Absorbed dose from a beta source as shown by thermoluminescence dosimetry

~a~on~

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~ Apph~ Ra~ahon ~ d l ~ t o ~ s

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pp ~ 5 ~ 7

P~gamon P ~

Pnn~d m G r i t Bn~n

Absorbed Dose a Beta Source as Shown by Th m umin cence Dosimetry A

G

WINTLE

and

M

J

AITKEN

R ~ e a ~ h L a b o ~ t o ~ ~ r A~haeolog~ O x ~ d U m v ~ ,

6 Ke~e Roa~ O x ~ d

(Rec~ved 30 September 1976) A depth-dose curve w ~ obtained ~ r a 9 ° S r - 9 ° Y b ~ a source using a fine grmn TL phosphor to measu~ ~ e absorbed dos~ alumlmum absorbers bong ~nt~posed between ~ e s o u s e and ~ e d ~ e ~ o r , the curve went ~rough a maximum at an absorber thickness of about 40 mg cm- ~ Th~s curve was then ased to p ~ the average dose r a ~ to various ~ k n ~ s of calcium fluoride which has a mrmhr absorpuon charac~nshc to Mum~mum, these values were compared with expenmen~Hy d e ~ n'uned dose ra~s Th~s work was done m connec~on w~th ~ e r m o ~ m m e ~ e n c e da~ng of flint and c a l c ~ ~n a ~ h a e ~ o g y and geo~gy 1. I N T R O D U C T I O N FOR THE therm~um~nescence (TL) dating of fl~nt ~nd c a l c ~ it ~s necessary to know the average dose dehvered to a t ~ n shce (of the order of 0 3 m m t h ~ k a n d 3 m m s q u a r ~ by the beta r a & a ~ o n ~ o m a ra&o~sotope source On the present work t h ~ was a th~n h y e r of 9°Sr-9°Y) ~ n c e an error ~n e v a ~ a ~ n g th~s average ~ v e s rise to a p r o p o r ~ o n a l error ~n the T L a g ~ ~t ~s demraNe to keep it b d o w a ~ w per cent O n e approach wouM be ~0 c a h b r a t e each s a m ~ e by g a m m a ~rad~a~on but t ~ s is not a convement roua n e procedure I n s ~ a d the variation of dose w~th depth has been measured e x p e n m e n t a l ~ ~n Murtumum and ~ o m th~s the average dose can be c ~ c u h ~ d for each thickness of ~ n t or c ~ o t e used The v ~ d ~ t y of th~s p r o c e d u ~ has been checked using shces of a ~ a n s p a r e n L ~ g h ~ e n ~ v a y TL p h o s p h o r (CaF~ D ~ , these checks included c a h b r a ~ o n with gamma l~adla~on

~ PREVIOUS WORK Several ~ u & e s cf absorbed b e ~ dose tn various m a t e n a ~ have been ~ p o r t e d ~ r 9°Sr-O°Y sources connected with t h o r use in r a ~ o t h e r a p ~ ~ a n d ~n such m d u ~ n M d e ~ c e s as beta-ray t h ~ k n e s s gauges ~ D e p t h - d o s e curves for a large 9°Sr-9°Y b ~ a source used for w h o ~ body ~ r a & a u o n s have been o b t m n e d w~th a flat l o m z a ~ o n c h a m b e r by H a y ~ t t l d 3) An m~ a l increase ~n dose was observed as perspex absorbers were interposed ~n front of the c h a m b e ~ followed by a decrease as a ~ e n u a ~ o n became d o m i n a n t , the m a ~ m u m dose c o ~ e s p o n d e d lo an absorber thickness of about 35 mg c m - ~ The bm~625

up has been explained as b o n g due to m ~ e a s e d obhq u l ~ of the electron paMs caused by m ~ p ~ e ~ lng as they p e n ~ r ~ e the sample (4 57, thus ~ e p ~ h ~n~h/umt M~kn~ and hence the energy ~ s ~ ~nc ~ a s ~ wlM ~epth •

