and supralinear above 40 Gy. Its
usable range was from 1 mGy to 10" Gy. Because of its
hygroscopic nature and high fading (40% in 17 days) it could not
become a practical dosimeter.
2.4 CaSOi :Dy/Tm TLD Phosphors
CaSCU :Dy and CaSOq :Tm phosphors were initially prepared in
small batches of 10-20 g in the laboratory by fallowing th&
method described originally by Yamashita et al.' 1 2' TT i->-i<=
method CaSCU :2HzO powder (analytical reagent grade) is dissolved
in cone. H2 SO4 (analytical reagent grade) along with appropriate
amount of . the required activator. The resulting solution is
evaporated at a temperature of about 270 - 300°C to dryness in
order to obtain polycrystalline phosphor. The complt.'t>- process
of evaporation takes about 4-5 hours in order to obtain clear
crystallites of the luminescent material. The product so
Table 1. Dosimetric characteristics of common TL phosphors.
TLD Phosphor Relativegamma raysensitivity
TL emissionspectrum
(nm)
Dosimetricpeak temp.
Detectionthreshold
Effectiveatomicnumber
Photonenergyresponse'a)
TL fading ofdosimetric peakat 25°C
LiF:Mg,TiLiF:Mg,Cu,P chips .
Li2B<07:CuLi2B<07:NnMgB<07:DyCaSO<:TmCaSCU:0yCaF2:MnCaF2 (natural)CaF2:DyMg2Si04:TbAl2O3:Si ,Ti<x-Al2O3:C s i n g l e
crystals
129
2-30.47
32385
231653
5
50
400360,410
368600
480,570450
480,570500380
480,570380,552
420
420
190210
215210210220220260260
200,240195250
190
5060
100.210
55
1010
55
10
1
M<5ynGy
tiGymGypGyyGypGypGyMGyMGyMGypGy
pGy
88
778
15151616161110
10
.2
.2
.4
.4
.4
.3
.3
.0
.3
.0
.0
.2
.2
1.351.00
0.90.91.5
10-1210-1213-1513-1513-154.53.5
3.5
5%/monthno fading
in one month9%/month10% /month<10%/month
1-2%/month1-2%/month10%/month3%/month10VYear<>>>3%/month5%/tvo weeks
3%/month ( r t )
(a) TL sensitivity at 30 keV relative to that at 1.25 MeV
(b) It was subjected to post-irradiation annealing at 100°C/20 min prior to readout
(in the absence of this treatment the TL fading in this phosphor is 25% per month)
(c) Detection threshold is highly dependent on photomultiplier tube used.
(d) Akselrod, M.S. et al. Radiat. Prot. Dosim. 31, *5 (1990).
obtained is repeatedly rinsed in distilled water in order to
remove the traces of acid adhering to the crystals of CaSO4:Dy /
Tin. The product is dried to remove truces of water and then
annealed at 600 - 700°C for 2h in order to stabilize its TL
characteristics. The resultant phosphor is ground and grains in
the desired grain size are obtained by using the sieves of
appropriate mesh size. The laboratory scale preparation helped
u s f3,35] to make samples with various concentration of the
activators so as to exploit these phosphors for various
dosimetric applications. For personnel monitoring use we arrived
at a Dy3+ concentration of 0.05 mol % in CaSO<i so that the
phosphor has optimum sensitivity to gamma rays and negligible
thermal neutron response (equivalent to 3.8 mGy of Co-60 gamma
rays per 1010 ncnr2). Table 1 compares the TL sensitivities of
some common TLD phosphors which are also available commercially
from many international manufacturers.
After standardizing the phosphor, we requested chemistry
Division, BARC to devise a process for the large scale
manufacture of the phosphor for its use in personnel monitoring.
Chemistry Division developed ci°J a large scale process for the
manufacturer of this phosphor; they make 900 g CaSO4:Dy (0.05
mol%) per batch. Presently, DRP is getting about 6 kg phosphor
per year from them. The phosphor being made by them is quite
satisfactory. For personnel and environmental monitoring use,
the selection criteria for a particular phosphor batch is the
following:
If the TL sensitivity of the supplied batches is within ± 5%
(lo) of the standard batch then they are selected; the standard
batch was chosen from one of the earlier batches (typically five
batches were taken for selecting the standard) when they were
compared with a small batch (10-20 g batch) prepared by us in the
laboratory. The TL sensitivity variation within a batch (intra-
batch sensitivity) should be less than ±5%(lo). Fig.l shows the
typical TL glow curve for a large size batch (made by Chemistry
Division) for a test dose of 0.1 Gy and Table 2 shows the
comparison of TL sensitivity of 6 batches made by l?rge scale
method.
Table 2 : Relative TL sensitivity of CaSCU :Dy powder (0-74 pm.size) from different batches (900 g each)
Batch number Relative sensitivity
Standard 1.00
1 1.02
2 1.05
3 1.01
4 1.00
5 1.01
2.5 Development of CaSOi:Dy Filled Glass Capillaries
After devising a process to make CaSO^:Dy TLD powder in
large quantities, it was essential to explore its use as a
dosimeter for large scale use in personnel and environmental
monitoring. The handling of loose powder in routine use is
somewhat cumbersome. Therefore, initially phosphor fillwcj glass
capillaries (yach of about 2 rnai iflia find 10 mm length and each
containing 20 ng CdSO* :Dy) were di-.-velopedr ' ' > . They were used
for various applications in medical physics particularly in,
patient desimetry, mapping of dos«-distribution fro™ teletherapy
rnachines.
For personnel monitoring use, a pocket dosimeter containing
two CaSO.i :Dy powder {grain size 74-210 pro) filled glass
capillaries was developed. In the pocket dosimeter'12', a 0.5 ,-ii.v
thick lead filter {cylindrical in shape) with 10% perforatLons
yielded a uniform response (within ± 10%) in the energy range 33
to 1250 kaV. The response of the pocket dosimeter was reported to
be linear in the exposure range (2.5 x 10"6- 2.58 x 10-MCkg-1
i.e. 10 mR to 1000R.
2.6 Development of CaSO< :Dy Embedded Teflon TLD Discs
Since the phosphor filled glass capillaries are somewhat,
fragile and hence need extra care during their use and handling
during readout. Subsequently, phosphor embedded teflon TLD discs
{each of diameter 13.3 mm and thickness 0.8 mm) were
developed1'33 . For this CaSCu :Dy powder ( in the grain size 0-74
j.m) and Teflon powder grade 7 A {average particle size 35 \iv\) nrt-
mixed in the ratio of 1:3 respectively. Discs each of weight 280
ing are made by cold compaction of CaSO4 :Dy and Teflon mixture in
a hydraulic press. These cold pressed discs are given a moulding
treatment at 400°C for 1 hr in order to give them strength and
flexibility. The mixing of CaSO*:Dy and Teflon powder is done at
liquid nitrogen temperature as the teflon powder is available
from manufacturers in the form of lumps and at ambient
temperature it is difficult to mix them uniformly by adopting
usual blending techniques used for mixing inorganic powders.
3. Applications in Radiation Dosimetry
3.1 Personnel Monitoring TLD Badge using CaSO< :Dy Embedded
Teflon TLD Discs
A TLD badge based on CaSO4:Dy Teflon TLD discs was
developedr'4'. TLD badge (Fig. 2) consists of a plastic cassette
containing three Teflon TLD Discs (13.3 mm dia and 0.8 mm thick)
that are mechanically positioned on circular holes {12 mm dia)
in an aluminium card. A thin paper strip with printed
information about the badge and the user is inserted in the badge
as a wrapper of the card along with a plastic pouch (t£tal
thickness the paper wrapper and plastic pouch is 13.5 mg/cm2).
There are three well defined areas in the badge corresponding to
the three TLD discs. These are {1) a circular filter combination
of 1 mm Al and 1 min Cu (the copper filter is nearer to the TLD
disc), (2) a circular open-window region and (3) a region with a
3.5 mm thick plastic filter over the TLD disc in the middle
portion of the badge. A clip attachment affixes the badge to the
user's clothing. Fig.3 shows the photon energy dependence of the
TLD badge. The usable gamma dose range of the TLD badge for
personnel monitoring applications is 0.2 mSv to 10 Sv. The badge
can be reused for more than 15 cycles. The badge is meant for
monitoring, generally, beta, gamma or X-, gamma-radiations in our
nuclear installations including nuclear power stations,
industrial, medical and research institutions. Presently, out of
the total of 38,000 radiation workers in the country, about
20,000 are covered by TLD monitoring and the remaining are
monitored with photographic film badge.
The readout procedure currently being used for the personnel
monitoring TLD cards leaves certain amount of TL as the residual
TL in the TLD cards. As a result of which the second readout'1S'
of the same disc (in the TLD card) is typically about- 10-11% of
the first readout (Fig.4). This residual TL is used to advantage
for verifying the first measured dose (by taking the second
readout) in certain situations when the personnel monitoring TLD
card receives gamma doses greater than 10 mSv.
The personnel monitoring TLD badge has been usedt*6•*7' in
two International Intercomparisons of Personnel Dosimeters. The
performance of our dosimetry system was quite satisfactory in
these intercomparisons.
With the help of locally available teflon moulding companies
we have developed t » «• • « *» 1 techniques for the manufacture of thin
discs (0.1 to 0.8 mm thick) by making either CaSO« :Dy teflon tape
or by moulding CaSOi :Dy teflon rods of desired diameter and
cutting out discs of required thickness. These thin dosimeters
were prepared 120,21] either by cementing 0.1 mm thin CaSO.) :Dy
teflon discs to the 0.7mm teflon (pure) base or by moulding
CaSO4:Dy teflon discs containing certain percentage of graphite
powder. These thin dosimeters helped'z*• s8J in improving thy
beta energy response (Fig.5) as compared to the 0.8 mm thick
CaSOn :Dy Teflon discs which are being used in the present TLD
badge.
For improving the performance of the present TLD badg*.- in
terms of faster readout for a large scale personnel monitoring,
efforts are being made either to develop TLD badge with thinner
detector or by using heating methods which would enable faster
readout of the dosimeters. Following efforts are being made :
(i) TLD cardt23' based on 0.4 mm thin tape incorporating
antibuckling device (0.4 mm thin sheet of mica) in order to
avoid buckling of the teflon tape during readout. This
design enables the TLD card readout in the presently
available TLD readers (without any major modification) with
lesser time and better reusability.
(ii) The hot air/hot nitrogen method of heating is being
developed by using modified TLD card design and using < 0.4
mm thin CaSO4:Dy teflon discs. Initial experiments in the
laboratory tend to prove that the readout time could be
reduced to 5-10 seconds from the present readout time of
60 seconds.
(iii) Laser (CO2 laser emitting at 10.6 pm) heating method and
using thin teflon discs (0.1 to 0.4) as well as thin
substrate dosimeters of CaSO4:Dy either on polyimide film or
on thin alumina ceramic base. Initial experiments using COz
laser beam and thin teflon discs (0.2 mm), it is possible to
perform readout of these dosimeters in about 15 seconds.
The development of thin substrates of CaSO* :Dy detector is
underway for their trial with CO2 laser beam.
3.2 Environmental Gamma Dosimetry
Due to photon energy dependence of CaSO«i :Dy teflon TL
dosimeters, their response should be compensated appropriately so
as to get nearly a flat response for low energy photons (<200
keV). An environmental dosimeter pack based on CaSO<:Dy phosphor
embedded teflon TLD discs was developed!2*' for background dose
measurements in our laboratory. It consists of eight CaS04:Dy
Teflon TLD discs which are sandwiched between a pair of 1 mm
thick copper discs (dia 50 mm) and contained in a plastic box of
2 mm wall thickness. The minimum measurable gamma ray exposure
with these discs is 3± 0.2 mR(lo). We participated in three
International Intercomparisons of Environmental Dosimeters held
in USA. Table 3 shows that the results of our participation in
these International Intercomparisons were quite satisfactory.
Subsequently efforts, have been made1s6> s 7 ] in our laboratory
to arrive at suitable filter thickness/combination alongwith
CaSOi :Dy detectors so that a compact dosimeters could fulfill the
ANSI requirementsi251 for environmental dosimeters. A TLD badge
containing pair of detectors were used ts6.s7]f. o n e under metal
filter combination (0.8 mm Al + 0.55 mm Sn + 0-35 rcm Cu) and
other under a composite filter (1.6 mm plastic + 0.8 mm Al). It
Table : 3. Results of the International Intercoiparison Experiieots onEmroniental Dosiieters""
Intercos- Field Exposure Laboratory Exposureparison — - -site DRF Hein of Organizers value DRP Mean- of a l l Organi-
value a l l value dosi ie ters zers(iR) dosi ie- Exposure Exposure UR) (iR) value
ter (iR| rate liR)
(iK! (pR/h)
S.Y.,1976 13.8+2.12 16.4+3.85 17.1+0.98 7.9 Id.4+2.94 18.8+3.82 21.3+1.1
Oak Ridge,1977 30.8+1.9 31.9 34.9 45.4 83.5+5.04 86.2 91.7Houston,Texas, 13.7+2.16 16.0+4.5 14.1+0.7 5.6 (I) 10.5+2.57 12.0+3.8 12.2+1.21979 (11)41.8+5.63 43.916.6 45.814.6
was found t h a t from the two s e t of d o s i m e t e r s ( i n the TLD b a d g e ) ,
the energy independent response (R) can be e v a l u a t e d by the
following equation:
R = 0.9A + O.I C (1)
where A is the readout of the disc under combined filter O.8 mm
Al + 0.55 nun Sn + 0.35 mm SS, and C is the readout of the disc
under 1.6 mm plastic + 0.8 mm Al filter. These readouts provide
response *R' with an uncertainty of ±20% in the energy range 27-
1250 keV. Thus there exists a possibility to develop an
environmental dosimeter based on CaSCM :Dy Teflon TLD discs.
