The decay of 47Sc

0883-2889/86 $3.00 + 0.00 Pergamon Journals Ltd

Appl. Radiat. Isot. Vol. 37, No. 9, pp. 973-978, 1986 Int. J. Radiat. Appl. Instrum. Part A

Printed in Great Britain

The Decay of 47Sc D . R E H E R , 1 H. H. H A N S E N , 1 R. V A N I N B R O U K X , 1 M . J. W O O D S , 2 C. E. G R A N T , 2 S. E. M . L U C A S , 2 J. B O U C H A R D , 3 J. M O R E L 3 a n d R. V A T I N 3 ~CEC-JRC, Central Bureau for Nuclear Measurements, Geel, Belgium, 2National Physical Laboratory, Teddington, U.K. and 3Laboratoire de M6trologie de Rayonnements Ionisants, Gif-sur-Yvette, France (Received 12 December 1985)

The decay data of 47Sc have been remeasured using various methods. The following results were obtained: the half life, tlj2 = (3.3492 __+0.0006)d, the y-ray emission probability, Pr = 0.683 + 0.004, the transition probabilities for the two/~ - branches, Pa~ = 0.316 ___0.006 and Pa2 = 0.684 + 0.006, the endpoint energies of the two /~- spectra, Ea~ = (600.5 +__1.9) keV and Ea2 = (439.0 + 1.6) keV, and the internal conversion coefficients, ct = 0.0045 + 0.0003, ctK = 0.00406 + 0.00021, and ~,+ = 0.00041 + 0.00010. These data have been compared with the results obtained from previous measurements.

1. Introduction The radionuclide 47Sc (decay scheme, see Fig. 1) is used in radiodiagnostics, e.g. for tumour imaging and in radiotherapeutic applications, t~-51However, several of the decay scheme data are not very accurately known, e.g. according to the most recent evaluation, t6) the v-ray emission probability, P~ has an uncertainty of ___3% and the K-conversion coefficient, ~z, + 2 7 % . If these data were known with better accuracy, 47Sc could also serve to calibrate the efficiency of solid state detectors at 159.39 keV. This is important for producers of the widely used radiopharmaceutical 123I, as its activity is normally determined by measuring the 159.00keV v-ray emission rate. In the frame of an international comparison for the determination of the activity concentration of a 47Sc 47Sc 7/2- 21 26

00 ~

3.3492 d

\Pp2 pp~ \\0.68, 0.316 \ \

~

159.39

712

5/2~7 Ti 22

25

Fig. 1. Decay scheme of 47Sc.

solution, tT) the decay data of this radionuclide were remeasured using different methods. In the following all uncertainties quoted are single standard deviations of the mean.

2. Source Material

The 47Sc solution used for the majority of the measurements detailed below was obtained from the Institut National des Radio616ments (IRE), Fleurus, Belgium and distributed to three standardizing laboratories, the National Physical Laboratory (NPL), Teddington, U.K., the Laboratoire de M6trologie des Rayonnements Ionisants (LMRI), Gif-sur-Yvette, France and the Central Bureau for Nuclear Measurements (CBNM), Geel, Belgium. The material used for the half-life measurements at N P L was obtained from Amersham International, U.K. Each laboratory determined the level of y emitting impurities using high purity Ge detectors. For the I R E material, 47Ca was found to be the major impurity at a level of less than 10 3 of the 47Sc activity concentration at the reference date. The only other impurity was 4SSc at a level of 2 × 10 5 at the reference date. The latter impurity had no influence on the present measurements, the results or their uncertainties. For the N P L half-life measurements, any 47Ca was chemically separated at N P L before use and the residual contaminants determined by V spectrometry both before and after the determination. N o significant contaminants were detected. 973

D . REHER et al.

974

3. Experiments 3.1. Measurements of the disintegration rate The disintegration rate, No, of the I R E solution was determined by each of the three laboratories using the 4nil-y-coincidence technique a n d liquid scintillation counting. G o o d agreement was obtained, the results being s h o w n in Table 1.