EXPERIMENTAL

DETAILS

The d e p t h - d o s e curve was obtmned by measuring Me dose absorbed ~n a thin h y e r of fine grmns ~ - 8 ~ m ~ a ) of CaF~ Dy (Harshaw T L D 200) d ~ p o ~ t e d on an ~ u r m m u m d~sc 0 4 5 m m thxck a n d 4 m m & a The absorbers were ~ s o 4 m m &a a n d were 50 a n d 4 6 0 ~ m ~ c k , Mey were Naced d ~ c t l y on ~ p of the fine gram d o ~ m ~ T e s ~ of the d e , h dose curve were carried out u ~ n g 100 ~m grains a n d shces of CaF~ Dy cut w~th a w~re saw ~ o m extruded ch~ps 3 2 × 3 2 x 091 m m V a r m ~ o n ~n thickness ~ r a 3 5 0 g i n shce cut ~n M~s way was ± 1 0 # m A~er lrradragon Me TL was m e a s u ~ d w ~ ~ e ~ a n d a r d TL d a ~ n g s y s ~ m (6~ The b e ~ source u ~ d was a 500 ~C~ p h q u e source of 9°Sr-9°Y, ~ p e SIP, ~ o m Me Ra~ochem~cal C e n ~ A m e r s h a m The acave m a ~ n ~ t a s an a ~ a of 1 cm ~ a n d a t h m k n e ~ of 0 02 m m It is d e p o u ~ d on a substrate of ~lver and ~s screened by 0 1 m m of ~lver Th~s screemng ~s suffioent to a b s o r b Me beta p a r a d e s ~ o m 9°Sr, w ~ c h have a m a x i m u m energy of 0 5 4 M e V , ~ r 9oy Me m a ~ m u m ~ n ~ g y ~s 2 26 MeV ~ r a & a ~ o n s w ~ e p ~ r m e d w~M Me beta s o u s e 1 6 2 m m above the 0 5 m m thick mchrome oven p h ~ or 15 7 m m above a perspex Nock F o r the g a m m a ~ a d m ~ o n s the ~ m ~ e s were p h c e d ~nmde small a ~ m l m u m c a p s ~ which had a w ~ l th~ckn~s of 2 m m

626

4 G Wmtle and M J 4ttken

th~s was found to be 8 7 ± 1 2°Umm F~gure 2 shows the c o ~ e ~ e d d e p t h - d o s e curve to be used in the case of a 350 #m shce The spot values shown on th~s curve were obtained by plamng the fine gram dommemr in succes~ve p e n n o n s m a 350 gm stack of alunumum absorbers These confirm the vM~d~ty of the correcnon made

% TE~S

4~

50 I

,)ol Depth

~1

2on~

~o

m g / c m -~

FIG 1 Fra~mnM depth~lose curve for a "°Sr 90y beta source umng fine gram CaF2 Dy dommeter and alumamum absorbers

4. D E P T H - D O S E C U R V E F~gure 1 shows the v a n a ~ o n of dose rate w~th absorber th~kness obtained u~ng the fine g r i n p h o ~ phor and the 4 r a m ~ a m e ~ r ~ u m m m m absorbers The ~ a d ~ a ~ o n s were performed ov the n ~ h r o m e heater plate The maximum dose rate occurs at a th~kness of about 40 mg c m - z B~ore th~s curve can be used to predtct the average dose rate rec~ved by samples of d~fferent thicknesses ~ o m th~s source the dependence of dose rate on d~stance ~ o m the source must be taken into account At a d~stance of 16 m m ~ o m the face of the source

12

OF THE P R E D I C I ' E D D O S E RATES

Following the procedure outlined in the previous sec~on, corrected curves can be obtmned for any thickness of materials h a ~ n g the same absorphon coeffiment One can then obtmn the raho between the average dose rate and the dose rate to fine grmns placed on the same ~rradlahon surface Th~s ~s done by lntegra~ng under the curve and d ~ d ~ n g by the s a m p ~ thmkness Some values are g~ven ~n the second column of Table 1 (0 To confirm the vahd~ty of these ca~ulat~ons CaF 2 D~ sltce~ 350 and 9 1 0 # m thick, and 100pm grams were o v e n a known g a m m a dose A~er measurement of the TL ~nduced, the shces and grmns were re-~rrad~ated w~th the beta source, for th~s ~rrad m h o n the s a m p ~ s were placed on a 045 m m thick alum~mum d~sc placed on the m c h r o m e plate to mm~mtze the effects of backscatter ~ o m the n ~ h r o m e By c o m p a n n g the TL m e a s u r e m e n ~ the average dose rates to the samples were obtmned Comb~mng these w~th the fine grmn dose rate obtained ~n the same gamma ~rrad~atlo~v~ the experimental dose rate rahos are obtmned and are gwen ~n the third column of T a b ~ 1 There is good agreement for the 100pm grmns and the 350 gm shce~ and reasonable agreemm~t for the 9 1 0 p m Mice The rat~o of measured and TABLk 1 Calculated and measured rahos of dose rates to slices of pamcular thicknesses divided by him grmn dose rate for same backing materud

o

g g

Slice thxckne~s I~ml IO~

I00

200

$00

Depth /microns

FIG 2 Depth dose curves for a 350 ~m shce la) as obtmned by placing up to 350 am of aluminlum on top ol the dommetcr (b) a~er a height correc~on ol 8 7°dram has been apphed to cur~e (aL the triangles on curve (hi are points obtained with the dommeter at various h~gh~ in a 3Nl~m stack ot alummmm absorbers

lt~ 2~ 3~) 350 4~/ 5~) ~Rt 7~ 8~ 9l~1 ~ou,

Average beta dose rate Fine grain dose rate lal Calculated ~b) Measured

Ratio (bF(a)

