Standardization of 59Fe and 131I by liquid scintillation counting

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH

Nuclear Instruments and Methods in Physics Research A 339 (1994) 402-407 North-Holland

Standardization of 59Fe and 131I by liquid scintillation counting Giinthcr

E.W.

Physikalisch- Technische Bundesanstalt, D-38116 Braunschweig, (;ermany

The CIEMAT/NIST liquid scintillation efficiency tracing method is frequently used to determine the activity of pure beta-particle emitters. With the programme EBEGA it can be extended to beta-particle emitters with no more than one coincident gamma ray per decay path. In this work it is shown that the method can also be applied to 5')Fe and ~311,which are beta-gamma emitters with gamma cascades.

1. Theory of the tracer method

T h e principle of the C I E M A T / N I S T liquid scintillation efficiency tracing m e t h o d is a c o m b i n a t i o n of a theoretical calculation of the counting efficiency and an experimental d e t e r m i n a t i o n of correction factors by m e a n s of a tracer nuclide, for example 3H. The activity is d e t e r m i n e d in 3 steps: 1) Count rates and the q u e n c h - i n d i c a t i n g p a r a m e t e r s are d e t e r m i n e d for a set of samples of the nuclide to be measured, and for a set of 3H s t a n d a r d samples, with a different quench. By combining these data, a c o r r e s p o n d i n g 3H efficiency is o b t a i n e d for each sample of the nuclide. 2) T h e efficiency of the nuclide is theoretically calculated as a function of the efficiency of the tracer nuclide 3H. 3) This relation is used in conjunction with the measured data to calculate the efficiency for the nuclide and an activity value for each single m e a s u r e m e n t . The p r o c e d u r e is described in detail in refs. [1-3]. The m e t h o d is commonly used to d e t e r m i n e the activity of beta-particle emitters with no more than one coincident g a m m a ray per decay path, but it is also useful for the s t a n d a r d i z a t i o n of b e t a - g a m m a emitters with cascades of several g a m m a rays. For the calculation of the overall efficiency e, the decay scheme must be split into individual decay paths. T h e overall efficiency e is the sum of the weighted efficiencics of the n decay paths: tl

with the decay path probabilities Pw as weights.

The detection efficiency of the wth decay path with a beta transition followed by n,. g a m m a transitions is

~-w= l - ( t - ~ . )

(2)

with the efficiency of the beta radiation of the wth decay path ~-~,,., and the efficiency for the ith g a m m a transition of the wth decay path e~,,. The beta efficicncies el~,, are calculated with the C I E M A T p r o g r a m m e E B E G A [4]. As a data set of constants for the energy transfer from electrons to the Ultima Gold scintillator was not available, the constants for the Hisafe I1 scintillator [5], which is similar, were used. In g a m m a transitions, conversion electrons and g a m m a q u a n t a must be t a k e n into account: ~,,., = .'~.,~

..... .,

+ ( 1 - a',.,)%.,,.,

(3)

with ot~,., reduccd conversion cocfficicnt O'tot/(l + cetot),

e.ce.w.i efficiency of the conversion electrons, e.v.~,i efficiency of the photons, all of the ith g a m m a transition. T h c efficiency of p h o t o n s e~,i,~., in Eq. (3) is 6"V..'.i = Pint.w.,~'i . . . . '.i,

(4)

with the interaction probability Pint.wa and the efficiency of the g e n e r a t e d C o m p t o n and p h o t o e l e c t r o n s e,nt.wa. T h e calculation with the E B E G A p r o g r a m m e includes a M o n t e Carlo calculation of thc interaction probability Pint,w., and the calculation of the efficicncics t:'int,,,.i and ec~..,, ,. The average X-ray and A u g e r electron energies and emission probabilities used in the calculation of e ....... i are taken from rcf. [6]. Thc following efficicncics were calculated as a function of the efficiency of tritium, fTr,,ccr" ef3w with E B E G A , ew.~ with E B E G A , (3) and (4), e,. with (2),

0168-9002/94/$07.00 ,~, 1994 - Elsevier Science B.V. All rights reserved

SSDI 0 1 6 8 - 9 0 0 2 ( 9 3 ) E 0 7 ( I 7 - Y

H (1 - ~:~.,), ~=1

403

E. I'E Giinther /NucL Instr. and Meth. in Phys. Res. A 339 (19941 402-407

4451 d

59 F'o

aa

ill'

44.51 d

1

~2....