3.2. Measurement of the half l(['e At C B N M , the half life was m e a s u r e d using three different detector systems: a calibrated NaI(TI) integral c o u n t i n g system, ~8~ a n d two calibrated high purity G e detectors from D S G (Detector Systems G m b H ) a n d Enertec, respectively. 19) Three sources were measured over a period of 10 days. With the NaI(TI) c o u n t i n g system, all detected events above a threshold o f 10 keV were c o u n t e d a n d a correction for the 47Ca impurity was applied. With the two Ge spectrometers no such correction was necessary because only the 159 keV y-ray peaks of the m e a s u r e d spectra were evaluated. At N P L , half-life m e a s u r e m e n t s were performed using a high pressure ionization chamber. A previously u n p u b l i s h e d m e a s u r e m e n t in 1976 was reevaluated a n d two new m e a s u r e m e n t s were performed. The c h a m b e r has been s h o w n to have no nonlinearity effects over the m e a s u r e m e n t range a n d corrections were m a d e for c o n t a m i n a n t s from a c o m b i n a t i o n of 7 spectrometry checks a n d meas u r e m e n t of the samples after the 47Sc h a d decayed. In each case, m e a s u r e m e n t s were taken over a period o f at least 5 half-lives a n d in excess of 5000 data points were a c c u m u l a t e d for each sample. Statistical tests (F-test) were p e r f o r m e d over each half life period to d e m o n s t r a t e consistency over the whole m e a s u r e m e n t period. A l t h o u g h the c o n t a m i n a n t levels were low, traces were f o u n d of the following radionuclides: 46Sc, 48Sc, -~tCr, t52Eu, 154Eu, 169yb a n d t92Ir. The results from all the m e a s u r e m e n t s are

collected in Table 2. The weighted mean is: tl, 2 = (3.3492 _+ 0.0006)d, the weights being the mverse of the squared uncertainties.

3.3. Measurement o / the 7-ray emission prohahili O' The 7-ray emission probability, P , has been calculated from P. = N~/N o, where N. is the n u m b e r of )' rays emitted from a source having a disintegration rate N 0. At L M R I , four sources were prepared for the 7-ray emission rate measurements. The efficiency of a 3 x 3 in. NaI(TI) (Quartz & Silice) detector was determined using 57Co a n d a~gCe sources measured by the 4~zfl- 7 coincidence method. At a distance of 20 cm with a threshold of 20 keV, the integral efficiency for 47Sc y rays whose energy is very close to that of t"~('e, was determined to be (0.746 _+_0.002)% using a linear interpolation. The same sources were also measured at a distance o f 12 cm from a 100 cm 3 GeLi (Princeton G a m m a Tech) whose efficiency had been calibrated with ~5-'Eu and ~3~Ba. The efficiency at the 47Sc y ray energy was f o u n d to be (0.845 ± 0.004)%. The d e t e r m i n a t i o n s of the 7-ray emission probabilities o f 47Sc were 0.687 + 0.004 and 0.689 _4-0.007 for Nal a n d GeLi respectively. F o u r sources were also used at C B N M in the ;'-ray emission rate measurements. The two efficiencycalibrated pure Ge detectors, from Enertec and DSG, were employed. The nuclides and their 7-ray emission probabilities used for the calibration are described elsewhere. ¢9~ F o r the m e a s u r e m e n t s described in this report, the points at 145.4 a n d 165.8 keV respectively from the decays of H~Ce and tV~Cc had the most influence on the calibration. The solid angles subtended by the detector were a b o u t 0.1 and 0.2 sr for the Enertec a n d the D S G detector, respectively. The source to detector distance was in both cases a b o u t 3 cm. The efficiencies for the 47Sc 7-ray were found to be (0.3115 + 0.0017)% a n d (0.685 ± 0.007)% for the two detectors m e n t i o n e d above.

