KJELLER REPORT

HIOH EFFICIENCY COUNTINO OF IEIKELIUM-249 I V A LIQUID SCINTILLATION TECHNIQUE

I N S T I T U T T F O R A T O M E N E R G I Kjeller - Norway

INSTITUTT FOR ATOMENERGI

KJELLER RESEARCH ESTABLISHMENT

K j e l l e r , Norway

MUH EFFICIENCY COUNTING OF BERKELIUM-.^

bi 1 A LIQUID SCINTILLATION TECHNIQUE

by

x) W.A. van der HeijdLn und V. r.raak

This work was carried out as part of a .joint project of Institutt for Atomenergi, Norway, and Reactor Centrum Nederland, the Netherlands.

January 19 TO

Present adress: Reactor Centrum Nederland, Petten (N.ii.) the Netherlands

ABSTRACT

A liquid scintillation techniqu" f T high efficiency counting of BK is described. A detection efficiency of about 9J? wan obtained.

C O N T E N T S

AK'.TRACT

L. i!iThOMJirrio:i i

EMlKBUillT 2

? . l Li rJui 1 .Joint i l ± a t i o n Counter ?

2..' ;fi;;-!'l .w i TI Proport ional Counter 3

3. :;oi,Lri'io;i:; 3

h. IMTU: r'-!:rAHA,'n:i i*

•;. HL;;":/:':; A:::, ••'.•.xmziou 5

5.1 rJai:-ricv '4 r Beta Propor t ional Counting 5

5.2 Liquid S c i n t i l l a t i o n Counting 5

5.3 Determination of the Detection C . . 2J49

Efficiency of Bk

6 . CONCLUSION rt

7 . ACKNOWLEDGEMENT 7

8 . REFEREI1CES 8

LIST OF TABLES

21)9 Table I Gas-flow u n beta proportional counting of Bk PhQ Table II Liquid scintillation counting of BK

HIGH EFFICIENCY COUNTING OF BERKELIUM-2U9 BY A LIQUID SCINTILLATION TECHNIQUE

by

W.A. van der Heijden and W. Kraak

1. INTRODUCTION.

For absolute measurements of beta emitting isotopes by Un pro­portional counting, the loss of electrons by absorption in the source and i t s supporting fo i l , makes a correction neceasary for most B-emitters. Operating the h TT proportional counter in conjunction with a gamma or X-ray detector, this correction can be obtained by the coincidence method (1) or, for pure beta emitters by the "efficiency tracing" technique (2) in which a suitable B~Y "tracer" i s added to the sample. The lacX of a

2U9 suitable B-r tracer for Bk (max. B-energy 125 KeV (3)) might be the reason why no standardization method is found in l i tera ture for this isotope* Impurities in the 3k-samples introduce another difficulty; even purification by the method of i-looi et a l , (h) leaves some inactive (probably organic) material in the samples.

Standardization by the liquid scinti l lat ion integral counting technique, f i r s t mentioned by Belcher {5)» is performed with a liquid scint i l la t ion spectrometer by extrapolating the integral bias curve to zero energy (6 - 10).

The detection efficiency for liquid scint i l la t ion counting of beta-active isotopes depends on a . o t :

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a) the energy of the beta p a r t i c l e b) the nature of the s c i n t i l l a t o r solut ion c) the photomultipliers d) the e lectronics of the spectrometer.

In t h i s work the l iqu id s c i n t i l l a t i o n determination of Bk i s studied; the resul ts are compared with those obtained by gas-flow h ti beta proportional counting.

2k9 The counting e f f ic iency of Bk under our experimental con­

dit ions was derived from a plot of the experimentally determined coun­t ing e f f ic iency of several standardized 8-emitters against the ir maxi­mum 6-energy.

2. EQUIPMENT.

2.1. Liquid Scintillation Counter.

Commercially available liquid scintillation spectrometers, Packard Tri Carb. Model 3303 and 3315 (1966) were used.

The spectrometers, supplied with two photomultipliers for co­incidence counting, have three channels, each of them equipped with two discriminators to set the energy "window". The lower discriminators were calibrated by plotting the discriminator settings versus the corre­sponding pulse heights produced by a pulse generator. As seen from fig. 1 straight line extrapolation of this plot gives the real zero of the discriminator as minus 1 (scale 0 - 100). The plot is linear above dis­criminator setting 2. The three discriminators of both spectrometers proved to be identical.