Recently Vinten Instruments (UK) have developed'26' an
environmental TL dosimeter based on four 0.4 nun thick Cascu :Dy
teflon TL discs and two sets of filters(i) 3 mm Cu and 0.9 mm Al
and (ii) 4 mm PTFE and combined response is obtained by using an
algorithm
D3 o = 0.1 Dp + 0.9 D (2)
where D30 is true dose from 30 keV to 3 MeV; DPis dose
indicated under plastic filter and Da is dose indicated under
metal filter. By using the above eq. (2) the dosimeter response
could be made energy independent within ±30% from 30 keV to 3
MeV.
3.3 Ultraviolet (UV) Dosimetry
In view of the NIOSH1871 report recommending standards for
UV exposures and the likely adoption of similar standards, it was
expected that new techniques and instruments will be needed to
measure low level U.V. radiation. Thermoluminescence radiation
dosimeters and readers have been developed . over the past 3
decades and are now widely used for the detection of ionizing
radiation in personnel and environmental monitoring as well as in
medical physics applications. Many TL phosphors have been
investigated as thermoluminescent UV radiation dosimeters. Among
them are CaSO* :Dy, CaSCU :Tm, CaSO« :Dy, Cut 29 J , Mg2SiO<i:Tb,
L^BiChiCu, and CaF2 :Dy (thermally treated at 900°C) . Table 4
gives'281 the comparison of intrinsic TL response of common TLD
phosphors to UV radiation of wavelength 254 nm.
Another technique that is generally adopted for UV dosimetry
is by using photo-transferred TL {PTTL). This involves [31-34]
exposing the TL phosphor to a high gamma dose and post-annsaling
at a higher temperature so that the main dosimetry peak is
completely erased but leaving a higher temperature peak as
residual TL (RTL) peak in the sample. On giving a UV exposure
(generally at 253.7 nm) to the treated sample, electrons/holes
trapped at the high temperature peak are released and become
mobile to get trapped at lower temperature TL peaks {which were
empty). The intensity of PTTL is proportional to UV exposure
within a certain range of UV exposure.
used for a number of TL phosphors.
CaSCU :Ttn, LiF:Mg,Ti and MgzSiC^iTb, etc.
PTTL technique has been
For example, CaSO< ;Dy,
Table 4. Intrinsic Response of TL Phosphors to 254 nm UV Radiation12a
Phosphor
CaSCM :Dy
CaSCu:Tm
CaSCM :Dy, Cu
Liz Bi 0- : Cu
AI2 O3 :Si,Ti
ActivatorConcentration
0 .1 mol .%
0.05 mol %
0.1 mol %
Origin
DRP, BARC
-do-
-do-
MatsushitaElectricCo., Japan
H.P. DivisionB.A.R.C.
Relative TL response
1
80
40f 2 9 3
50[3 °'
30
CaF2 :Dy (TLD-2C0)
2 SiO-t :Tb
Harshaw chemicalCo., U.S.A.
Dai Nippon ToryoCo. Ltd., Japan
308
3.4 Microwave Dosimetry
Microwave radiation which forms a significant part of the
non-ionizing radiations and has been in use in telecommunications
and diathermy, has now entered into modern kitchens through the
extensive use of microwave ovens. Recently, in India also such
ovens have been marketed by some manufacturers for domestic use.
Microwaves have in,sufficient quantum energy to cause ionization
in biological systems but the absorbed energy is transformed into
kinetic energy in the absorbed molecules thereby producing tissue
heating. This, in turn, can result in biological damage when an
excessive exposure to microwave radiation occurs. With a view to
develop a microwave dosimetry system, the effect of microwave
radiation on certain thermoluminescent phosphors has been
investigated^34 1 .
Gamma irradiated TL phosphors (in powder form) when exposedto microwave radiation (using CW diathermy unit : 2425 + 25 MHz,maximum irradiance level 2000 Wm- 2), a reduction in gamma inducedTL output was observed. The percentage reduction in integratedTL outputs (integrated between 25-300°C), for 1 h microwaveexposure at an irradiance level of 500 Wm-2 is given in Table 5.
Table 5. Reduction in integrated TL output of variousphosphors for microwave irradiation. Gamma exposure: 2.58 x 10"3 Ckg-l; microwave exposure : Ihat 500 Wm-2 < 3«>
Phosphor Percentage reduction
BaSO-i :Tb 78
CaSO4 :Mn 96
MgF2 :Mn 9
MgF2:Tb 25
MgFz :Tm 60
The reduction varies from one phosphor to another. CaSO-i :Mn
exhibits maximum reduction (96%) whereas BaSO« :Tb shows a
reduction of 78% for a similar microwave exposure. Reduction in
integrated TL output of BaSOa:Tb was proportional to the
microwave radiant exposure (product of the irradiance level and
the exposure time) in the range 250 - 2000 Wm-2.
3.5 Neutron Dosimetry
3.5.1 Thermal Neutron Response
TL phosphors such as LizB^O-rMn, Li2B4O-:Cu LiF:Mg,Ti (TLD
100) and 6LiF:Mg, Ti(TLD 600) are highly satisfactory for the
dosimetry of thermal neutrons. Table 6 shows the thermal neutron
sensitivity of some common TL phosphors'3!. Since many other TL
phosphors have negligible sensitivity to thermal neutrons, pairs
of dosimeters can be used for mixed field (gamma ray and thermal
neutron) dosimetry. Very low thermal neutron doses can also be
measured by mixingf3sJ TL phosphors (e.g. CaSOi:Dy or Mg2SiO<i:Tb
etc. having high sensitivity to alpha or beta or gamma rays with
pure chemical powders containing elements of high thermal neutron
cross-section (6 Li2 SO4 etc).
3.5.2 Fast Neutron Dosimetry
In view of the current concern for low dose effects of high
LET radiations and the consequent proposals for increasing the
quality factor for neutron irradiations by a factor of 2, a lot
of importance is being attached to neutron monitoring.
One of the methods of fast neutron dosimetry is based on the
activatison of some of the constituent elements of the material
using threshold nuclear reactions!36 I . The TL detectors are
annealed after neutron irradiation to erase the TL induced during
irradiation. Then they are stored to permit self-irradiation
from the internal radioactivity (induced due to neutron
irradiation) for time comparable to the half-life of the
activated element. The post-irradiation induced TL provides a
measure of neutron dose. Among the various available TL
phosphors, CaSd :Dy (using the 3 2S (n, p ) 3 2 P reaction) is most
suitable (o(n,p) at 14 MeV = 250 mb); the 3aS(n,p)3zP reaction
offers the broadest energy range (2.5 - 14 MeV). The 14.3 days
half-life" of 3 2P is advantageous for long exposure time and for
handling after neutron irradiation. The minimum measurable fast
neutron dose by this method*37i was of the order to 0.04 Gy. The
sensitivity of this method has been increased by mixing small
amounts of CaSO<:Dy phosphor (grain size less than 75 um) with
pure sulphur powder'381. The fast neutron efficiency for pellets
with 0.1% CaSO«:Dy has been found to be about 100 times that of
bare CaSO*:Dy powder {Table 7). This method is quite sensitive
and can be used for low level fast neutron dosimetry. However,
this cannot become a practical device for large scale personnel,
monitoring due to tedious procedure of post-irradiation annealing
and subsequent long duration of storage for accumulation of TL in
the sample.
Table 6. Thermal neutron sensitivity of some TL Phosphors' 3 • s3>
Phosphor Main reaction Thermal neutronresponse in Co-60equivalent dost?in tissue (Oy)per 1010 n cm-2
CaS04:Dy (0 .05 mol%)
Liz B« O7 :Mn
LiF(TLD-600)
LiF(TLD-lOO)
LiF(TLD-700)
CaFz :Dy
1 6 <Dy(n ,
6 L i ( n , a )
6 L i ( n , a )
6 L i (n ,a)
6 L i ( n , a )
l 6 « D y ( n ,
T ) t » D y
3HLi
3H
3H
3H
T ) 1 6 S D y
3 . 8
3 .
15
3
1 . 1
5 . 9
X
9
. 2
. 3
X
X
1 0 - 3
io-z
10-3
3.5.3 Albedo Neutroja Dosimetry
In the recent years, TLD albedo neutron dosimetry has become
the most acceptable tool. This technique has two main
advantages: (i) it has no energy threshold and (ii) sufficiently
small doses of neutrons can be measured by using suitable TLDs.
Fig. 6 compares the energy response of albedo technique with that
of the NTA and the CR-39 detectors.
In the albedo method we practically measure the albedo
thermal neutrons and therefore employ a pair*39' of TLDs, one
sensitive to thermal neutrons and the other insensitive to them
as much as possible. In order to develop an albedo technique for
neutron dosimetry in addition to normal CaSO<t :Dy Teflon TLD
discs, thermal neutrons sensitive CaSO*:Dy Teflon discs were made
by adding 6Li-compound (non-TLD grade) to the normal discs*4°5
Table 7. TL accumulation efficiency of the CaSO*:Dy - mixed sulphurpellets, CaSO4 :Dy loose powder and CaSO*:Dy embeddedteflon discs <5 s >
Type of TL per Gy for Accumulated TL per Gy Effici-dosimeter 90Sr-90y for fast neutrons ency %
Ep E u R.i /Efs
10 pel le ts feach lg) 1 0.026 2.6with 0.1% CaSO<t10 pellets (each of lg) 11.5 0.094 0.82with 1% CaSO4 :DyCaSO* :Dy loose powder - - 0.025CaS04:Dy embedded - - 0.013teflon discs
during mixing. Initial experiments (Table 8) with CaS0i:Dy
teflon discs mixed with 25% 6LiF by weight show that the thermal
neutron response of these discs is 2.2 equivalent Gy of Co-60
gamma rays per 1010 ncnr2 which is about l/7th of the
corresponding thermal neutron response of LiF TLD-600. It may be
noted that the thermal neutron response of CaSO4:Dy Teflon discs
is l/4th the response of LiF TLD-700 under similar conditions
(table 6). Therefore, the net differentiation between thermal <
neutron sensitive and insensitive CaSO*:Dy detectors is
compareable to that between TLD-600 and TLD-700. A pair of TLD-
700 and TLD-600 detectors are used in the albedo neutron TLD
badge developed by M/s. Harshaw/Filtrol, USA. Since the TL
sensitivity of CaSO4:Dy is 38 times higher than that of TLD-600,
CaSO«:Dy Teflon discs loaded with 25-30% 6LiF would give
satisfactory response as an albedo dosimeter. The detailed
results of a preliminary experiment*«°> with CaSO* :Dy Teflon disc
detectors are reported elsewhere.
Table 8. Thermal neutron responses of different typesof Teflon TLD discs'«°>
Teflon Discs Equivalent Gy of 60Co gamma
rays per 1010 n cm-2
CaS04 :Dy 2 . 5 x 1 0 - "
6LiF + CaSO< :Dy 2.2LiF + CaSO4:Dy 0.46
LizB-jO? + CaSO< :Dy 0.26
* Lakshmanan A.R. and Bhatt R.C. Int. J. Appl. Radiat.Isot. 28, 665 (1977)
3.6 High Level Gamma Dosimetry
Although the TL output of the dosimetry peak ( " 230°C)
in CaSOi:Dy and CaSOiTm saturates at a gamma dose level of about
2 x 103Gy, the high temperature peak (~400°C) saturates^ - ^ J at
a dose level of about 106Gy (Fig. 7); the TL vs gamma dose
response curve for the high temperature peak rises sublinearly in
the dose range 102 to 106 Gy. Alternatively, photo-transferred
TIJ (PTTL) respose for the dosimetry peak could also be used as it
avoids infrared problems which are prevalent during the readout
of such high temperature TL peaks. For CaSO4:Dy, the PTTL
response for the dosimetry peak saturates at about 5 x 10s Gy.
The materials that were tried for high level gamma dosimetry were
caSOi:Dy, CaSO<:Tm and Li2B«O7:Mn. The saturation dose for high
temperature TL peaks in CaSO^:Tm (400°C) and Li2B4O7:Mnf* * 3
(370°C) were 10$and 10s Gy respectively. The corresponding PTTL
response at the dosimetry peak saturates at doses of 3 x 105 and
10" Gy.
CaSO* :Dy samples containing high Dy-concentration (2 mol %)
show better TL vs dose characteristics and lesser tendency f«ai
for saturation against gamma radiation dose as compared to the
samples containing normal Dy-concentration (0.05 mol % ) . Thus
high temperature TL peak (" 400°C) in CaSOt:Dy (2 mol %) does not
show saturation in its TL vs dose response even upto the studied
dose of 3 x 106 Gy.
4. Recent R & D Efforts leading to New Techniques in
TL Radiation Dosimetry
Investigations have been done in our laboratory in order to
study the effects of additional dopants (co-dopants) on the TL
characteristics of CaSO4 :Dy and CaSO-j :Tm phosphors. Effect of
co-doping in CaSO<:Dy phosphor for augmenting its TL
characteristics as well as for increasing TL sensitivity were
studied. Activator concentration dependent effects were also
studied.
4.1 Effect of Monovalent Cations on the TL Characteristics of
CaSO< :Dy and CaSOi :Tm Phosphors.
It has been observed*43] by us earlier that the addition of
Na* in CaSOi:Dy reduces the intensity of dosimetry peak (230°C)
but enhances the intensity of low temperature {"130°O pe*k (Fig.
8). This study was basically initiated to see the effect of
monovalent charge compensators on the TL characteristics of
CaSOi:Dy phosphor as well as to arrive at the Na4 concentration
below which it does not affect the useful characteristics of the
phosphor. Na* in concentrations < 0.02 mol % in CaSCU :Dy (0.1
mol%) does not affect the TL characteristics of dosimetry peak.
:20:
In addition, at a suitable concentration of Na* (0.5 mol %)
in CaSOi:Dy (0.1 mol % ) , the intensities of low and high
temperature peaks become comparable as well as they are also well
separated. Therefore, due to relatively high sensitivity and two
peak structure, the ratio**61 of low^and high temperature TL
peaks can be used to estimate the elapsed time after acute gamma
exposure {Fig. 9) as well as the magnitude of radiation exposure
from the high temperature TL peak intensity.