Table 1. Results of the 47Sc activity measurements Activity concentration on 15.10.83 Oh U T Participant

47Sc (kBq/g)

~ C a impurity (% of 4~Sc)

LMRI NPL CBNM

596.2 _+ 0.5 596.7 _+ 0.9 595.3 + 0.9

0.084 ± 0.005 0.067 ± 0.008 0.078 ÷ 0.004

Weighted m e a n

596.1 + 0.5

1/.079 + 0.004

Table 2. Results of the half-life measurements Detector

t~ :(d)

NaI(TI) integral counting G e ( H P ) E N E R T E C ()'~sg) G e ( H P ) D S G (7~9) Ionization c h a m b e r Ionization c h a m b e r Ionization c h a m b e r

3,3500 3.3480 3.3525 3.3478 3.3491 3.3483

+ 0.0006 +_ 0.0031 + 0.0015 + 0.0010 + 0.0019 ± 0.0006

Weighted mean

3.3492 ± 0.0006

Measured at C B N M (1984) C B N M (1984) C B N M (1984) N P L (1976) N P L (1985) NPI. (1985)

The decay of 47Sc Table 3. Results of the P~. measurements Detector ENERTEC Ge(HP) DSG NaI(TI) Ge spectrometer (PGT) Weighted mean

Pr

Measured at

0.678 +_ 0.005 0.671 _+ 0.008 0.687 _+ 0.004 0.689 + 0.007 0.683 + 0.004

CBNM (1984) CBNM (1984) LMRI (1984) LMRI (1985)

The results of the measurements are given in Table 3. The weighted mean of the four measurements is: P~,.= 0.683 + 0.004.

975

P~, = 0 . 3 2 6 + 0 . 0 1 4 and P~2 = 0'674 -+ 0.014 the quoted uncertainties being standard deviations. Details on the treatment of a composite fl spectrum can be found elsewhere. (42) By extrapolating the Kurie plots of the partial spectra to their intercepts with the abscissa, the fl-ray maximum energies were determined to be E~, = (600.5 + 1.9) keV and Ea2 = (439.0 + 1.6) keV. The difference of these energies of (161.5 + 2.5) keV is in satisfactory agreement with the 7-ray energy. 3.5. Measurement o f the K-conversion coefficient

3.4. The fl-ray emission measurements A sector-type double-focusing fl-ray spectrometer (type described by Sakai ~j°)) from JEOL was used at C B N M to measure the two components of the fl spectrum. Three spectra from one source were measured. Both components were separated by subtracting the high-energy part from the total spectrum (Fig. 2a). The fl-ray-emission probabilities P~, = N~,/(N~, + NB2), i = 1, 2, were calculated from the count rates N~, and N~2 obtained by integration of the partial fl~ spectra derived from the corresponding Kurie plots (Fig. 2b). The results are

r

As can be seen from Fig. 2 the K-conversion electron peak can clearly be distinguished in the spectrum measured with the beta spectrometer. However, the resolution of the peak and also the total number of counts in the peak relative to that of the fl- continuum was not sufficient for an accurate determination of ~K. Therefore a K had been measured using the open Si(Li) detector of the low energy x-ray and electron spectrometer, LEXES. ol) In the energy region between 127 and 166keV the efficiency of LEXES was calibrated using the conversion electrons emitted after the 139Cedecay. The efficiency curve was

_.

!

f

i: 2

I

1"

t-.~..#

-:

(a)

....... u~.:

] 0,164

0.540

0.916

1.292

1.668

2.0/,4

ELECTRON MOMENTUM , 1] = mPc 2

(b)

o

,

,

2.0

1.5

ELECTRON

ENERGY

R..-"Ekin

moC2

.I

Fig. 2. (a) Electron spectrum measured with the fl ray spectrometer. (b) Kurie plots.