Because of the marked increase in noise with 5.ncrease in tempe­rature, the liquid scintillation counter was operated at 9°C (at lower temperatures solidification of the scintillator occurred). Samples were measured in 20 ml screw-capped glass vials.

3 -

The t i n e elapse between sample preparation and measurement was su f f i c i en t t o allow the samples t o reach temperature equilibrium and to avoid the influence of phosphorescence on the measurements.

2 . 2 . Oas-flov U it Proportional Counter.

The apparatus UBed vas delivered by Frieseke und Hoepfner G.M.B.H., Erlangen Bruck. I t i s provided with a gas-flow counter of type FH 51 and a sca ler with power unit o f type FH 90, The counter was operated at 3750 vo l t s and pure methane was used as counting gas .

Sample p l a t e s , on which the a c t i v i t y was measured, were prepared from a solut ion of VYHS* in cyclohexanone. A drop of t h i s solut ion i s allowed to spread on a clean water surface, the solvent evaporates and the resu l t ing f i lm i s l i f t e d o f f from the water by means of a metal frame.

p The p l a s t i c film weighing 5 - 1 0 ug/cra was coated with gold at both s ides ( to improve the conductivity and to lower the probabil ity of film breakage when the sample i s dried under a heat lamp). The gold was sublimated on the film by e l e c t r i c a l heating at about 1200 C, and a pressure of approxi­mately 10 mm Hg. This treatment increased the weight by 20 - 30 vg

2 per cm .

3 . SOLUTIONS.

For a l l our measurements a l i q u i d s c i n t i l l a t o r so lut ion of the following composition was used:

0 .7 w/v % a - tU ' - t -buty lpheny lK^V' -b ipheny ly l l - l .S . ' t -oxad iazo le (butyl PBD)

10 w/v % naphtalene

1 v / v f b i s - (2 -e thy lhexy l ) phosphoric acid (HDEHP), the primary solvent was p-dioxane.

VYHS i s a so lut ion of a p l a s t i c material obtainable from DuPont.

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To s t a b i l i z e the sample-sc int i l la tor mixture HDEHF vas used

instead of trioctylphosphine oxide ( 1 3 ) , s ince HDEHP shoved t o be

superior under certain circumstances.

Radioactive solut ions used are:

A_ Solutions supplied by the Radiochemical Centre Amersham:

1) S in 0.01 M n i t r i c ac id , containing as carrier 25

micro gram of sulphur as Li„S0. per gram of so lu t ion .

Act iv i ty 10.1(3 uC/g of so lu t ion .

go 2) Hi in 0.01 M n i t r i c acid with 25 Pg nickel as NiCl„

per gram of so lut ion . Act iv i ty 0.^86 uC/g of so lu t ion .

B_ Solutions supplied by Packard, Dovners Grove, I l l i n o i s , USA:

1) Toluene labe l led with C . Act iv i ty 1|.37 x 1 0 5 * 1.0 %

dpm/ml.

2) Toluene labe l led v i th H . Act iv i ty 2.77 x 10 * 1.0 %

dpm/ml.

C_ A berkelium-2U9 s o l u t i o n , stored in our laboratory s ince 1962,

was used for the counting experiments. Before use the solution

was purif ied by an extraction chromotographic procedure as

described by Kooi et a l . (U).

It. SAMPIÆ PREPARATION.

Samples for counting were prepared by weighing the act ive so lut ion

on a micro balance (accuracy ca. 10 ug) . The following technique was

applied: a pipet v i t h a capi l lary t i p , closed at the other end v i t h a

small rubber t e a t , vas f i l l e d sr that no so lut ion vas present in the c a p i l ­

lary t i p ( to avoid evaporation). The p i p e t , handled v i t h tweezers , was

weighed before and after dropping a small amount of the ac t ive so lut ion on

a counting p la te or in a v i a l containing l iqu id s c i n t i l l a t o r so lu t ion .