The effect of addition of monovalent cations (Na4) has been
investigated in other CaSO*:RE3 + systems. The addition of Na* in
CaSO<:Tm and CaSO«:Sm reduces the dosimetry peak but enhances the
low temperature peak similar to that observed earlier in CaSO4 :Dy
doped with Na* , although the quantitative factors were somewhat
different.
4.2 Effect of Co-dopants in CaSO< ;Dy TL Phosphors
4.2(a) Recent characterization studies in our laboratoryt«7 J
have shown that the linear dose-response relationship and simple
glow curve structure of materials like LiF:Mg,Cu and Li2B.»O7:Cu
(unlike the bulk of TL materials) could be attributed to copper
doping/codoping. In view of this, the materials development
studies have been undertaken to analyse the likely influence of
such impurities (Cu2< and others including rare earths) in
favourably augmenting the TL characteristics of some common TL
materials. Therefore, samples of CaSO4:Dy3 • ,Cu2+ were prepared
and their TL properties have been investigated^« 8 i . Addition of
copper in concentrations less than 0.1 mol% in CaSO^ :Dy (0.1
mol*) effectively eliminates the conspicuous and broad high
temperature peak structure above 275°C normally present in CaSO4
samples doped with Dy alonet33'. CaSO4:Dy,Cu samples show
extended linear response up to 102 Gy (in CaSCU :Dy, the onset of
supralinearity starts from the gamma of 1-2 Gy). For uv
dosimetry (Table 4), with proper manipulation of Cu2 •
concentration (> 0.2 mol %) CaSO«:Dy, Cu could be made as one of
the very sensitive TL materials (> 40 times of CaSO4:Dy).
Table 9. Radiation induced sensitization (S/So)values for CaSO«:Dy samples along withhigh temperature residual TL (RTL) peakintensities. Pre-dose was 103 Gy, Post-annealing treatment 300°C, 1 h and testgamma dose 1 Gy.
Sample
CaSCU :Dy
1 0.05 mol %
2
3
4
5
6
0.1 mol %
0.2 mol %
0.5 mol %
1 mol %
2 mol %
S/So value
2.33
1.86
1.67
1.44
1.82
1.62
RTL intensity(arbitiary units)
22.53
22.71
21.87
12.00
11.37
8.08
4.2(b) It is well known that the optimum concentration of an
individual dopant (e.g. Dy3 • in CaSOi) is determined by the onset
of concentration quenching; beyond this concentration the TL
sensitivity declines. One possible wayt*9J of overcoming this
limitation in sensitivity is to co-dope CaSO4:Dy with another
activator which after gamma irradiation has a TL sensitivity ^nd
glow curve shape comparable to that of Dy (when doped
individually in CaSO« matrix). Initial experiments in this
direction using Tm3• as a co-dopant show that CaSO*:Dy (0.05mol%)
doped additionally with 0.011 mol % of Tm3• show 14 % increase in
:22:
TL sensitivity when compared with either CaSO4;Dy (0.05 mol %) or
CaSO<:Tin (0.011 mol % ) . Efforts are underway in order to
increase further the TL sensitivity of CaSO<:Dy by trying various
combinations of dopants.
4.3 Activator Concentration Dependent Radiation Induced
Sensitizatison and LET Effects in CaSO* :Jy and CaSOi :Tnt
Phosphors.
The radiation induced sensitization factors (S/So) for the
dosimetric peak in CaSOi:Dy and CaSO^:Tm decrease slowly with
concentration (Table 9) from the starting concentration of 0.05
mol %. The decrease of S/So values with increasing activator
concentration yielded extended linearitytso' and lesser
supralinearity in both the phosphors.
In CaSOi:Dy, concentration dependent LET effects have been
observed!s!3 for irradiations (in the grain size range 74-210 pm)
to Am-241 alpha particles and Co-60 gamma rays (Table 10). The
peak height ratio (alpha to gamma response) of the dosimetric
peakc9l 1 for 1.0 mol % doped Dy samples was 1.7 times that of 0.1
mol % Dy doped samples. Similar to CaSO<:Dy, concentration
dependent LET effects have been observed!"2' in CaSO4:Tm. The
corresponding ratio for 1.0 mol % doped Tm samples was 1.5 times
that of 0.1 mol % Tm doped samples. Thus these phosphors could
be exploited for high LET dosimetry.
Additionally, in CaSO4:Dy dependence of TL peak width on
stopping power has been investigated*s3' in the LET range 0.55 to
237 keV pnr* . Analysis of peak width parameters (i.e. TL peak
widths at a quarter (w, ) , half (w,) and three quarters {W3/4 )
of the peak maximum as a function of radiation dose) shows that
the amount of increase in peak width decreases with increasing
stopping power. That is, peak width is maximum for radiation
:23:
Table 10. TL sensitivity of CaSO-i :Dy (height of peak II)for alpha and gamma radiations as a function ofDy concentration. The sensitivity values areonly relative and not absolute because thesample thicknesses used are slightly largerthan the a-particle rangefs* ' .
Dy-Concentration
(mol%)
0.01
0.05
0.1
0.2
0.5
1.0
2.0
5.0
Alpha
2.26
6.82
7.78
9.59
14.06
13.97
12.78
8.05
Height of peak
Gamma
27.6
73.0
71.0
70.0
80.0
75.0
78.0
49.0
II (arbitrary units)
Alpha
Gamma
0.032
0.093
0.110
0.137
0.176
0.186
0.164
0.164
Alpha
Gamma(Normalisedat 0.1 mol%)
0.75
0.85
1.00
1.25
1.60
1.69
1.49
1.49
with low stopping power (0.55 keV unrl ) whereas, no increase in
peak width is observed for radiation with very high stopping
power ("240 keV pnr* ) . This dependence of peak width on stopping
power can be used as a measure of LET of radiation. However, the
additional dependence of peak width on the radiation dose at low
LET has to be considered^s"•'«I.
:24:
5. Conclusions
In addition to the various applications of the TL phosphors
in the field of radiation dosimetry stated above, numerous
studies have been done, in DRP, on CaSO4:Dy, CaSO-i:Tm,
Li2B<0?:Mn, Li2B4O7tCu, LiF:Mg,Ti, CaF2:Tm and many other common
TL phosphors in order to understand the basic TL phenomena,
nature of trapping entities involved in the TL process, role of
non-radiative processes on the TL behaviour of materials,
mechanism of radiation induced sensitization and supralinearity,
activator concentration dependent TL behaviour, photo-transfer TL
(PTTL), study of emission spectra of various TL peaks in
materials, determination of TL parameters for various TL
materials etc. In addition, we have gained from the numerous
detailed investigations on CaSO«:RE3', CaFz:RE3< LiF:Mg,Ti and
AlzO3:Ti TL phosphors carried out in Health Physics Division,
BARC for understanding the basic TL mechanism in these materials.
All these investigations have given us confidence to use
CaSO<:Dy , CaSCU :Tm and other TL materials for various dosimetrio
applications as well as in the development of new and sensitive
TL materials. In future we wish to develop a-Al2Oa:C, CaSO-j :Dy
and CaSO4:Tm phosphors co-doped with suitable metal ions ss
sensitizers of luminescence and rare earth doped double sulphate
systems.
Acknowledgements
I am grateful to Dr. U. Madhvanath and Dr.S.J. Supe for
encouragement and helpful discussions. I wish to thank Or. A.S.
Pradhan for going through the manuscript and giving suggestions
for modification.
:25:
References
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14. Vohra, K.G., Bhatt, R.C., Bhuwan Chandra, Pradhan, A.S.,Lakshmanan, A.R. and Shastry, S.S. Health Phys. 38., 193(1980).
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17. Bhatt, B.C. Paper presented at IAEA/RCA workshop onIntercomparison of Personnel Dosimeters held at JAERI,Tokai-mura, Japan, 22-26 October, 1990.
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(1980) .
22. Lakshmanan, A.R., Bhuwan Chandra, Pradhan, A.S., and Supe,S.J. Radiat. Protect. Dosim. .17. 49 (1986).
23. Lakshmanan, A.R., Kher, R.K. and Madhvanath, U. Radiat.Protect. Dosim. 3C>, 179 (1990).
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27. National Institute for Occupational Safety and Health(NIOSH), Criteria for a Recommended Standard OccupationalExposure to U.V. Radiation, HSM. 73-11009 (1972).
28. Nagpal, J.S. Radiat. Effects Letters 43, 173 (1979).
:27:
29. Srivastava, J.K. Bhatt, B.C. and Supe, S.J. Paper presentedin National Seminar on Thermolumineseence and itsApplications, Feb. 7-9, 1991. M.S. University of Baroda,Baroda - 390 001, India.
30. Lakshmanan, A.R., Bhuwan Chandra and Bhatt, R.C. Radiat.Protect. Dosim. 1_, 191 (1981).
31. Mason, E.W. Phys. Med. Biol. 16., 303 (1971).
32. Sunta, CM. and Watanabe, S.J. Phys. D : Appl. Phys. 9, 1271(1976) .
33. Nambi, K.S.V. and Higashimura, T. Proc. 3rd. Int. Conf. onLuminescence Dosimetry, Riso Report 249 (ARC, RJso,Denmark?, p. 1107 {1971).
34. Nagpal, J.S., Varadharajan, Geetha and Gangadharan, P. Phys.Med. Biol. 27_, 145 (1982).
35. Ayyangar, K., Bhuwan Chandra and Lakshmanan, A.R. Phys. Med.Biol. 19, 656 (1974).
36. Pearson, D.W. and Moran, P.R. USERDA Technical Report COO1105-227 (1975).
37. Bhatt, R.C., Lakshmanan, A.R., Bhuwan Chandra and Pradhan,A.S. Nucl. Instrum. Meth. 152, 527 (1978).
38. Fradhan, A.S., Bhatt, R.C., Lakshmanan, A.R., Bhuwan Chandraand Shinde, S.S. Phys. Med. Biol. 22, 723 (1978).
39. Storm, E., Buslee, P.L., Blackstock, A.W., Littlejohn, G.J.,Cortez, J.R., Fultyn, R.V. and Lawrence, J.N.F. Radiat.Prot. Dosim. 1, 209 (1981).
40. Pradhan, A.S., Bhatt, B.C. and Bhatt, R.C. Bulletin ofRadiat. Prot. (India), 13L, 47 (1988).
41. Alsmiller, R.G. and Barish, J. Health Phys. 26, 13 (1974).
42. Bhuwan Chandra, Bhatt, R.C. and Supe, S.J. Int. J. Appl.Radiat. Isot. 32, 553 (1981).
43. Lakshmanan, A.R. and Bhatt, R.C. Phys. Med. Biol- 2_4, 1258(1979).
44. Pradhan, A.S., Bhatt, R.C. and Supe, S.J. Int. J. Appl.Radiat. Isot:. 3_1, 671 (1980).
45. Bhuwan Chandra, Shinde, S.S., Lakshmanan A.R. .-m<:3 Blvitt,R.C. Phys. Stat. Sol. (a) 1JD3, 599 (1987).
:28:
46. Lakshmanan, A.R., Bhatt, B.C. and Bhatt, R.C. Radiat. Prot.Dosim. 27, 15 (1989).
47. Srivastava, J.K. and Supe, S.J. J. Phys. D : Appl. Phys. 22,1537 (1989).
48. Srivastava, J.K. Bhatt, B.C., Bhatt, R.C. and Supe, S.J.Presented at 3rd CECRI Research conference on Luminescence;,12-14 January, 1990, CECRI, Karaikudi, India.
49. Bhatt, B.C., Srivastava, J.K. and Shinde S.S. Paperpresented in National Seminar on Thermoluminescence and itsApplications, Feb. 7-9, 1991, M.S. University of Baroda,Baroda 390 001, India.
50. Bhuwan Chandra and Bhatt, R.C. Nucl. Instrusm. Meth. 164,571 (1979).
51. Lakshmanan, A.R. and Bhatt, B.C. Radiat. Prot. Dosim. 2J5, 31(1988).
52. Bhatt, B.C., Shinde, S.S. and Bhatt, R.C. 9th Int. Conf. onSolid State Dosimetry, Vienna, Nov. 6-10, 1989 (Abstractonly).
53. Srivastava J.K. and Supe, S.J. J. Phys. D: Appl. Phys. 13_,2337 (1980) .
54. Shinde, S.S. and Shastry, S.S. Int. J. Appl, Radiat.Isotopes 30, 75 (1979).
55. Pradhan, A.S. Radiat. Protec. Dosim. 1, 153 (1981).
56. Pradhan, A.S. and Bhatt, R.C. Nucl. Instrum. Meth.166., 497 (1979) .
57. Vohra, K.G., Pradhan, A.S. and Bhatt, R.C. Health Phys. 4J3,391(1982).
58. Charles, M.W. In Proc. 5th Int. Conf. on LuminescenceDosimetry, Sao Paulo, Brazil, p313(1977).
25 100 200
TEMPERATURE (*C)
300 400
Fig.1. Typical TL glow curve of CaSO^ .'Dy powderfor gamma dose of 0.1 Gy.
FIG. 2 . T L D BADGE SYSTEM
oCARD HOLDER CARD TLD BADGE
(Front view)
1. COPPER FILTER2. TEFLON TLD DISC3. OPEN WINDOW4. TRANSPARENT PLASTIC
WINDOW5. STAINLESS STEEL CUP
ASSEMBLY OF TLD BADGE(Back view partially open)
10
LL)
I
UJ
UJDC
0.10.01 0.1
PHOTON ENERGY (MeV )
Fig.3. Photon energy dependence of the TLD badge( a ) response under the Al*Cu filter,(b) response under the open window.
S 5
II III Iv v
Readout times
vl vll
Fig. 4. T L output of a card exposed to10 R of ^Co gamma rays and read outrepeatedly without subjecting to annealingor exposure.
HI
zoQ.CO
UJ>
UJCf.