976

D. REHER et al.

y - R A Y S A N D L , M , ,,, CONV. ELECTRONS

K - CONV. 12

% x z z

,< I

l,J [3_

z o

ENERGY 80 I

0

120 i

(keY) 160

200

i,

I

512

256

768

CHANNELS

Fig. 3. 47Sc spectrum measured with the open Si(Li) of LEXES, found to be nearly i n d e p e n d e n t of energy a n d effectively linear in this region as was observed by others (12 ]4) a n d the energy of the 478c K-conversion electrons (E K = 154 keV) is very close to one of the calibration points (EK(139Ce)= 160keV). Hence, a linear interpolation was used to determine the detection efficiency for the 47Sc K-conversion electrons. Part of a 47Sc spectrum measured with the open Si(Li) detector is s h o w n in Fig. 3. The right h a n d peak contains the ?, rays a n d the L, M . . . . conversion electrons. We subtracted the 7-ray c o n t r i b u t i o n from the spectrum by remeasuring the sources with a n AI a b s o r b e r of 0.245 m m thickness to stop all conversion electrons and correcting for the a t t e n u a t i o n o f the 7 rays in the aluminium. The difference spectrum should contain the K-conversion electron peak a n d the L, M . . . . -conversion electron peak at 160 keV. The K peak can clearly be seen a n d can also be evaluated. The L, M . . . . peak however, c o n t a i n e d too few c o u n t s and its structure could not be used in the evaluation by a peak fitting program. Therefore, only a n upper limit N L M . . / N K ~ 0.1 could be established. The evaluation of the peak c o u n t rates was carried out on the R N g r o u p R N D A S c o m p u t e r (PDP11/34y TM using the p r o g r a m E L E G A N T ~16) which evaluates multiplets o f peaks c o n t a i n i n g electron and p h o t o n peaks. The K-conversion electron emission probability PeK = N~K/No was found to be P~K = 0.00277 + 0.00014 where N~K is the n u m b e r of K-conversion electrons emitted from the source. With 2K = PeK/P~. the K-conversion electron coefficient was determined to be :~K =0.00406_+ 0.00021. The uncertainties stated are s t a n d a r d deviations, of which the most i m p o r t a n t c o m p o n e n t came from the d e t e r m i n a t i o n of the K-conversion electron emission rate.

4. Discussion The uncertainties on the individual results corres p o n d to s t a n d a r d deviations of the mean. They include all r a n d o m a n d systematic uncertainties which were pooled statistically. In the cases where a weighted m e a n (using the reciprocal squared uncertainties as weights) was calculated from the values of the different m e t h o d s from the different laboratories the largest of both, the internal or external e r r o r ] ~7~ was chosen as the uncertainty of the final value. Only when this uncertainty was smaller t h a n the smallest uncertainty of the individual data we took the latter as the uncertainty of the final value. The d e t e r m i n a t i o n of the b r a n c h i n g ratios P/~, a n d P& was carried out in a direct way (~-spectrometry) but has also been m a d e in an indirect way using the results of the y-ray and electron spectrometry. To determine P,I a n d P , : from the latter m e t h o d s we h a d to deduce the total conversion coefficient ~ from the ~K m e a s u r e m e n t . F r o m the m e a s u r e m e n t with L E X E S we k n o w t h a t c~K/c~e is a b o u t 10. This is

T a b l e 4. T h e o r e t i c a l i n t e r n a l c o n v e r s i o n d a t a f o r t h e 47Sc d e c a y f r o m R e f . (22), Z = 22, E7 = 159.39 k e V Quantity uK cqj cq. 2 ~L3 :q C~rOr ~K/~l KK~ c-i,

MI 5.201 - 10 4.651 • 10 8.489.10 4 . 3 9 6 " 10 4.780-10 5.679" 10 10.9 5.17-10 4.75.10

E2 ~ 4 ~ ~ 4 3 ~ 4

5.070" 10 4 . 3 6 4 - 10 1.523'10 2 . 1 7 2 " 10 4.734-10 5 . 5 4 3 " 10 10.7 4.80"10 4.49.10

~ : , is t h e i - c o n v e r s i o n e l e c t r o n p r o b a b i l i t y : ~q = c£/(I + ~TOT)-