59 F e ~"

9'7 9"4 '1

{ '6'2 .~_.~_'

>

P5.......

-I

~1 f12"

~3-

7 3 7. /~1 4 7~ ~

...:iLL.....

II

~4"" ®~5-"

59 C 0

rl

9'2

~7 .'51~'6 9",5 /~3 ~'3 72 76 75

stable

Ill 111 59 C o

stable

Fig. 1. Decay scheme and split decay scheme of 5';Fe.

and e with (1). For interpolation p u r p o s e s a leastsquares fit of the relationship b e t w e e n e a n d en,c~ , with the polynomial 2

= E ~,(~+~ .... )m

(5)

m--0

is used.

2. Standardization of 59Fe 59Fe is a b e t a e m i t t e r with several b e t a a n d g a m m a transitions [7]. T h e simplified decay scheme a n d the split into s e p a r a t e decay p a t h s are shown in Fig. 1. T h e efficiency calculations were d o n e for tritium efficiencies in the region of e T. . . . . = 0 . 3 t O 0.6. m

tritium efficiency of 0.5 is r e p r e s e n t a t i v e for unq u e n c h e d samples in the U l t i m a Gold scintillator. T h e wide region of eT,,c~, is not necessary in practical use, but differences in the comparison of h i g h - q u e n c h meas u r e m e n t s with the predicted value may reveal incorrect calculations. T h e results of the calculation are shown in Tables 1 a n d 2 and in Fig. 2. D u c to the high energy the efficiency calculated only by taking into account the beta rays is 96.8% for u n q u e n c h e d samples. W h e n the emitted g a m m a rays a n d conversion electrons arc also included in the calculation the efficiency is e n h a n c e d by only 0.2 to 97.0%. T h e efficiency d e p e n d e n c e obtained was a p p r o x i m a t e d by the polynomial (5) with the coefficients k 0 = 0 . 8 8 7 2 , k 1 = 0 . 2 6 1 4 , and k 2 = -0.195.

Table 1 Beta and gamma transitions for 59Fe: Energies E k and E m, beta-particle efficiency e~k, reduced conversion coefficient or~, interaction probability Pint,,,, and efficiencies of generated Compton and photoelectrons, photons, conversion electrons, and the gamma transition eint.m, e.v.m, ece.m, •m (for ~'Tracer = 0.5) Beta transitions k E k [keV] 1 2 3 4 5

83.3 130.8 273.4 465.8 1565

Gamma transitions m E m [keVl 1 2 3 4 5 6 7

eak

142.65 192.34 334.99 382.5 1099.251 1291.596 1481.7

0.8296 0.8940 (I.9559 0.9788 0.9975

O/~n

Pint,an

Eint,m

E',t,an

Cce,an

~m

0.0070 0.0070 0.0000 0.0000 0.00(O 0.0000 0.0000

0.1268 0.1146 0.0952 0.0904 0.0636 0.0604 0.0598

(I.8611 0.9130 0.9682 0.9750 0.9965 0.9973 0.9973

0.1092 0.1046 0.0922 0.0882 0.(}633 0.0603 0.0596

1.000 1.000 1.000 1.tYO0 1.000 1.000 1.000

0.1154 0.1109 0.0922 0.0882 0.0633 0.0603 0.0596

V. STANDARDS, 75Se AND tY21r

404

I-. W (;iinther / N u c l . Instr. and Meth. in Phys. Res. A 330 (1994) 402-407

Table 2 Efficiency calculation for ~'~Fe decay with the decay path probability p,., the efficiency of the decay path e:,., and the contribution to the total efficiency p , e , . (for e:T,,,c~,= 0.5) w

path

p.

e.