1.0
0.8
0.6
0.4
0.2
n
y
A
/B
/
I
r
//
2 M TL11 I
u VQ
/
/
32 90P Sr/Y1 1 i
0.2
BETA ENERGY E M A X IMeV)
Fig. 5.. THE ENERGY DEPENDENCE OF TL DOSIMETERS
TO BETA RADIATION.(A) 0.025mm LiF TEFLON
DISC BONDED TO 0.2 mm TEFLON BASE I Charles
1977).(B) 0.1mm CaSO .'Dy DISC ON 0.7mm PURE
TEFLON BASE AND (C) 0.8mm CaSO^ :Dy TEFLON
DISC.
io3
i 10*
io5
16 V
10 I I
TLD-ALBEDO
1.0
0>
13
0.1 ItilDC
0.01
103 10* Id1 101 10'
NEUTRON ENERGY (MeV)
Fig.6. Energy dependence of T L D albedo, CR-39and NTA Films.
210
190
170
> 150H;
2130 —
110UJ
5 soUJ
70
50
30
10
O -GAMMA RESPONSE(250°C Peak )
X - PTa(25O°C Rak)
O -RTL(400°C P«a<)
£ ~RTL(485°C Pec*)
10 10 10 10
GAMMA DOSE ( Gray)
10
20.000
16.000
12,000
8000
4.000
5x10
F ig . 7. RELATIVE TL INTENSITIES OF THE GLOW PEAKS IN CaSO^ :Oy {0.05 moUS) AS A
FUNCTfON OF PRE-GAMMA DOSE. MULTIPLICATION FACTOR INDICATED ON THE CURVES
WAS APPLIED TO BRING THE INTENSITY WITHIN THE SC/UE.
130 #C Peak
225 °C Peak
375 °C Peak
0.5 1-0 2.0
Na CONCENTRATION (mol •/•)
Fig. 8. Effect of Na4 concentration onTL intensities of (a) A130°C,(b)«225#Cand • 375 *C TL peaks In CaSO4 'Dy(O.imot0 / .).
joO
8
2.0
1.8
1.6
u 1 A
o
o? 1.2inCM
2 1.0o1 0.8
0.6
0.4
(25* T O
t lh )«10(1 .625 -R) /0 .43
34*3 C
t(h)sK)(1.36-R)/0.43
10 10'
POST IRRADIATION INTERVAL, t (h)
Figure. 9. Dependence of glow curve area ratio (25-180 °C / 180-300'C)of CaSOA (0.1mot*/o Dyt 0.5mol% Na) on post-irradiation intervalfor dosemeters stored at RT and at 34£3°C. The plot between Rand log t is linear in both cases.
PAPER - 3
PRODUCTION OF CaSO^ : Dy PHOSPHOR FOR DOSIMETRIC APPLICATIONS
R.K. Iyer
PRODUCTION OF CaSO4;Dy PHOSPHOR FOR DOSIMETRIC APPLICATIONS
R.K.IYER
Applied Chemistry Division
Bhabha Atomic Research Centre
Bombay - 400 085
Solid state dosimeters based on thermoluminescence are
now being widely used in place of film badges for personnel
monitoring. CaSO<:Dy phosphor is one of the prominent materials
used for this purpose due to its high gamma ray sensitivity,
remarkable stability under extreme climatic conditions and
relatively low cost. To meet the requirement of this phosphor for
the countrywide personnel monitoring programme undertaken by the
Division Of Radiological Protection) B.A.R.C., work was taken up
to develop a suitable process for preparation of the phosphor in
large batches (300g or more per batch). The method of preparation
involves dissolution of the requisite quantities of Ca.SO4.2H2O
and Dy2O3 in cone.H2 SO<, removal of H2SO4 by evaporation,
isothermal annealing of the product at 700°C followed by grinding
and sieving to get powder of particle size <75 vim (average
particle size " 38um). The gamma ray sensitivity of the phosphor
prepared on a large scale by the open evaporation method differed
considerably from batch to batch due to contamination with
impurities. In addition, release of the sulphuric acid into the
environment resulted in severe corrosion and pollution problems.
Hence detailed studies were carried out to circumvent these
problems and a simple process was developed employing a simple
distillation assembly with, air as carrier gas and gradient
heating. The sulphuric acid is recovered and a major part of it
is recycled. The process gives a pure product of superior quality
with batch to batch variation in gamma ray sensitivity within ±
5%. With a 10 litre assembly, 600g of the phosphor can be
produced per batch. The phosphor is now being produced in
Chemistry Division, B.A.R.C. and supplied to the Division Of
Radiological Protection (6Kg/annum) for making CaSCU:Dy teflon
discs and other dosimetric applications.
INTRODUCTION
During the last three decades, a new type of dosimeter has
been added to the family of radiation measuring devices(1).
These are solid state dosimeters based on a phenomenon called
thermoluminescence(TL).The choice of a TL phosphor for dosimetric
use depends on several factors and hence only a handful of them
find commercial application. LiF:Mg,Ti; BeO:Na,Li; MgB4O7:Tb ;
Li2B4O7 :Mn,Si; AI2O3 ; Mg2SiO4 :Tb ; CaF2 :Mn; CaSO* :Tm and CaS04
:Dy are some of these phosphors. Among these, lithium fluoride is
the most widely used TL phosnhor.• The disadvantages of most
lithium fluoride phosphors are : (i) they can be reused only
after a relatively complicated and time consuming annealing
procedure (ii) permanent loss in sensitivity after exposure to a
high radiation dose (2). In later years, use of CaSO*:Dy phosphor
in gamma ray dosimetry attained prominence due to its
comparatively low cost and remarkable stability under extreme
climatic conditions(3,4). It is 20 to 30 times as sensitive as
LiF:Mg,Ti and radiation doses as low as 0.3 mrad can be
detected(5). It is used successfully in environmental dose
measurement in tropical(6) as well as arctic countries(7).
The Division of Radiological Protection, B.A.R.C. developed
TLD badges for personnel raonitering using CaSO4:Dy phosphor. The
estimated annual requirement of the phosphor in 1975 was 5 to 10
kg. Due . to the exhorbitantly high cost of the imported phosphor
(present cost is approx.. Rs 4 lakhs/kg), the Pure Materials
Section of Chemistry Division, B.A.R.C. was entrusted with the
task of developing a suitable method for production of the
phosphor.As a result of detailed studies,a simple process was
developed for preparation of the phosphor in large batches(8).
The salient feature of the process is the use of air as carrier
gas and gradient heating for removal of sulphuric acid by
distillation.The highlights of the above process are
incorporated in a review on thermoluminescence of calcium based
phosphors(9).
The specifications of the phosphor are i) high gamma
ray sensitivity ii) particle size < 75 um (average particle size
38ura) and iii) batch to batch variation of .gamma ray
sensitivity within ± 5%, The phosphor should be of high purity
since impurities such as iron act as poisons while impurities
like manganese are sensitive activators which cause fading of TL
output.
PREPARATION
The details of the work carried out for developing the
above process and also the standardised procedure for preparation
of the phosphor are given below.
The method of preparation of the phosphor on a few
grams scale has been reported earlier(10-12).Yamashita et al(10)
prepared the phosphor by dissolving the required quantities of
CaSO4.2H2O and Dy2O3 in the minimum quantity of sulphuric acid
and evaporating the sulphuric acid at about 300°C. Though the
method is very simple, major problems are associated with the
preparation of the material on a large laboratory scale.
Open Evaporation
400 g of CaSO4.2HaO and 0.217 g Dy2O3 were dissolved in 3
litres of Cone.H2SO4 in a shallow china or silica dish and heated
using a burner inside a fumehood.The problems encountered were :
i) corrosion of parts of fumehood and environmental pollution due
to the copious fumes of H2SO4 ii) increase in volume due to
absorption of moisture on leaving the partially evaporated
solution overnight iii) occassional breakage of dish during the
final stages of evaporation and iv) contamination of the product
with external impurities. The batch to batch variation in gamma
ray sensitivity was ±10X(Table I).
Table I
TL sensitivities and spectrographic analyses of CaSOi:Dy samples
Sample
No.
Time
for
of H2
required
removal
SCU (Hrs)
Spectrographic
Fe Pb
(in parts per
analyses
Mg Mn
million)
Relative res-
ponse to gamma
radiation
12
3
4
5
6
830
35
35
16
16
10
. 30
50
50
25
10
<5<5
<5
<5
<5
<5
1010
• it;
10
25
10
<5CS
<5
<5
10
<5
1.00.9
0.85
0.8-*
1.0
* peak structure of glow curve altered.
Remarks: Sample Nos 1 to 4: prepared by open evaporation.
Sample Nos 5 and 6: sulphuric acid removed by distillation.
Distillation
Bumping was severe during distillation of the solution.
Distillation in presence of air as carrier gas
This was reasonablly satisfactory, but occassional bumping
was observed during the later stages of distillation due to
increase in concentration of solution.
When use of air as carrier gas was coupled with gradient
heatingi distillation was smooth and no problem was encountered.
METHOD STANDARDISED
The details of the method standardised for obtaining the
phosphor of the required purity and also the recovery of
sulphuric acid by distillation under suitable conditions for a
typical batch, are given below:
Batch scale :300 g
Calcium sulphate dihydrate :400 g
(BDH Analar grade)
Dysprosium oxide(99.99%) :0.217 g
Cone. Sulphuric acid :4 litres
The dysprosium oxide was dissolved in 400 ml of hot
sulphuric acid and transferred to a 5 litre 2 necked round
bottomed flask. 2.85 litres of sulphuric acid were added to the
flask which was then heated on an isoraantle to about 200°C. A
slurry of the calcium sulphate dihydrate made in about 750 ml of
cone.sulphuric, acid was added in small quantities to the contents
of the flask with constant stirring. * The heating was continued,
with occasional stirring till a clear solution was obtained.
Distillation of the sulphuric acid without bumping was
achieved by using air as carrier gas and gradient heating. The
assembly employed is shown in Fig.l. Dry air was drawn through
the system at the rate of 1.5 to 2 litres/min. using a water
suction pump. The isomantle used contained two heating coils with
independent controls so that the temperature at the bottom and
the side could be controlled as desired. Heating tape was used to
heat the top part of the flask. The temperatures Tl, T2 &nd T3
at different parts were monitored using thermocouples. Tl and T2
were maintained at 275-300°C and 325-350°C respectively. A double
walled condenser was used to condense the vapours. During
collection of the first fraction (*200ml) T3 was 290-310°C.
The temperature subsequently rose to 316-318°C. The distillation
rate was 250-275 ml/hour.When there was no liquid in the flask,
both Tl and T2 were raised to « 3750c to decompose the calcium
bisulphate formed into CaSC-4 and H2SO4.Nearly 98X of the
sulphuric acid was recovered out of which more than 90X was 35
-36 N.This was recycled for processing subsequent batches.The
time required was 16 hours.
The resulting crystalline product of calcium sulphate
( doped with dysprosium) was powdered and heated to remove the
residual sulphuric acid. The product was then isothermally
annealed in air at 700°C for two hours, ground and sieved to get
powder of particle size < 75um (average particle size 38um).The
powder was then blended and homogenised using a horizontal
blending machine.
CHARACTERISATION OF THE PHOSPHOR
This was carried out by the Division of Radiological
Protection. The method consists of exposing a sample (100 mg) of
the phosphor to a known and fixed dose of Co60 gamma rays and
measuring the TL output, 24 hours after irradiation, using a
'Thermoluminescent Reader' assembly. The suitability of the
phosphor is determined by comparision of the TL peak structure
and the total TL output with those of a standard. The glow curves
of the sample and the standard are given in fig.2. These were
obtained at a non-linear heating rate, the average heating rate
being 14°/sec.
The conditions of preparation of the phosphor were
optimised such that the batch to batch variation in gamma ray
sensitivity was within ±5%.
EFFECT OF VARIOUS PARAMETERS ON GAMMA RAY SENSITIVITY
Gamma ray sensitivity of the phosphor is considerably
affected by the concentration of impurities during the process of
its preparation as well as the particle size of the final
product. It has been found that the presence of manganese and
iron beyond the limits of 5 and 10 p.p.m. respectively in the
final product is detrimental to the sensitivity of the phosphor
(Table I). These impurities generally creep in from the bulk of
the sulphuric acid used and get concentrated during its removal
by distillation (although they may be present at sub p.p.m. level
in the bulk used). Therefore one has to be careful to chovje the
starting materials of the desired purity, especially with respect
to iron and manganese. The effect of particle size on the
sensitivity of the phosphor has. been studied in detail
earlier(13,14). The optimum particle size suitable for TLD badges
is <75u (average particle size ~ 38um).
The phosphor is now being produced in Chemistry Division,
B.A.R.C and supplied to the Division of Radiological Protection
(6Kg/annum) for making CaSOi :Dy teflon discs and other dosimetric
applications.
Methods of preparation of the phosphor similar to the one
given above have been reported subsequently (15,16). However, the
basic feature remains the same. In these, the sulphuric acid
obtained by distillation is neutralised with NaOH. It is not.
advisable to do this since 90% of the sulphuric acid recovered
has a strength of 35-36N which is suitable for reuse.
The work reported here was carried out in Chemistry
Division, B.A.R.C in collaboration with S/S Y.W.Gokhale,
S.K.Gupta, S.G.Deshpande and S.S.Gupta and Dr G.S.Rao.
REFERENCES
1. K. Becker, Solid State Dosimetry, CRC Press, Cleveland, Ohio,
U.S.A.(1973).
2. K.P.Doppke and J.R.Cameron, Report COO-1105-119, USAEC,
Phys.Med.Biol., i£i 571 (1970).
3. A.S.Pradhan, Bhuvan chandra and K.Ayyangar, Proceedir>,;» >i
National Symposium on Thermoluminescence and its Applications,
R.R.C, Madras, India, 409 (1975),
4. Teledyne Isotopes, 50, Van Buren Avenue, Westwod, N. J. 07675,
U.S.A.
5. K. Becker, Progress in Luminescence Dosimetry, Science, 177
539 (1972).