16 21 23

19 ± 004

O~

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27 ± i)(~

(~

22 N) 18 15 12

tN

(M

±

0{~

The fine grain dose rak error is +_~",,

t)9{

Absorbed dose from a bern source as shown by TL do~me~v

TABLF 2 Comparison of ca~uhted and experimental dose rates in a 350pm CaF2 Dy shce w~th and w~thout alumlmum absorbers Absorber thickness (#m)

With absorber W~thout absorber x 100 Ca~u~ted Expertmental

100 250 450

I00 7 93 5 805

100 93 80

Experimental reproduobd~ty for these measuremen~ was ±2°° c a ~ u h ~ d beta dose rates are ~ v e n ~n the fourth column 01) An m&rect check of the u~hzaUon of the depthdose curve was also made The average dose rates recewed by a 3 5 0 ~ m shce through 100, 250 a n d 4 5 0 p m of a l u r m n m m w ~ e measured and c o m p a r e d w~th that o b t m n e d when no absorber was present Table 2 compares lhese values w~th those predicted by the d e p t h - d o s e curve, there ~s e x c d ~ n t agreement Th~s experiment was carried out on both the mchrome p l a ~ a n d on the perspex base, the resuRs were the same w~thm the experimental erro~ confirm~ng the n e g h ~ b l e effect of backscatter on the ra~os b ~ n g measured 0u) E x p e n m e n ~ were carried out m which two 350 # m shces were ~ r a & a t e d on top of each otheL both on m c h r o m e a n d perspex They then had their TL measured s e p a r a b l y a n d the resuRs are ~ v e n tn F~g 3 In both cases the r a u o of the TL from one

627

shce a~er ~t h a d been ~ r a & a ~ d under the o ~ e r shce to that when ~t had been l ~ a ~ a ~ d on ~ own at the same ~ s m n c e ~ o m the source was 0 81 ± 0 0 2 Th~s ~s Io t e compared w~th tt~e t h e o ~ u c a l raUo of 0 85 o b t m n e d from the a p p r o p n a ~ curves 0v) F r o m the d a m m F~g 3 it ~ s a ~ o pos~Me to o b ~ l n the percentage effe~ of the b a c k s c a t ~ r ~ o m the n ~ h r o m e Nate and from perspex for a 3 5 0 ~ m shce T ~ s ~s done by c o m p a n n g the TL ~ o m a shce lrra&ated on nmhrome or perspex w~th that ~ o m ~ e same ~ e ~ a & a t e d on top of the second shce A~er the h ~ g h t c o w e c n o n has been a p p h e & the effect of the n ~ h r o m e N a ~ ~s to ~ncrease the totM average dose rate by 10°~ whereas the effect of the perspex ~s to decrease ~t by 7°~

6. C O N C L U S I O N The work reported here ~ m ~ s that the depth~dose curve obtmned for Murmmum can be s u c c e ~ f u l ~ empMyed to p r e & ~ the average dose rates to m a t e r ~Ms w~h a u m d a r bern absorption r e s p o n ~ e g CaF2, • nt a n d c a l o ~ It m u ~ be remembered however that the observed TL depends also on the e x ~ n t of s d ~ a b s o r p ~ o n of ermtted hght H e n c ~ m the a p p h c a t m n to T L d a h n g of m a ~ n a ~ such as fl~nt a n d c a l o ~ th~s effect must Mso be c o m ~ n e d in the m ~ g r a l

Acknowledgements--~nanc~M support

On nlChrome TL

TL

I ~1

Io6

I

21

by the Nuffidd Founda~on ~s gra~ful~ acknow~dged The ~lces were cut by Mrs N a ~ h e G a r ~ n Gamma ~rradmnons were carried out at the E n ~ n m e m M and M e d ~ S ~ e n ~ s D i a t o m AERE Harwe~ the N ~ n ~ Ra&~o~c~ P~c~on Boat& Harwell and ~ e C h u r C H H o s ~ k Ox~rd We are g m ~ l to P r o C t o r J H Boag ~ r b n n ~ n g to our a~en~on ~ e exphna~on of the b m l ~ u p effect and ~ r helpful &~usm~n

112

REFERENCES On perspex

I I

,ooo ~] 91

t [

,o,: ~1

9~

FIG 3 NormM~ed TL measuremen~ from two 350#m shces ~ a & M e d m jux~po~non

1 RUDEN B I and BENGTSSON G Phvs Med B ~ ! ~ 186 (1974) 2 FRANOS T M and SEYMOUR R NatlonM Ra&olo~cM Protec~on Board Report, NRPB-R4 (1972} 3 HAYmTTLE J L Br J Ra~ol 3% 297 (1964) 4 BOAG J W In Manual on Radmtmn Haematoloq~, p 31 IAEA, V~nna (1971) 5 BOAG J W Br J Ra&ol, 4 ~ 229 (1972) 0 AITKEN M J and FLEMING S J in T o ~ c s m Ra&atton Domm~ry Supplement 1 I (1972) 7 W~NTLE A G and MURRAY A S Archaeom~ry 1% 95 (197~