p,,ew

1 2 3 4 5 6 7 8 9 sum

13F~7 [5?/4",/5 132-),y,/s [32~,~3,6 1327~2"ys 133%

0.00061 0.00021 0.0027 0.0092 0.0006 0.4271 0.0289 0.5280 0.0018 0.99912

0.8397 0.8544 0.9099 0.9119 0.9219 0.9585 0.9633 0.9801 0.9975

0.0005 0.0002 0.0025 0.0084 0.0006 0.4098 0.0279 0.5180 0.01)18 0.9695

~3"Y2'¥5

134~5 135 Fc-59

3. Experimental The m e a s u r e m e n t s were carried out in a t e m p e r a t u r e - c o n t r o l l e d (12°C) C a n b e r r a Packard m o d e l 2200CA liquid scintillation counter with an external 133Ba source for the d e t e r m i n a t i o n of the quench par a m e t e r tSIE. Nine samples with solution masses in the range of 40 to 80 mg and a carrier content of 25 ng Fe 3 ~ were prepared• For each one an a m o u n t of 0•25 ml of H D E H P (diethylhexyl phosphoric acid) or 30 mg T O P O (tri-n-octylphosphinoxide) was used as a complexing agent to prevent adsorption of the radionuclidc on the wall• In some cases the water was removed from the radionuclidc solution by adding 1 ml of propanol to the mixture and subsequent drying in a microwave oven. T h e n l(I ml of the Ultima Gold scintillator were added and after cooling to about 12°C, each of the samples was counted five times for 180 s. The results are shown in Fig. 3. Variable amounts of the fluores-

iO0

:i

0

99" ,It,

II-

0

98

54.

5'.5 ~ " T r Q c e

r

':-

tr'l

Fig. 3. Results for unquenched samples of 5'~Fc. a is the specific activity obtained with I,SC, a u, is the specific activity obtained with the ionization chamber. One data point represents the result of one measurement. The first result of a measurement is shown with the respective uncertainty bar (random uncertainty).

cence q u e n c h e r carbon tctrachloride were then a d d e d and the counting p r o c e d u r e was r e p e a t e d (Fig. 4).

4. Results The best group of samples with the highest results and the best stability in time are those p r e p a r e d after removal of the water• This had also been our experience in earlier SSFe m e a s u r e m e n t s . But some chemical problems still remain: the count rates decrease with time, therefore some adsorption of hydrolysed iron c o m p o u n d s is suspected• With this group of samples the specific activity of the solution agreed within 0.2% with an activity value obtained using an ionization c h a m b e r that had b e e n calibrated by m e a s u r e m e n t s on 59Fc solutions standardized by 4,-rl3-~/ coincidence counting•

99 • . • . ,

98 E

8g

..,.."

97"

• ••••••••'•••°'••

•°

FO

96" .,J u -I z

I00

• • •

c~

9,5-

99

94"

'i

i!!

0

93 2O

3'o ~" T

4'o t" , o , o e

go r

6'0

Ll~

Fig. 2. Detection efficiency of SgFc and ~311,vs efficiency of H.

9 8 3'o

5'0 ~r T

r ~ o e

r

tn

Fig. 4. Results for quenched samples of S'~Fe.

405

E.W. Giinther / Nucl. Instr. and Meth. in Phys. Res. A 339 (1994) 402-407

@

8 ~

719 7~n

7"/10

(~

/31 71 ~/19

,611 77111. fie 7t4 710 : 1

d

131i&,~---...~. ,,, ,,~ ..." &.". t~'"..~\

"" ]'

|[|"

76

131Xe

",

ZlO

',, 715 ,,' ; ,

®

/

,,,,',,,,,,. / @ ", ', ,, : &/@'

,,

" ./.7-, 11:'4 I 11-"3 ,3%m I 1.93

,@

76

;

B3"'/

==>

,%

77 71 /@

~t"" B2"'.

t7 .... y

I 'i i \\ I I1 \1 1 1

<%.

<--,

,

8021

,q: M_..e

f14 77 /71 //

,

I l l Is73,%: I1

d

stabil

mlXe

stabil

I

Fig. 5. Decay scheme and split decay scheme of 131I.