6. K. Becker, R.H. Lu and P.S.Weng, Proc Third Int.
Conf.Luminescence Dosiraetry,Riso-Rep. 249, Vol.2, Danish
AEC,Riso,Roskilde,960 (1971).
7. B.L.Nielsen and V.Mejdahl, Riso-Rep.219,Danish
AEC,Riso,Rokilde(1970).
8. G.S.Rao, R.K.Iyer, Y.W.Gokhale, S.K.Gupta, S.G.Deshpande and
S S.Gupta, Preparation of CaSO4; Dy phosphor Report No.
BARC/I- 591, Bhabha Atomic Research Centre, Bombay -
400 085 (1980).
9. C.M.Sunta, Nucl. Tracks, 10, 47(1985).
10.T.Yamashita, N.Nada, H. Onishi and S. Kitamura, Proc.2nd Int.
Conf. on Luminescence Dosimetry, USAEC-CONF 680920,4(1968).
U.K. Becker, Nucl. Instrum. Meth. , ifiit 405 (1972).
12.K.Ayyangar, Bhuwan Chandra and A.R.Lakshmanan, Phys.Med.Biol.,
H , 656 (1974).
13.Bhuwan Chandra, K. Ayyangar and A.R. Lakshmanan,
Phys.Med.Biol., ZX, 67 (1976).
14.A.S.Pradhan and R.C.Bhatt, Nucl. Instrum. Meth., 161. 243
(1979).
15.L.L. Campos, Journal of Luminescence, £&, 481 (1983).
16.J. Azorin and A. Gutierrez, Health Physics, ££, 551 (1989).
AIR—=5
F = - * T O WATERSUCTION PUMP
ISOMANTLE
FK3.1 EXPERWENTAL ASSEMBLY FOR PREPARATION OF CaSO4 : Dy PHOSPHOR
> 2
HI
enEXPOSURE : 2-5 R OF CoY-fWSAVERAGE HEATING RATE: 14*/SEC.
210 *C
STANDARDSAMPLE
15 30 45TIME (SECONDS)
FIG. 2 GLOW CURVES OF Ca$O4*Dy PHOSPHOR
PAPER - *
PREPARATION AND CHARACTERISATION OF RARE EARTH
BASED PHOSPHORS
G. Alexander
PREPARATION AND CHARACTERISATION OF RARE EARTHBASEP- PHOSPHORS
BY
G. Alexander,Uranium Extraction Division,
Bhabha Atomic Research Centre,Bombay - 400 085.
Abstract
Rare Earth phosphors of the Europium .activated typo findextensive application in colour T.V. tubes and mercury vapourlamps. Potential uses of Terbium activated phosphors are alsofound in medical radiology involving X-rays. Several crores atRupees of precious foreign exchange are spent every year for i tsjimport. The technology for the preparation of these phosphors isa closely guarded secret. The basic raw materials of the highpurity Rare Earths are now being produced witin the country bythe Indian Rare Earths in active collaboratin with the UraniumExtraction Division of BARC. With a view to commercially exploitthese raw materials for indigenous production of the valuablephosphor, appropriate facilities have been created for thepreparation and characterisation of these phosphors. Some of thephosphors successfully synthesised are the Eu(3+) activatedYttrium vanadate, Yttrium oxide. Yttrium oxysulphide and Eu(2+)activated Aluminosilicate, Tb(3+) activated Yttrium oxysulphideetc. The emission and excitation spectra of the phosphors havebeen examined and their quantum and c^thodoluminescentefficiencies have been evaluated.
I. Introduction
Of late, interest in luminescent materials based on rareearths either as host lattices or as activators is increasing.The year 1964 marked the. beginning of a new era in phosphorpreparations and applications based on rare earths. The longfelt need of the colour T.V. industry to have a narrow band redemitting phosphor with the correct eye fesponse characteristics
was realised with the discovery of Eu(3+) activated YV04 byPalilla etal.(l) Thereafter, the emergence of a broadtheoretical framework based On crystal field theory as applied torare earth crystalline lattices incorporating rare .earthactivators coupled with the modern purification methods ofindividual rare earths, resulted in the multifaceled growth ofthe rare earth phosphor industry covering many areas of presentday applications. Several variants of the original YV04:Eu(3+)have since appeared. New phosphor host lattices with various.combinations of activators and co-activators have beendiscovered. A peculiar feature is that host lattices played asignificant role in deciding the colour of the emission likethe blue emission of Eu(2+) and the i«d emission of Eu(3+) indifferent crystalline matrices. Characteristic red emission ofEu(3+) and the green emission of Tb(3+) have led to the developmentof many efficient group of phosphors incorporating these ions forthese two colours. The task of a phosphor scientist is to findthe right type of crystalline matrix to incorporate the. activatorin optimum concentrations with suitable combinati< 'i-s ofco-activators which will help to enhance the omissioncharacteristics of the activator, for the sole purpose ofextracting maximum efficiency of emission of a particular colour.The modern crystal field theory gave a thrust to thee.se
developments.
Applications in which rare earth phosphors are put to useare numerous. Mention must be made at this stage that yttriumthoi"~h strictly not a rare earth is clubbed with other rareearths primarily because of its close chemical identity andbecause of its ability to form crystalline compounds of similarcrystal structure like Gadolinium, lutetium etc. Tonic siso-i ofyttrium also matches closely with that of the rare earths. Inaddition it is optically inert and in this respect helps not tointerfere with any activator emission. Activator ions likeEu(3+), Tb(3+) etc. find easy substitution in lattices formedwith yttrium and other rare earth compounds with almost identicallattice surroundings. An examination of the emissioncharacteristics of Eu(3+) in Gadolinium and yttrium vanadatebears ample testimony to this fact (2). Thus modern phosphorterminology does not made any distinction between yttrium andother rare, earths and treats both as belonging to the samefamily. Mixed crystal lattices like (Y,Gd)2O2S., (Y,Gd)2 03 etc.are easily realizable with varying ratios of these
A broad subdivision can be made of rare earth phosphors ascoherent and incoherent rare earth optical materials. In thecoherent group, we have Ndodymium activated Yttrium AluminiumGarnet as the shining example of a laser material. Singlecrystals of YVC4 and Y2O3 have also been found to be excellentlaser hosts for ions like Eu(3+) and Nd(3+). Area of our presentactivity is centred on the development of incoherent group ofmaterials such as powder phosphors, mainly from the ind<jenoussources of raw materials. Rare earth oxide materials o.f up tofour nine purity are now available from Indian Rare Earths. Themost important new application of Rare Earth phosphors hasoccured in the fields mentioned below:1. Cathodoluminescent devices2. Fluorescent lamps3. X-ray image intensifier tubes and dosimetry films4. Thermoluminescent dosimeters5. Up converting phosphors
Since the bulk of the phosphor requirement is in the fieldcathodoluminescence and fluorescent lighting, such phosphors willbe highlighted in this presentation. The mechanism ofluminiscence being the same in X-ray and cathodoluminescentphosphors and similar phosphors are also used in X-rays, thisalso will be briefly discussed.
II. Phosphors for Cathodoluminescent Scieens:-
Phoophors for cathodoljminescent applications such us incolour T.V. tubes, only the red emitting europium activated typehave so far been found successful. No replacement for the (Zn,Cd)S type phosphors for the blue and green primary colours hasbeen found acceptance yet. This is so mainly because (Zn, Cd)Svariety have unmatched efficiency figures and superior colourcharacteristics. However, one area in which the Tb activatedgreen rare earth phosphor holds promise is in its saturationcharacteristics. The emission efficiency of this phosphor islinear with respect to current density to a great, extent where asthat of (Zn, Cd)S type is not. Non linearity with current densitymay lead -to errors in colour balance - a limitation standing inthe way of improved picture quality by increasing current.density. Thus search is still on for an improved Tb(3+) activatedgreen phosphor. After the intioduction of YV0^:Eu(3+) in W64 asa red colour primary phosphor, new materials of the Eu(3+)
activated variety like Y,03 and Y2O2S with superior efficiencyvalues have been discovered. Addition of several co-doi»>inL-; suchas Tb, Bi, etc. in these lattices have been tried for efficiencyimprovement. The relative brightness figures of these phosphoi sare given in Table I. By the present reckoning,Y202S:Eu(3+) phosphor appears to have an edge over others.Cathodoluminiscent efficiency of several phsophot .-, with Eu and Tbactivation are given in Table II. All the above phosphors withEu(3+) activation give narrow line emission in the range 611 to627 nm.
Table I (Red emitting Phosphors)
Material Relative Brightness Relative Enoi <jy el f ii-i
Zn ,Cd nS:Ag
:Eu3 +
Y2O3:Eu"
Y2O2S:Eu3 +
48%
.57%
100%
100%
180%
86%
100%
120%
(Data Taken from Electro. Chem. tech. Vol. 4, 21-24 (1966))
Table II
Material Cathodoluminescent ef f icit-ncy
YV04:Eu(3+)
Y203:Eu(3+)
Y202S:Eu(3+)
LaO2S:Tb(3+)
6dO,S:Tb(3+)
5 . 4 - 7 . 1 %
6 . 5 - 1 0 . 3 % ( 1 2 )
16% ( 1 3 , 1 4 )
13ut,
18%
Ill Phosphors for fluorescent lighting
Rare Earth phosphor* are extensively used as colourcorrectors in High Pressure Mercury Vapour lamps. H.P.M.V lampsgive intense light emission in the blue, green and yellow regionbut defficient in the red. It also emit intensa ultra violentradiation which goes uruitlised. The net effect is that goodcolour retention is affected in addition to a loss in energyefficiency. Eu(3+) activated red phosphor has come in handy as acolour corrector in H.P.M.V. lamps. The inner surface of theenvelope of the lamp is coated with a powder layer of thephosphor which absorbs the ultraviolent radiation and emit redlight, thereby compensating for the deffic iency of the lamp inthe red region of the spectrum. The desirable properties of asuch a phosphor are (1) high quantum efficiency (2) a broad andstrong ultraviolet absorption band (3) high quenching temperatureof luminescence. High quantum efficiency is essential to ensure alarge visible light output for all the ultraviolent radiationthat is absorbed by. it'. Quantum efficiency is defined as thenumber of emitted photons to that absorbed by it. Quantumeffciency as high as 90% have been realised for YV04:Eu(3+)phosphor. Fig. 1 shows the excitation spectrum of this phosphorwhich is broad and covers the entire region of the mercury vapourlamp ultraviolent emission. A notable feature of the Eu(3+)activated YV04 and Y203 is that its luminous efficiency remainsunaffected at high temperature, an essential criteria forselection for HPMV lamps since its operating temperature is above300*C •
IV Rare Earth phosphors for X-ray Application
The basic similarity of the excitation mechanism of cathoderays and X-rays in powder phosphors led to its adoption in X-rayintensifying screens and image intensifier tubes„ The twoimportant properties which suggest most strongly the use of rareearth phosphors in X-ray intersifying applications (4,5) are(1) the high cathode-ray efficiency of the terbium activated raiu
earth oxysulphide and(2) the high average atomic number and the consequent increase in
absorption coefficient for the Lanthanum and Gadoliniumbased compounds.The reported cathode ray efficiencies for
La2O2S.Tb(3+) - 13% and Gd2O2S:Tb(3+) - 18% compare favourablywith the till now used (Cd, Zn)S: Ag of 15 to 20% in x-ray
intensifiar tubes.' The replacement of (Cd,Zn)&.A:; by one or theother of the terbium activated oxysulphide should increasesubstantially the number of X*ray photons absori .•<•_ i by the X-rayintensifier input screen and may lead to a significant increasein net light output from the tube for a given operating conditionand incident X-ray flux. The two important advantages resultingfrom the use of rare earth oxysulphidos are (1) increase in thefractional utilisation of incident X-ray photon, tho majorbenefit of which is a lower dosage for equivalent data content inthe output image (2)substantial net gain in tube brightness atany given dosage level due to easier development of rare earthphosphor powder of the required particle size.
Since 1896, CaWO4 is the only phosphor used in X-ray filmintensifying screens. Though it . has a relatively high x-rayabsorption coefficient, its light conversion efficiency of 3 to5% is very poor. The two rare earth • oxysulphidesLa20j,S:Tb(3+) and Gd2O2S:Tb(3+) have very high light conversionefficiencies in comparison. However, its major disadvantage isthat the present day X-ray film developed solely for CaWQ4(emission ir. the blue) is not sensitive to the green emissionfrom these phosphors. If a green sensitive film equivalent to thepresent blue sensitive one is made available, the X-ray exposurereduction by as much as a factor of 20 could become feasibl«(5).This is a phenomenal gain considering the risk factors thataffect health. Indeed green sensitive film is already on its wayto development.
V An introduction to the theory of Rare Earth Luminescence
Tripositive rare earth ions when incorporated in crystallinematrix possess unique spectroscopic properties. The 4f shell isincomplete and the electrons in it are protected by the 5s and 5psubshells from interaction due to surrounding ions. he energylevel transition within the 4f shell are spin and paarityf orbidden(7,8,9) .• The crystal field splitting of the electronicenergy levels is of the order of 100cm" whereas that in atransition metal ion is of the order of 104cm~J. Inspite of thispoor crystal field interaction, many rare earth ions like Eu(3+),Tb(3+), Dy(3+) etc, are found to be optically active. The reasonfor this observed phenomenon is the strong spin orbit coupling inconjunction with the weak crystal field interaction - leading toa partial lifting of the spin and parity prohibition rules. Forefficient luminescence to occur from such ions, the followingconditions are to satisfied.
(1) Activator ions, should occupy a lattice site devoid ofinversion symmetry.(2) Activator ion should have"*comparable ionic size to that ofthe host lattice cation which it replaces and also similarcrystal structure.(3) Activator concentration should not exceed a certain maximumpercentage of the host cation.(4) A certain classes of ions known as sensitisers in verysmall concentrations will enhance the emission efficiency of theactivator whereas certain others will quench the activatoremission. Hence, purity of the materials used for preparation ofthe phosphor.