Pipets were coated beforehand with dimethyl-dichloro-si lane t o avoid ad­

sorption of a c t i v i t y at the g lass surface.

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The counting f o i l s for b it proportional counting vere pretreated -2

with a v e t t i n g agent; for t h i s a so lut ion containing 10 ug insul ine per ml was used. Before drying the act ive so lut ion on the counting f o i l , one drop (10 - 20 ug) of a 1:10 di luted solut ion of Ludox HS (a c o l l o i d a l s i l i c a so lut ion) was added to promote the formation of very small, c r y s t a l s . The l i q u i d of the sample vas careful ly evaporated by gently heating under an infra-red lamp.

5. RESULTS AKD DISCUSSION.

5 . 1 . Gas-flow k n Beta Proportional Counting.

The resu l t s of eight gas-flovr U n proportional measurements of the berkelium solut ion are summarized in Table I . Ho corrections for se l f -absorption are included. The arithmetical average value for the a c t i v i t y was 1.301* pC per gram o f berkelium so lu t ion . In our eight measurements the maximum deviation from t h i s average value was 8.1 %.

5.2. Liquid Scintillation Counting.

The samples were counted at a certain high voltage se t t ing (the one with the highest e f f i c i ency and the best ra t io between counting rate and background was chosen), and at ten to th ir teen different, d i s ­criminator s e t t i n g s . Plots were made of counting rate versus discriminator s e t t i n g and by extrapolating t o the real zero of the discriminator the a c t i v i t y o f the samples was obtained. This s t r a i g h t - l i n e extrapolation was done by the l e a s t square f i t method s ince i t i s more r e l i a b l e than graphical extrapolation ( f i g . 2 ) .

The data o f ten samples counted with the older l i q u i d s c i n t i l l a t i o n spectrometer (model 3303) are given in Table I I . The arithmetical average a c t i v i t y was 1.562 pC per gram of berkelium s o l u t i o n , while the maximum deviation from the average a c t i v i t y was only 0.9 %• I t should be noted

: i .

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that the average activity i s 20Jt higher than in the case of gas-flow 2̂ *9 proportional counting, while the detection efficiency of Bk in the

newer spectrometer (TriCarb Model 3315) is s t i l l 10? higher. Striking too ia the difference in the scattering of the resul t s . The range (in percentage of the average value) in which a l l the values were found, was 15.l*Jf for gas-flow and l,55f for liquid scint i l la t ion counting. By taking special precautions (12), the reproducibility can be utterly increased.

pho 5.3. Determination of the Detection Efficiency of Bk .

2U9 The counting efficiency for Bk was studied with our newer liquid scint i l la t ion spectrometer (model 3315). Sompj.ec of four standar­dized, low energy, pure beta emitters, H , Ni , C , and S were counted by the described liquid scint i l lat ion technique. The detection efficiencies derived from these results were plotted against the maximum 8-energy of the isotopes. This plot shows a straight l ine (f ig. 3) from which, a l ­though there is no theoretical justification as yet , a 935É counting eff ic i -

249 ency for Bk was derived.

Compared to Goldstein's results (10) our plot shows and important increase in counting efficiency, probably caused by the higher efficiency of butyl-PBD (11) compared to PPO and t'ae use of better photomultipliers.

CONCLUSION.

A relatively simple, rapid and accurate high efficiency liquid scint i l la t ion counting method for Bk was worked out.

The detection efficiency of this method was about 30Jf higher than in the case of k n (S-proportional counting. The standard deviation for the liquid scint i l la t ion measurements was 0.3 % (= l a ) , while for gas-flow proportional counting 1.8 St was found.

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7. ACKNOWLEDGMENT.

This work was carried out as part of a jo int project of I n s t i t u t t for Atomenergi (Norway) and Reactor Centrum Nederland (the Hetherlands). The authors are grateful to t h i s I n s t i t u t e ' s Isotope Department for kindly putting the gas-flow proportional counter at the ir d i sposa l , and for the ir helpful guidance during the measurements, and to Mr. K. Joon for his in teres t and valuable d i s ­cussions •

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8. DEFERENCES.

1. Gunnink, B., Colby, L., and

Cobble, J.W.