T h e s t a n d a r d deviation of the specific activity obtained with the C I E M A T / N I S T m e t h o d is 0.4%. T h e main c o m p o n e n t s are due to c o u n t i n g statistics, backg r o u n d variation, sample stability a n d comparison with the 3H tracer (0.3%), and to radioactive impurities a n d half-life u n c e r t a i n t y (0.2%).

5. Standardization of tat l 131I is a beta e m i t t e r with several beta a n d g a m m a transitions [8]. Again, the simplified decay scheme is split into s e p a r a t e decay paths (Fig. 5). A small fraction of the 1311 decay leads to 131mXe. T_he activity ratio

Table 3 Beta and gamma transitions for 1311: Energies E k and E,,,, beta-particle efficiency %k, reduced conversion coefficient o~, interaction probability Pint,m and efficiencies of generated Compton and photoelectrons, photons, conversion electrons, and the gamma transition emt.,n, ev.,n, ecc.,,,, e,n (for eTraccr = 0.5) Beta transitions k E k [keV] 1 2 3 4 5 6

%k

0.9458 0.9575 0.9621 0.9830 0.9839 0.9889

247.9 303.9 333.8 606.3 629.7 806.9

Gamma transitions E m [keV]

Otto

Pint,m

't:'int,m

E'*,m

6cc,m

Em

1 3 4 6 7 10 11 12 14 15 16 17 18 19

0.6109 0.9808 0.1942 0.0476 0.0476 0.0338 0.0320 0.0320 0.0223 0.0177 0.0088 0.0047 0.0093 0.1XM6

0.1430 0.1201 0.1179 0.1047 0.11)19 0.0978 0.0966 0.0963 0.0917 0.0894 I).0832 0.0764 0.0759 0.0731

0.6468 0.8861 0.911111 0.9568 0.9598 0.9671 0.9666 0.9666 0.9730 11.9773 0.9850 0.99118 0.9909 0.9924

0.0925 0.1064 0.1061 0.1 Of}1 0.0978 0.0946 0.0933 0.0930 0.0892 0.0874 0.0819 0.0757 0.0752 0.0726

1.0000 1.0000 1.0(R~ 1.0000 1.0000 1.0000 1.0000 1.0000 1.00011 1.0000 1.{XI(X/ 1.0000 1.0(030 1.(XXX)

0.6469 0.9828 0.2797 0.1430 0.1408 0.1252 0.1223 0.1220 0.1095 0.1035 0.0900 0.0800 0.0839 I).1)768

m

80.183 163.93 177.21 272.49 284.30 318.09 324.62 325.78 364.48 404.80 502.99 636.973 642.70 722.893

V. STANDARDS, 75Se AND 19Zlr

406

E. I,E Giinther /Nucl. Instr. and Meth. in Phys. Res. A 339 (1994) 402-407

Table 4 Efficiency calculation for )3)I decay with the decay path probability p,~, the efficiency of the decay path e w, and the contribution to the total efficiency pwe~ (for eTracer = 0 . 5 ) w

Path

l 2

13t-yw 13f¥ D't)~

3

6t~10~'t~

4

131~to~ t t~ l

5 0 7 8

132~u,

9

10 11 12 13 14 sum

p~

~2"Y12~4 [~3"~ 17 133%'Yt4 133"/6"Y 7"Y l 134')' 14 134~'7 ";/I

e.

p..e.

0.0179 0.0022 0.0006 0.0002

0.9499 0.9825 0.9853

0.0170 0.0022 O.O{X)6 0.0002

0.0037 0.0026 0.0715 0.0005 0.00004 0.834

0.9613 0.9731 0.9651

0.0035 0.0025 0.0690

0.9711

0.0005 0.0000

0.9575

0.9902 0.9848 0.9948 0.9884 0.9889 0.9828

0.0650

[55"/4 [3(, -y3 1-131

0.0007 0.0010 (0.0(X)2) 1.0(X)0

100

o 0 T-4

ti ) t

99

@ 0

98

3'0

4'0 ~r T r o o e

go r

Ln

Fig. 7. Results for quenched samples of 1311.