In the light of the above v requirements fo< activatoremission, the characteristics of a typical phosphor likeYV04:Eu(3+) with reference to the energy level diagram of theEu(3+)(1) ion will be briefly discussed below. The» energy leveldiagram of Eu(3+) ion is given in Fig. 2.
There are 6 electrons in the 4f shell of the Eu(3+) ionwhich is less by one electron for a half filled stable state. Theexcitation of the Eu(3+) ion will result in the jumping over ofan electron from the surrounding oxygen ion to the 4f shell,which is termed as the charge transfer state, the state of theexcitation being 4f72p~1. The charge transfer state slowly losesits energy to the lattice in small packets as phonons and finallyarrives at the luminescent level. Luminescent levels are denotedby black half circles. There are 4 such levels - SD O to5D 3 from- where luminescent transitions can occur to the groundlevels 7FO to
7Ffi. .Symmetry considerations of the lattice, willdecide the criteria for transition between these levels. Anexample of an electric dipole transition from 5 Do to 7F2 willalways result in red emission irrespective of which lattice theion is located. Thus the charge transfer state play a part in theenergy of the excitation but tha emission, colour is alwaysindependent of this state, the site symmetry consideration forthe electric dipole transition 5no~*>
?'r2 is alwayssatisfied in the yttrium vanadate lattice which crystalises inthe tetragonal system with zircon structure. Europium alsocrystallises as orthovanadate with the same crystal structure andsimilar lattice constants, hence Eu3+ can readily substitute forY in the YV04 lattice.
The energy level transitions within the 4f shell is given inFig 2. All fluorescent transitions are found to originate fromthe 5D O level and the terminating levels except 7FQ are all
degenerate. A case of the degeneracy of the 7F 2 level beinglifted by the crystal field of YV04 is shown in this fig- andresults in a group of linens which are very closely spaced. Twomost prominent lines in this group are 615nm and 6l9nm andalmost 70% of the energy of emission occurs here. In additionthere are emission bands centred at 698nm due to the electricdipole transition 5De--
7F4 and 595nm due to the magnetic dipoletransition Do~~ (% • °ne typical feature of the electric dipoleemission is that it is characterised by a larger than usual levellife time of the order of 10 3 sec. which is indicative' of theforced nature of the transition.
VI Preparation of rare earth based phosphors
a)GdV04:Eu(3+), YV0,,:Eu(3+)
The phosphor is synthesised by mixing stoichiometricproportions of Gadolinium or yttrium oxide with a little excessof V205» and Eu203 as a small percenage of Gd or Y and firing attemperatures above 1000°C. Mixing of the constitutents is carriedout in a ball mill and on a laboratory scale by using a mortarand pestle. The compound YV04: Eu(3+) is formed by the process ofsolid state reaction. Purity of the starting material is 99.99%(obtained from Indian Rare Earths), Vanadium Pentoxide >99.9% andEuropium oxide >99.9%. After firing at temperature above 1000'Cfor 2 to 3 hours, the material is taken out, ground to a finepowder and washed in ammonia solution for the removal of anyunreacted excess vanadium pentoxide. After rinsing several timesin distilled water, a second firing for one hour is carried out.Measurements are carried out after grindiny' the material to afine powder.
b) Y203:Eu(3+)
Europium activated yttrium oxide phosphor is prepared bydissolving both Y203 and Eu2Os in appropriate concentrations inconcentrated' nitric acid and precipitating the homogeneousmixture of both yttrium and europium oxalate by the addition of aconcentrated solution of oxalic acid. The mixed oxalate is driedand fired.for 1 hour at 800*C in a quartz crucible and refired at1000 C for 3 hours. Since there is no flux used for thepreparation, no cleaning process is involved in the preparationof Y203 Eu(3+) phosphor.
c) Y202S:Eu(3+)
The Y202S:Eu(3+) phosphor*is prepared by the sulphur carbonateflux technique. The chemical constituentsY203,Eu203, Na2C03 and excess quantity of sulphur overstoichiometric compositons are mixed together and loaded in aplatinum crucible and fired under nitrogen atmosphere in a quartzchamber using an R.F. heater as the heating source. Thetemperature of firing is about 1100°C for about 2 hours duration.The material after cooling to room temperature is ground to afine poweder and washed in distilled water a number of times anddried.
d) A12O3: SiO2 : Eu(2+)
An aluminosilicate phosphor activated by the Eu(2+) ionhaving quantum efficiency close to unity with emission peaking at450 nm was described by A. Wacht(lO). From quantum efficiencyvalues and emission characteristics, the phosphur appears to beof excellent quality as a blue component lamp phosphur. Thisphosphor is prepared in the laboratory by the addition of excessalcoholic NH4OH to a solution of Al(N03)3 + Eu(N03)3 i-Si(OC2H5)4 in about 70% alcohol. The gel thus formed is waterwashed, dried at 950'C and fired in air at 1250*C for 2 hours.After grinding the product to a fine powder, it is refired inhydrogen atmosphere in a quartz chamber at 1100'C for 2 hoursusing an R.F. heater. The phosphor is cooled to room temperatureand ground to a fine powder.
VII Evaluation of Phosphor Characteristics
Phosphors have to be evaluated for its efficiency andspectral emission charactristics. In our laboratory, facilitieshav been created for ' the measurement of photo andcathodoluminescent efficiencies and cathodoluminescent spectra ofa phosphor.
a) Quantum efficiency
An apparatus- has been designed and fabricated for themeasurement of quantum efficiency of a phosphor using aphotodiode as a sensing element. A block diagram of the apparatusis given in Fig. 3. It consists of a high pressure mercury vapourlamp as a source of ultraviolet light, an interference filter toisolate a narrow band of ultraviolet radiation from the mercury
vapour lamp, a quartz lens to focus ultraviolet light to t^ephosphor plaque and the silicon phototfiode electrometercombination to detect and m&asure radiation intensity. Theintensity of the u.v. light used'for excitation of the phosphoris measured by reflecting it ^ith MgO powder. The quantumefficiency is computed(ll) using the formula
o = (f /i-r)*(iuv/ivi)*(c2/cl)
where " ("" is the reflectin coefficient of MgO for U.V.radiation "r" is the reflection coefficient of the phosphor foru.v. radation. I and Iv. are the currents produced by thephotodiode per photon p&r sec. for ultraviolet and visibleradiation respectively, Cx and C2 are the measured currents bythe electrometer detector combination for ultra violet andfluorescent radiation signals respectively.
b) Cathodoluminescent efficiency and spectra
A demountable all metal cathodoluminescent apparatus hasbeen designed and fabricated to record the cathodoluminescentspectra and cathodoluminescent efficiency of the phosphors. Theapparatus consists of a S.S vacuum system which can give a vacuumof the order of 10"6mm of Hg, an electron gun which can give abeam current of a few microampers at a continuously variableacclerating voltage' up to 20kv, a 1\4 meter Jarrel Ashmonochromabor to scan the cathodoluminescent spectra and arecorder to record the spectrum. The phosphor is deposited on analuminium plate of lxl inch and mounted on a cubic block ofcopper. Four such samples can be mounted on the block at the sametime which can be rotated from outside and made to face thedefocused electron beam at an angle of 45°. The light emitted bythe phosphor enters' the •entrance slit 'of- the tnonochromatorthrough a quartz windiow and the monochromator output is fed to arecorder for recording the spectra. The cathodolumeniscentefficiecncy of the phosphor can be computed by measuring the beamcurrent, accelerating voltage and the intensity of the lightemitted by the phosphor. A block diagram of the apparatus isgiven in Fig. 4.
VIII Results
Quantum efficiency values of the phosphors prepared in our
10
activated variety like Y203 anc' Yi°2S w* t n superior efficiency
values have been discovered. Addition of several co-dopants suchas Tb, Bi, etc. in these lattices have been tried for efficiencyimprovement. The relative brightness figures of these phosphorsare given in Table I. , By the present reckoning,Y202S:Eu(3+) phosphor appears to have an edge over others.Cathodoluminiscent efficiency of several phsophors with Eu and Tbactivation are given in Table II. All .the above phosphors withEu(3+) activation give narrow lino emission in the range 611 to627 nm.
Table I (Red emitting Phosphors)
Material Relative Brightness Relative Energy efficiency
Zn 2Cd eS:Ag 48% 180%
YVO4:Eu3f 57% 86%
Y203:Eu3* 100% 100%
Y202S:Eu3+ 100% 120%
(Data Taken from Electro. Chem. tech. Vol. A, 21-24 (1966))
Table II
Material Cathodoluminescent efficiency
YVO4:Eu(3+) 5.4 - 7.1%
YiO3:Eu(3+) 6.5 -10.3% (12)
Y202S:Eu(3+) 16% (13,14)
LaO2S:Tb(3+) 13%
GdO2S:Tb(3+)
11
laboratory are given in Table 3. Detailed preparation techniqueshave been studied so' far for YVO4:Eu(3+) phosphor only. Theoptimum quantum efficiency obtained for the phosphor is 85 - 90%which compares favourably with the value reported in theliterature. Few preliminary experiments carried out forY203:Eu(3+) has yielded a quantum efficiency of 60%. Thequantum efficiency value obtained forA12O3:SiO2:Eu(2+) is of the order of 50%. A lot more work is yetto be done on the preparation of Y202S:Eu(3+)• The evaluation ofcathodoluminescent efficiency is also at the preliminary stages.The recorded cathodoluminescent spectra of YV04:Eu(3+) taken inour apparatus is given in Fig.5.
Table III
Material Quantum Efficiency
YVO4 : Eu(3+) 70 - 90 %
Y203 : Eu(3+) 50 - 60 %
A12O3 : SiO2 : Eu(3+) 50 %
Y2O2S : Eu(3+) very low
Acknowledgements
The author wish to thank Shri K.S. Koppiker, Head, UraniumExtraction Division and Shri U.R. Marwevh, Head, ProductDevelopment Section for all the support and technical guidanceextended for this programme. Author also wish to thank Shri S.D.Samant, Health Physics Division, BARC for many encouragingdiscussions and technical help extended, for this work.
References
1) Levine A.K. and Palilla F.C.Appl. Phys. Letters 5,118 (1964)
2) Palilla F.C.. Levine. A.K. and Rinkevin M.J.J. of Elec. Chem. Soc. 112, 776 (1965)
12
3) Bril A and Wanmaker W.L.Philips Tech. Rev. 27, 22 (1966)
• * •
4) Wickershiem K.A., Alves R.V. and Buchanan R.A.IEEE Trans. NS-17, 57 (1970)
5) Wang S.P. etalI-EEE Trans. NS-17, 49 (1970)
6) Buchanan R.A., Finkelstein S.I. and Wickershin K.A.Radiology 105, 185 (1972)
7) Blasse G. and Bril APhilips Tech. Review 31, 304 (1970)
8) Judd B.R.Phys. Rev. 127, 750 (1962)
9) 0 Felt C.S.J. Chem. Phys. 37, 511 (1962)
10) Watchel A,J. Elec. Chem. Soc.. 116, 61 (1969)
11) A paper on the design of the apparatus and evaluation of•quantum effi"c"6ncyi'tJf'~iTrof"g«f«««*i««»t*«k*#«»fj»+>6>ai>hors is due forpublication by the author.
12) Buohanan R.A. et alJ. Appl. Phys. 39, 4343 (1960)
13) Shrader R.E. and Yoecom P.N.J. of Luminescence 1,2, 14 (1970)
14) Lehmen WJ. Electro. Chem. Soc. 1164 (1970)
13
35
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FIG. 5.
PAPER - 5
DEVELOPMENT OF LIQUID SCINTILLATION CHEMICALS IN BARC
K.A. Noras and CM. Paul
DEVELOPMENT OF LIQUID,SCINTILLATION CHEMICALS IN BARC
K. A. NORAS and C. M. PAULChemical Engineering DivisionBhabha Atomic Research Centre
Bombay-400 08b
INTRODUCTION
One of the oldest techniques for the detection of radiation
is by the observation of scintillation light produced by
the action of radiation on certain materials. The scintillation
phenomenon is employed as a useful method for the detection •and
measurement of a wide assortment of radiations. Various types of
scintillators are available but this paper discusses the
development in BARC. of only liquid organic solutions as
seintillators.
In liquid scintillation counting, the radioactive labelled
sample is usually dissolved or kept in suspension in an
appropriate solvent. A small percentage of a primary solute
added fluoresces when excited by energy tr«tns±er processes taking
place within the mixture. Sometimes a secondary solute is added
as a wavelength shifter to tailor the emission spectrum for a
particular purpose. The sample, solvent and solute ifluori
mixture is taken in u glass or plastic vial and photomultiplier
tubes detect the light emitted from the vial. Electrical pulses
are produced and the pulse height is proportional to the energy
of the initiating particle. The liquid scintillation counting
technique is widely used for counting low level beta activity
such as from tritium and carbon 14. However, a number of
isotopes which emit radiations other than betas are also counted
by this method.
To assay tritium in heavy water moderated reactors,
development work on the preparation of scintillation chemicals
for use in liquid scintillation counting was undertaken. Their
preparation encompasses the field of synthetic organic chemistry
and as these chemicals have commercial importance most of the
methods of preparation reported in literature are patented and
hence other methods of synthesis were developed for some
1
chemicals. Over the years, the work carried out could be
classifiod as follows:
(1/ preparation of scintillation solvents
(2) preparation of scintillation solutes
(3) purification
(4) analytical and counting data of the scintillation chemicals
prepared.
1. SOLVENTS
Whereas a number of solvents have been used in liquid
scintillation counting, toluene and dioxane are those most
commonly used for non-aqueous and aqueous radioactive samples
respectively. The preparation of scintillation grade naphthalene
used as a secondary solvent in liquid scintillation counting is
also included in this paper. Naphthalene is used along with
dioxane.
The preparation of these scintillation grade solvents was
essentially the purification of the respective commercial grade
solvents.