2. Campion, P.J., Taylor, J.G.V.,

and Merritt, J.S.

3. Vandenbosch, S.E., Diamond, H.,

Sjoblom, R.K., and Fields, P.R.

k. Kooi, J., Boden, E., and

Wijkstra, J.

5. Belcher, E.H.

6. Steyn, J.

7. Duftschmid, K.E.

8. Flynn, K.F., and

Glendenin, L.E.

9. Horrocks, D.C. and

Studier, H.

10. Goldstein, G.

11. Scbmid, K., and

Anliker, R.

12. Vaninbroukx, B. aad

Spernol, A.

13. Joon, K. and Deurloo, P.A.

Analyt. Cbem. 1959, 31, 796

Int. J. Appl. Badiat. Isotopes

I960, 8, 8

Phys. Bev. 1958, 115. 115

J. Inorg. Nucl. Chem. 196A,

25 ?300

J. So. Instr. 1953, 10, 286

Proo. Phys. Soc. 1956, A69_, 865

ANL-Trans-216

Phys. Rev. 1959, ll£, 7&U

An<L. Chem. 196l, .J3, 615

Nucleonics 19É5, 23., no. 3, 67

Int. J. Appl. Radiat. Isotopes,

1967, 18, 307

Int. J. Appl. Radiat. Isotopes,

I965, l£, 287

Int. J. Appl. Badiat. Isotopes,

1965, 16., 33U

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TABLE I

Gas-flow l*n t ieta p ropor t iona l counting of Bk'

Sample Sample weight

i n grams

Sample a c t i v i t y i n c.p.m.

A c t i v i t y i n

c .p .m. /g

A c t i v i t y i n

UC/g

Deviat ion of average va lue

%

A 0.006910 21,1*67 3,106,657 1.399 + 7 .3 B 0.015319 1*6,808 • 3,055,552 1.376 + 5-5 C 0.022905 63,163 2,757,608 1.21*2 - U.B D O.O228T8 60,901 2,661,990 1.199 - 8.1 E 0.012616 36,1*58 2,889,822 1.302 - 0.2 F 0.017956 51,1*63 2,866,061 1.291 - 1.0 G 0.019078 56,271* 2,91*9,680 1.329 + 1.9 H 0.012616 36,282 2,875,872 1.295 - 0.7

TABLE I I

Liquid s c i n t i l l a t i o n counting of Bk'

Sample' no .

Sample weight in grams

Sample a c t i v i t y in c.p.m.

Ac t iv i t y i n

c .p .m. /g

A c t i v i t y i n

vC/g

Deviat ion of average v a l u e , %

1 0.020392 70,835 3,1(73,666 I .565 + 0.2 2 0.015696 5lt,2lt6 3,l*56,0l»0 1.557 - 0 .3 ' 3 0.025909 90,61*1* 3,1*98,553 1.576 + 0.9 1* 0.023636 81,679 3,1*55,703 1.557 - 0.3

5 0.0181)88 63,890 3,1*55,755 1-557 - 0 . 3 6 0.011035 38,201 3,1*61,803 1.559 - 0.2

7 - 0.017)122 60,236 3,1*57,1*68 1.557 - 0 .3 8 0.019383 • 66,830 3,1*1*7,867 1.553 - 0.6

9 0,bl8032 62,503 3,1*66,227 1.561 - 0 .1 10 0.019668 68,806 3,1*98,373 1.576 + 0.9

40 50 50 70 80 90 100 DISCRIMINATOR SETTING » -

FIGURE 1. CALIBRATION OF THE LOWER DISCRIMINATOR

^ v [20000 - X.

IN C

P.M

.-

— > I-15000

> O <

10000

1 1 i i i l X i ' l 5 6 7 8

DISCRIMINATOR SETTING -

FIGURE 2. INTEGRAL COUNTING-CURVE OF .Bk 249

SS 1001- -14 S' 35

o u LL LL LU

Ni 63

asoi-a

i i i i i i, 0 20 Z5 60 ' 80~~ 100 120 140 160 MAXIMUM BETA ENERGY. KeV *

FIGURE 3 RELATION BETWEEN DETECTION EFFICIENCY AND MAXIMUM BETA ENERGY.