0.8212 0.0647

0.0(X)7 0.0010 0.(XX)2 0.9833

1 3 t m x e / t ' ~ t l d e p e n d s on the time difference b e t w e e n source production and counting. T h e activities can be calculated by B a t e m a n equations: ,4 1 + A() e -tn(2)t/TI

A x~ = A o (0.0080) - -TI ( e ~(2)' / T, _ C In(e)t/"l'xc ), T t - Tx,.

prepared. Eight hours later the samples are measured. T h e activity ratio is then 0.0002. T h e results of the efficiency calculation are shown in Tables 3 and 4 a n d Fig. 2. T h e efficiency calculated only by taking into account the beta rays is 98.0% for u n q u e n c h e d samples. W h e n the e m i t t e d g a m m a rays and conversion electrons are included, it is increased to 98.3%. T h e efficiency d e p e n d e n c e o b t a i n e d was approxim a t e d by the polynomial (5) with the coefficients k 0 = 0.9389, k 1 = 0.1391, and k 3 = - 0 . 0 9 9 .

(6) with ,4t the activity of t3tl at the time t, Axe activity of 13tmXe at the time t, A o activity of t31l at t = 0 , T t half-life of 13]I (8.021 d), Txe half-life of 131mXe (11.93 d) and t the time differencc since separation. It is assumed that all 13tmXe is removed w h e n the sample is

&oo

t

99' o

!

54~ 4

54.. :2 d"

T

t-cloe

54. r

Six samples with drop masses of a b o u t 100 mg were p r e p a r e d using lO ml of the Ultima Gold scintillator. T h e addition of 1 mg sodium t h i o s u l p h a t e p r e v e n t e d oxidation and loss of iodine. M e a s u r e m e n t s with a n d without the addition of c a r b o n tetrachloride were carried out as described for 5'~Fe.

7. Results

.)

0

6. Experimental

6

tn

Fig. 6. Results for unquenched samples of ) 3 t l . a is the specific activity obtained with LSC, ate is the specific activity obtained with the ionization chamber. One data point represents the results of one measurement. The first result of a measurement is shown with the respective uncertainty bar (random uncertainty).

T h e r e is no significant difference between thc resuits for u n q u e n c h e d (Fig. 6) and q u c n c h e d samples (Fig. 7). T h e specific activity of the solution agreed within 0.2% with an activity value o b t a i n e d using an ionization c h a m b e r that had b e e n calibrated by meas u r e m e n t s on t31I solutions s t a n d a r d i z e d by 4"rr[3-~, coincidence counting. T h e s t a n d a r d deviation of the specific activity obtained by LSC is 0.4%. T h e main c o m p o n e n t s of the uncertainty are due to decay data (0.2%), c o u n t i n g statistics, b a c k g r o u n d variation, sample stability and comparison with the 3H tracer (0.2%), and to radioactive impurities and half-life uncertainty (0.2%).

E.I,K. Giinther / Nucl. Instr. and Meth. in Phys. Res. A 339 (1994) 402-407

References [1] A. Grau and E. Garcla-Torafio, Int. J. Appl. Radiat. Isot. 33 (1982) 249. [2] E. Garcla-Torafio, A. Grau Malonda and J.M. Los Arcos, Comput. Phys. Commun. 50 (1988) 313. [3] A. Grau Malonda and E. Garcia-Torafio, CIEMAT Report no. 616 (Madrid 1988). [4] E. Garcia-Torafio (CIEMAT Madrid), private communication (1988).

407

[5] J.M. Los Arcos and C. Borras, CIEMAT Report no. 646 (Madrid 1990). [6] E. Browne and R.B. Firestone, in: Table of Radioactive Isotopes, ed. V.S. Shirley (Wiley, New York, 1986) Appendix C. [7] F. Lagoutine, N. Coursol and J. Legrand, Table de radionucl6ides, LMRI (Paris) Sheet Fe-59 (1984). [8] F. Lagoutine, N. Coursol and J. Legrand, Table de radionucl6ides, LMRI (Paris) Sheet 1-131 (1984).

V. STANDARDS, 7SSe AND t92Ir