(a) Purification of toluene : Commercial toluene contains
methyl thiophene (thiotoluenea) b.p. 112" - 113' 0 which camiol
be separated by distillation. The separation is achieved by
making use of the fact that sulfcmation of methyl thiophene at
.30° C with cone, sulphuric acid takes place more readily as
compared to toluene. Thus, commercial toluene is treated with
about 10% of its volume of cone, sulphuric acid at 30° C. The
lower acid layer is separated and the procedure is repeated until
the acid layer remains colourless. The toluene layer after
separation is distilled over small amounts of sodium hydroxide
pellets to remove last traces of acid. Finally, toluene is
refluxed over metallic sodium and fractionally distilled at
110.6" C.
(b) Purification of dioxan-s Commercial dioxane contains small
amounts of acetaldehyde and glyool acetal together with water.
On keeping, the acetal tends to undergo hydrolysis and the
liberated acetaldehyde leads; to some peroxide formation.
Purification involves the removal of peroxide by refluxing
dioxane with 5% ferrous sulphate in dilute hydrochloric acid
solution followed by drying with sodium hydroxide pellets and
removal of aqueous layer. Finally, it is rei'luxed over sodium
metal and then fractionally distilled at 101.5" C.
(c) Purification of naphthalene : Commercially available
naphthalene contains thionaphthene as impurity. It was purified
by fractional crystallization in ethyl alcohol (95%). The
procedure was repeated 3 to 4 times to get the requisite
purity.
These chemicals were being supplied to- various units of DAE
but later discontinued with the availability of AR grade which is
cheaper and comparable to the scintillation grade.
(2) SOLUTES
Although a number of compounds serve as seintillators, only
a few find practical use. The principal solutes used in liquid
scintillation counting for monitoring tritium in heavy water
moderated reactors is a combination of primary solute PPO i 2. £•
diphenyl oxasole) and secondary solute POPOP (.1,4 di-2(5 phetiyl
oxasolyl) benzene. Other solutes have specific applications.
The preparation of a few scintillation solutes prepared by the
group are as follows:
(a) p-Terphenyl
This primary scintillation solute was obtained from
commercially available Santowax F (a mixture of o, m and p-
terphenyl). A unit was setup to produce 5 kg of p-terphenyl per
month based on the differential solubility of the isomers in
benzene.
(b) 1,1,4,4- Tetraphenyl 1,3 butadiene
The preparation of this secondary solute involved the
following steps:
(i) preparation of diethyl succinate from succinic acid and
ethyl alcohol
(ii) the interaction of diethyl succinate with phenyl magnesium
to give 1,1,4,4 tetraphenyl butane diol-1,4
(iii )dehydration of the diol with acetic aci:f containing 6%
sulphuric acid to give 1,1,4,4 tetraphenyl 1,3 butadiene.
(c) 2,5 Diphenyloxaaole (PPO)
A ne.v and convenient method(1,2) with overall high yield was
developed in the Chemical Engineering Division of BARG for the
synthesis of PPO. The method involves the conversion of hippurie
acid by phosphorus pentachloride to hippuryl chloride and the
acylation of benzene with hippuryl chloride (using anhydrous
aluminium chloride as catalyst) to give the intermediate N-phen-
acylbensamide, which is cyclodehydrated to PPO by cone, sulphuric
acid.
This new method of synthesising PPO has an advantage over
the methods reported earlier in literature because it eliminates
the use of lachrymatory w-bromoacetophenone and avoids the
difficult experimental conditions of making PPO via asalaetone.
(d) 1,4 Di-2(5 phenyloxaaolyl) benzene (POPOP)
It is also synthesised by a new method{2) developed in the
Chemical Engineering Division of BARC. It involves the acid
hydrolysis of N-phenacylbensamide to give w-amino acetophenone
hydrochloride which is then coupled to terephthalolyl chloride in
presence of aqueous sodium carbonate to yield N, H' diphenaeyl
terephthalamide which is subsequently cyclodehydrated by cone,
sulphuric acid to POPOP.
The main advantages of this new method of synthesising POPOP
are fewer steps in synthesis and higher yield.
(e) Other substituted oxaaoles and bisoxasoies
The method developed for the. synthesis of PPO and POPOP was
extended to the preparation of other substituted oxaaoles and
bisoxasoles, some of them for the first time. NPO, BFO, TFO.
BBO, TOPOT and 2, 2' p phenylene bis 5-p ethyl phenyl oxasole
(3,4) are examples of scintillation solutes prepared in the
laboratory of the Chemical Engineering Division.
(£) Oxadiasoles
The oxadiaaoles commonly' used in liquid scintillation
counting via. 2 phenyl 5 (4 biphenylyl) 1,3,4 oxadiasole (PBT>.?
and 2,5 phenyl 1,3,4 oxadiasole (PPD) were also successfully
synthesised. The method consists essentially of the following
steps:
(i) preparation of benahydraside from ethyl bensoate and 8b%
aqueous hydrasine hydrate.
(ii'i coupling of ben,3oyi hydrasine with either p-phesiyl bensoyl
chloride or bensoyl chloride or benaoyl chloride in presence of
pyridine to give 1 bensoyl-2-t4-p-phenyl benaoyl) hydrasine or
1,2 bensoyl hydrasine respectively.
(iii)dehydration with excess phosphorus oxyeliloride to give FbD
or PPD respectively.
(g) Organo tin compounds
These are compounds which do not scintillate but are loaded
in liquid or plastic scintillators for gamma detection like
iodine-125. Tetra-n butyl tin, triphenyl 4 ethyl phenyl tin and
tetraphenyl tin were synthesised.
(h) 9 Vinyl anthracene
A new method of synthesis was developed for preparing this
high potential sciritllator. The method was based on the
methylenation of 9 anthraldehyde by adding a benaene solution of
9 anthraldehyde along with an ethereal solution <.>£ ruethylene
iodide to magnesium amalgan>( 5 ) .
(3) PURIFICATION
Scintillation chemicals need a very high degree of purity
for optimum performance in liquid scintillation counting. Most
of the purification procedures involved crystallisation through
different solvents. Soxhlet extraction and final purification
through a modified Sangaster's apparatus which enveloped
extraction, adsorption and crystallisation all in one with a
minimum quantity of solvent.
(4) ANALYTICAL AND COUNTING DATA ~
(i) All compounds synthesised were subjected to a quantitative
determination of some of the constituent elements and the
observed values were comparable to the calculated values.
(ii) Melting points were determined in open glass capillaries on
Buchis' melting point apparatus and found correct.
{iii)Infra-red spectra of the compounds were recorded either in
the form of a mull in Hujol or KBr pellet on a Parkin Elmer Infra
Cord 137 model.
(iv) Proton magnetic resonance spectra were recorded on a Varian
A BOA model NMR spectrometer.
(v) Absorption and fluorescence spectra were recorded on an
Aminco Bonman Spectrophotofluorometer and found matching-
(vi) The relative performance of the scintillation solutes were
determined on LS spectrometer of Packard Instrument Co (Model
3255) and found acceptable.
CONCLUSION
Though hundreds of organic compounds have been classified as
scintillation solutes PPO is till today the most popular primary
organic scintillator. However, a recently reported compound PHP
(1 phenyl 3 mesityl 2 pyra^oline) offers promising potential as a
solute in liquid and plastic large volume scintillators.
ACKNOWLEDGEMENT
The authors wish, to thank Shri S. Sen, Director, Chemical
Engineering Group, E^RC and Shri V. S. Keni.. Head, Chemical
Engineering Division, BARC for their keen interest and
encouragement in the development of these scintillation
chemicals.
REFERENCES
1. S. D. Paul, D. L. Dhane, K. A. Noras and A. U. Mushrif,
J. Ind. Chern. Society 49, 579 (1972).
2. D. L. Dhane, K. A. Noras, A. U. Mushrif, G. M. Gaiki and
S. P. Hirve, BARC Report No. 792 (1975).
3. D. L. Dhane and G. M. Gaiki, BARC Report No. 1030 (1979).
4. D. L. Dhane, K. A. Noras and A. U. Mushrif,
BARC Report No. 1217 (1984).
5. D. I. Dhane, K. A. Noras and G. M. Gaiki,
BARC Report No. 835 (1975).
PAPER - 6
DEVELOPMENT OF PLASTIC SCINTILLATORS IN BARC
A.N. Rangarajan and CM. PauJ
DEVELOPMENT OF PLASTIC SCINTILLATORS IN BARC
A.N. Rangarajan and C M . Paul
Chemical Engineering Division
Bhabha Atomic Research Centre
Bombay - 400 085
INTRODUCTION
The plastic scintillator as a nuclear particle detector is
unique amongst different types of detectors. It is ehemically
inert, mechanically stable and can be prepared or moulded in
different sizes and shapes. The simplicity of its manufacture
and comparatively low cost are added advantages. The process for
the large scale production of plastic scintillators with
polystyrene as base has been successfully developed in Chemical
Engineering Division, BARC. The basic materials are locally
available. Machined and polished pieces of various sizes of
plastic scintillators were supplied to various research
laboratories of BARC, universities and other research
institutions. Besides larger blocks of 50 x 50 x 5 cm were
supplied to TIFR and NRL, for studies in cosmic rays.
PLASTIC SCINTILLATOR SOLVENTS
Monomeric styrene is a viscous liquid which polymerises on
heating to form a solid thermoplastic polymer polystyrene. The
vinyl group <-CH=CH2) forms the basis of all aromatic polymers.
Although vinyl toluene is a more efficient scintillation solvent
(10-15% higher) than polystyrene, the latter is preferred because
of its low cost and availability in the country. In acrylic base
scintillators employed for high transparency applications methyl
methacrylate monomer is used as a solvent.
SOLUTES
P-terphenyl (PTP) is an efficient primary solute with a
1
relative scintillation efficiency of 30% at an optimum
concentration of 1-3%. Its emission spectrum ( max) is 360 nm.
Since this emission is illmatched to S 11 photomultiplier
response, a secondary solute or a quartz window photomultiplier
is used to improve the scintillation efficiency. 2-phenyl-5- (4
biphenylyl) 1,3,4 oxadiazole (PBD) appears to be slightly more
efficient primary solute than PTP, although it is costlier.
TPB (1 I1 4 4' tetraphenyl butadiene) was the first
efficient substance discovered for use as a secondary solute in
plastic scintillators. Its emission spectrum ( max 445 nm)
matches well with the response of Sll photomultipliers. Though
Anthracene as a single crystal is the best known organic
scintillator, it is a mediocre solute for plastic scintillator.
However its derivatives with alkyl, aryl and methoxy groups in
meso positions are found to be efficient solutes. P-
guarterphenyl is another suitable secondary solute for use along
with PTP. The advent of oxazoles and 1,3,4 oxadiazole
derivatives and similar compounds led to their exploitation as
primary and secondary solutes for plastic scintillators. 4 4'
Diphenyl stilbene (DPS) and 2,5-Di-(4-Diphenylyl)-oxazole (BBO)
are the best secondary solutes. 1,4 Di-2-(5 phenyl oxazolyl)-
benzene) (POPOP) is also commonly used as a secondary solute with
PTP.
PRODUCTION OF PLASTIC SCINTILI.ATORS
Polymer grade styrene (purity 99.9%) is available with M/s.
Polychem Ltd., Bombay. The high purity scintillation solutes
like PTP, POPOP, TPB, PPO are being produced in the Chemical
Enginnering Division of BARC.
Styrene monomer is vacuum distilled to remove inhibitor and
oth3r oxygen bearing impurities. To fabricate a 50 x 50 x 5 cm
scintillator, 16 litres of styrene monomer along with 150 gms of
PTP (1%) and 7.5 gms of TPB (primary and secondary solutes
respectively) with 2 gms of Zinc stearate (mould releasing agent)
is polymerised at 140°C in a stainless ste??.! i factor vessel. The
reactor vessel is placed in an oil bath heated to 140°C. The
reactio'n starts within an hour with violent boiling. Being an
highly exothermic reaction, plenty of heat is liberated. But the
thermostatically regulated oil bath and the. condenser help to
control the react icr., Two more boilings are observed within an
hour, after which a jelly like ir.ass is formed. The heating will
be continued for another 22 hours to maximise the polymerisation-
It also helps to reduce the residual monomer content to as low as
0.1 to 0.2%. The presence of residual monomer reduces the light
output. The reactor vessel is transferred from the oil bath to a
water bath to shock .cool and release the block from the mould.
The blcok is annealed in a current of Nitrogen at 105°C for 12
hours to remove the internal stresses and strains and to improve
the mechanical properties.
CHARACTERISTICS OF A PLASTIC SCINTILLATORS
1. Composition Styrene monomerP-terphenyl
TPB
97.46%1%
0.05%
2. Resolution 20%
3. Decay time 3 n sec
4. Light output 60% of anthracene
5. Wavelength of max emission 425 nm
6. Molecular weight 80000-1,20,000 units
HEAVY ELEMENT LOADED PLASTIC SCINTILLATOR
In recent years, there has been an increasing demand for
scintillating plastics that show improved efficiency towards low
energy r-ray detection. Because of their low density and low
atomic number of its constituent elem^ntu (C & H) organic
scintillators have much lower r-ray absorption (especially at low
energies) than inorganic phosphors like Nal <Tl) or CSI (Eu).
Bur organic scintillators possess short decay time (3-4 n sec).
Since r-ray absorption in the photoelectric effect region is
proportional to the fifth power of the atomic number that is Z ,
it is reasonable to expect that by incorporating even small
amounts of heavy atoms into the matrix, a significant improvement
in the photosensitivity and resolution will result.
Since Tin (Sn) and Lead (Pb) have large cross section for
absorption of r-rays (Pb 1.68 x 10~ cm2/atom Sn 3.36 x 10~22
2cm /atom) incorporation of Tin or Lead will be desirable. Tin
compounds yield plastics with better clarity and even upto 10% of
elemental Tin could be loaded in PVT scintillator. Organo Tin
compounds were observed to quench the light output much less than
organo lead compounds. In addition, lead compounds have poor
solubility.
The organo Tin compounds like Tetraphenyl Tin and Triphenyl
4-ethyl phenyl Tin, which show good photoelectric peak resolution
in PVT or polystyrene plastics have been successfully synthesised
in the Chemical Engineering Division. A few plastic
scintillators incorporating these compounds have also been
prepared on an experimental basis.
POLYMETHYL METHACRYLATE SCINTILLATORS (PMMA)
Methyl methacrylate which is structurally similar to
nonaromatic liquid solvents like dioxane is found to be inferioi
in respect of energy transfer. But PMMA's high transparency and
good mechanical properties make them attractive as a base
material for scintillator.1 By using Naphthalene as a secondary
solvent (3%) energy transfer can be enhanced. Buty PBD and bis
MSB are being used as primary and secondary solutes respectively
(PTP is not suitable as a solute with naphthalene due to the
similarity of their spectra, thereby inhibiting energy transfer).
The primary and secondary solutes have been i-ynthesised in the
Chemical Engineering Division. A few experimental pieces of PMMA
scintillators with high transparency have made in glass vials.
PMMA (e>lso known as lucite, plexiglass or perspex)
scintillators because of their high transparency and clarity are
useful as light pipes. They are placed between the scintillator
and photo cathode and serve in helping to prevent trapping of the
light in the scintillator.
Plastic scintillating fibres command much attention these
days for applications in Hodoscopes, calorimeters, imaging trays
etc.
A STANDARD LIGHT SOURCE OF LOW INTENSITY
In thermoluminescence studies and in many photomultiplier
applications, a need exists for a standard reference light source
of constant output Cor frequent calibration of the photometric
system. A solid light source of scintillating plastic doped with14a C compound appears to have a distinct advantage over liquid
light source. It has a luminous intensity of 2 x 10
candles/cm and possesses long term stability and a very small
temperature coefficient.
To a pre-polymerised phosphor in a glass vial is added 0.1814mg of C labelled benzoic acid (25 uCi). The mixture after
stirring well is sealed in,a current of Nitrogen. Polymerisation
will be continued for 24 hours at 50-60'C. After completion of
polymerisation, the vial is broken to remove the plastic. It is
cut to the desired size and polished. Experiments with inactive
benzoic acid have been successfully carried out.
ADVANTAGES OF PLASTIC SCIBTTILLATORS
As no single scintillator has all the desired properties,
one chooses a scintillator which is most suitable for specific
applications. Some of the advantages of using plastic
scintillators are:
(1) For large size scintillators where it is impossible or too
expensive to use crystalline phosphors like anthracene or
stilbene.
(2) Simple to prepare, easy to machine and have good mechanical
and optical properties.
(3) The absence of hygroscopy and being a stable solid,
facilitates ease of handling, packaging and transportation.
(4) Compared to liquid scintillators, they have better thermal
stability and can be used over wide range of temperature (-
190°C to 70°C).
(5) They can be used in a wide variety of shapes like filaments,
thin films to count fission fragments, annular cylinders for
coincidence measurements.
Production of plastic scintillators (polystyrene base) of
50 cm x 50 cm cross section in the thickness range of 5 cm, 10 cm
and 15 cm was successfully demonstrated in the Chemical
Engineering Division of BARC. These scintillators are being
regularly supplied to various organisations to meet their
specific demands.
ACKNOWLEDGEMENT
The authors acknowledge their sincere thanks to Shri S. Sen,
Director, Chemical Engineering Group and Shri V.S. Keni, Head,
Chemical Engineering Division for their keen interest and support
in this work.*********************************
PAPER - 7
PLASTIC SCINTILLATOR SPONGE AND THIN FILM DETECTORS
DEVELOPED FOR CONTINUOUS ON-LINE MONITORING OF
TRITIUM IN WATER AND AIR
A.N. Singh, C.K.G. Nair and M. Rathnakaran
PLASTIC SCINTILLATOR SPONGE AND THIN FILM DETECTORS
DEVELOPED FOR CONTINUOUS ON-LINE MONITORING OF
TRITIUM IN WATER AND AIR
A. N. Singh, C.K.8. Nair a.nd M. Rathnaliaran
EIMVIRONNENTAL ASSESSMENT SECT I ON
BHABHA ATOMIC RESEARCH CENTRE
TROMBAY, BOMBAY 40O 085,
ABSTRACT
The paper describes the development of plastic sc::int.i 13 at or
based detectors -for continuous on line monitoring of tritium in
air and water. The detectors are prepared -from 5 uni thick plas-
tic scinfci 1 lator -films -for which a technique has been developed
that gives consistent results- The water detector shows a sen-
sitivity of 0.5 nCi/ml of tritium in water and the air detector
shown- a sensitivity o-f 1 uCi/rn3 o-f tritium in air. Both the
detractor;; have a -fast response, The detectors are in the -form
of- a flowcell made of perspex , coupled to a matched pair o-f
photonu.il tipi iers and the measurement is made-:' in the coincidence
mode o-f counting- With the introduction of mi ceo-computer based
background compensation, effect of- interfering gaseous
radionucl ides like A-41 does not a-f •feet the tritium measurement
in the air considerably. In presence of 2000 uCi./m13 o-f A-41
concentration in the air, the interference in the tritium chan-
nel is brought, down to 0.5 DAC level o-f tritium. The monitors
are designed for continuous on line operation.
1. INTRODUCTION
Continuous monitoring of- tritium presents difficulties
because? of two principal reasons. The first is due to the ex-
tremely low beta energy of tritium <1S keV ma;:..) and the second
is clue to the presence'.of usually large amounts o-f interfering
radi onucl :i des of much higher beta energy,. The low beta eneryy
could be detected only i f the sample is brought in intimate? con-
tact with the detecting surface. It has been evaluated that on
a detecting sur-face of 1 cm 2 sur-face area immersed in tritiated
water o-f 1 uCi/cm51 concentration only 62 tritium betas will
strike the surface every minuts* '•> . Detection of tritium with
reasonable sensitivity thus requires as large a detector surface;
area* as possible, in as small detector mass as -feasible, to keep
the gamma background low at the same time. For tritiated water,
a plastic scintillator sponge detector has been dev&loped that
combines these properties. <3J> The Sponge is prepared from 5 urn
thick plastic scintillator films and combines a surface arc??, o-f
3Odo cm15 in just i gm. For tritium in air detectors,about 4500
cma -film surface contained in a 65 ml flowcell has been
employed. The difference in wat&r and air detectors is mainly
in the former having wet operation and the latter having dry
operation. For reducing the effect of interfering
radionuelides, electronic cance11 ation of larger ©nergy pu1ses
and compensation technique for the residual background using
microcomputer have been employed.
2. Preparation of 5 urn Plastic Scintillator -films
The films sirs made on the surface of distilled water over
which a solution of the plastic scintillator is poured. The
material is dissolved in a solvent mixture consisting of 90%
ethyl acetate and 107. arnyl acetate. In 100 ml solvent, 3 g/ns
of the solute in the form of thin chips is dissolved. The solu-
tion gets ready within an hour. For each film sheet, 5 ml of-
the solution is measured in a measuring cylinder and poured in
quick succession over the surface of distilled water placed in a
tray of 30 cm ;•: 30 cm x S cm (depth). The solution rapidly
spreads over the water surface., After about 2 minutes, a large
number- of holes of 'varying sizes, start forming over the film
surface. The film gets- ready for removal in about 5 minutes. It
is removed by spreading •&. clean sheet of white paper an the film
surface and lifting it up. 20 such films each weighting 150 mg
and having surface area (each side) of about 250 cm2 can be
prepared from 100 ml solution. The f:i). ms are- then allowed to
dry on the paper, after covering it by another paper to prevent
the -film -from getting dirty.
3. Preparation o-f Detectors
For water detectors the cell volume is 15 ml, each ir.ell has
a diameter of 5 cm and is encased in aluminium. It consists of
two parts viz. the container and the lid with a neoprene 0-ring
in between. The two parts can bo screwed together airtight. A
nozzle on each of the two parts is provided -for water inlet and
outlet. To make the sponge; packing, the dry film is taken out
of the paper and is folded a number of times. It is then wetted
in distilled water and placed in the container part with a clean
forceps. It is positioned properly using a glass rod. For
satisfactory results, 7 or 8 such film sheets are stuffed one by
one and the lid is .tightened. After 3-4 hrs, the film packing
gets converted into a spongy disc of 48 mm dia. The holes on
the wet film surface play a role to convert the packing in to a
form of sponge that is highly porous. It permits water flow of
about 300 ml/miri in siphon mode, at i meter head of water. Fig i
shows the photograph of,the flow ceil filled with the detector.
The cell is then mounted on a pair of matched photomultipliers
type EMI 9635 A, face to face.
The cell for air detectors has a larger volume of 65 ml and
it. is stuffed only with dry film sheets. In the absence of
water wetting as was the case for water detectors, the sponge
formation does not take place. 6 to 10 films are used for each
det&ctar, depending upon the desired sensitivity. The outer
diameter of the aluminium clad of the flow cell detector is 95mm
and it is suitably t&pBrezd to conform to the requirements of 51
mm dia photomultipliers..
4. Performance of the Detectors
1. Tritiated Water Detectors:-
The important parameters to be studied are (a) flow of
sample water (b) tritium response as a function of number of
film sheets packed and (c) linearity of response for various
concentraicms o-f tritium activity in the water,
a) Flow of sample water
The detectors offer negligible resistance to flow of water
and d'o not require a peristaltic type of pump to push the water
through the detector. The detectors function quite well in
syphon mode of operation. Table-I gives the water flow rates at
various pressure heads.
Table I : Table showing the flow of water through the
detector as a function of the pressure head.
Water Flow Rate
(nil per min)
10
27
56
SI
105
120
<b) Tritium Sensitivity as a function of number of film sheets
Fig. 2 shows a plot of the count ratts obtained with various
numbers of film sheets used to prepare the sponge. There is
regular increase up to 7 film sheet after which rise becomes
slow saturating at 10 sheets.
(c) Linearity of Response
Tritiated water of various concentrations was flown through
the detector and the count rate for each concentration obtained.
The results are shown in fig.3. The response is linear.
2). Detectors for Tritium in Air
Using this detector along with the new microcomputer based
electronic system described later, it is possible to con-
tinuously monitor tritium even in presence of large levels of
gaseous interfering radi anticlines like A-41. The detector has a
• No.
1
2
3
4
5
6
Pressure
• (cms of
10
25
50
75
100
110
Headwater)
very fast response, <a few seconds only) and the sensitivity is
quite high.
The observed response is as follows:
Coincidence counting efficiency : 257.
Detector background
Observed count rate
Interference rejection
Sensitivity in presence of
2000 uCi/m* of.A-4i'
Percent of A-41 rejected
Equivalent to 1 uCi/m3
of tritium in air
50 counts/min for
luCi/nr3 tritium
80'/. rejection by pulse
height discrimination
Residual rejection by
background compensation
10 uCi/m* (0.5 DAC)
of tritium in air
99.5"/.
Gamma response
Other features
5. The Electronic System
Extremely low, a 6cm
thick lead shield could
eliminate it.
Simultaneous measurement
of A~4i activity.
Negligible attention in
continuous operation.
The electronic system incorporates the low and high voltage
power supplies, coincidence analyser and microcomputer based
counting and background compensation circuits. The entire
electronics has been accommodated in a standard siae small
cabinet. A tritium window has been provided for preferentially
separating tritium pulses from interfering pulses. The compen-
sation circuit involves digital subtraction of a predetermined
fraction of the counts above the tritium window (the upper
channel) from the counts in the tritium channel to offset the
spillover component of the interfering activity in the tritium
channel. The counting rates of the two channels (the tritium
channel and the interference channel), the compensated count
rate and the? conversion into DAC levels of tritium is printed on
a built-in minipri nt.er» The various parameters such as counting
times, number o-f readings to be averaged, the compensation
•factor, the conversion factor etc. are inserted through a built-
in keyboard. The parameters once entered are locked by the
software and cannot be disturbed unless the reset switch is
pressed, in which case the parameters will have to be entered
again.
The detector photomuitipl ier assembly .is incorporated in a
separate box. Provision is made for air inlet and outlet as
also the? power supplies for the phatomultipl ier and the -follower
circuit. A micropump of 1 1/min sampling rate, working on 6
volts DC is used for sampling. If necessary, the capacity of
the pump may be increased, though it is not essential. Fig.4
shows the photograph of phomultip!ier detector assembly with
the micropump and electronics. A flow cell without detector is
shown separately. For sampling in a remote mode, a pump of
higher capacity will have to be employed.
6. Conclusions
Plastic scinti11ator film detectors are found to be
suitable for 'tritium in water and air. The electronic: system is
common to both. In the present form, the? tritium monitors baaed
on these detectors ar& bound to be extensively used for the
monitoring of tritium in our heavy water type nuclear power
reactors.
7. References
1. Feinendecjen, L.E. (1967) "Tritium Labeled Molecules in Bio-
logy and Medicine (Academic Press, New York.)
2. Singh-, A.M., Rathnafcaran, M and Vnhra, K.G. , "An on-Line
Tritium-in-water Moni tor ".Nucl . i'nstr. and Meth A 236 < 1985)
159.
FIG. 1 . PHOTOGRAPH OF FLOWCELL ANDPLASTIC SCINTILLATOR SPONGE.
0 5 10 15NUMBER OF PLASTIC SIMULATOR FILMS
FIG. 2. COUNT RATE AS A FUNCTION OF NUMBER OF FILMSWITH 100nCi/ml OF TRITIATED WATER.
oo
i-zIDOo
50 100 150 200 250 300
TRITIUM ACTIVITY CONCENTRATION - TiCi / ml
FIG. 3 . ACTIVITY CONCENTRATION VERSUS COUNTRATE WITH 8 FILMS.
FIG.4. PHOTOGRAPH OF TRITIUM-IN-AIRMONITOR .
Published by : M. R. Baiakrishnan Head, Library & Information Services DivisionBhabha Atomic Research Centre Bombay 400 085