-' 4 ~ a ~

:

emission

The decay of 47Sc

977

Table 5. 47Sc data from literature and the results of the present work. The figures in brackets indicate the uncertainties Reference Measurements: Krisberg1231 Aten1241 Cheng~tS~ Corktl91 Duval~2S~ Marqueza6i Lyon1271 Nichols12s) Graves(291 Lidofsky0°) PoularikasOl) Hontzeas(32~ Misra(331 Konijn1341 Bakt351 Meadows1361 RavnOTJ Wooda~l Heath 1201 Mommsen~381 This w o r k : Evaluations: Helmer/391 Halbert14°) Lagoutine161

E~, (keV)

P~I

610 520 622(5) 640(39)

1 0.34(4) 1

490 620(30) 596(10) 600(2) 610(5)

eB, (keV)

PlY2

g (keV)

Pr

~t

ctK

fi/z(d) 3.43(3)

0.72 0.36 0.40 0.26

435(8) 280 460(20) 430(5) 439(2) 450(10)

0.66(4) 0.28 0.66(3) 0.64 0.60 0.74

160 185(7) 159.5 210 157(7) 167(2) 158(2) 160(5) 163 159.2(5)

5/4 0.0036(9)

0.66(3)

3.4 3.40(5) 3.44 3.44(5) 3.45(10)

0.73(4)

3.3(1) 3.45(6) 3.38(9) 3.40

K/L = 10 approx. 0.01

0.685(27)

ct/~tK= 1.12 3.345(3) 3.422(4) 159.39(5) 159.381(15)

600.5(19) 0.316(6) 439.0(16) 0.684(6) 600.5 0.32 600.5(20) 0.32(2)

441 0.68 441(20) 0.68(2)

confirmed by the works of C h e n g ~jS) a n d Cork, °91 a l t h o u g h because their statements are more or less assumptions, a large uncertainty of 2 5 % has been used for this value. F r o m this ratio we derive the L, M, ... conversion coefficient ~L+ = 0.00041 + 0.00010, a n d the total conversion coefficient ct = 0.0045 + 0.0003. Using Pr = 0.683 +__0.004 the b r a n c h i n g probability to the excited l e v e l becomes Pc: = PrO + 0c) = 0.686 ___0.006. The c o m p l e m e n t a r y b r a n c h i n g probability is Pc, = 0.314 + 0.006. These values agree well with the beta spectrometer measurements, a l t h o u g h the latter are less accurate. O u r final result is Pc, =0.316___0.006 a n d Pc2= 0.684 + 0.006 which is the weighted m e a n from the results of b o t h methods. Theoretical internal conversion coefficients were calculated with E;. = 159.39 keV, which is the average o f the most accurate values given by H e a t h ~2°) a n d W o o d . ~2~) In Table 4 we calculated the theoretical predictions for M1 a n d E2 multipolarities from the tables of Band a n d T r z h a s k o v s k a y a ~=) using spline interpolation. The theoretical predictions for a transition of M1 character are a b o u t 2 0 % higher t h a n our experimental results, whereas for a n E2 these predictions are more t h a n one order of m a g n i t u d e greater t h a n our findings. Hence, we m a y assume that the transition p r e d o m i n a n t l y has M1 character. In Table 5 o u r results are c o m p a r e d with those of other authors~8 2J.2_~38~ a n d with three evaluations. (6'39'4°)It is r e m a r k a b l e that m a n y of the quoted b r a n c h i n g ratio m e a s u r e m e n t s are given without u n c e r t a i n t i e s - - e v e n the 1977 Nuclear D a t a Sheet evaluation ~4°) avoids allocating uncertainties to these quantities. The m o s t accurate results have uncertainties of 4 % or more. Hence, the uncertainty

0.0045(3)

3.341(3) 0.00406(21) 3.3492(6)

159.39(5) 0.690(25) 159.381(15) 0.68(2) 0.0036(9) 159.391(15) 0.68(2) 0.0036(9)

3.39(4) 3.351(2) 3.351(2)

0.683(4)

0.0033(9)

allocation of the most recent evaluation <61is possibly based on the spread of the published data. O u r value for the y-ray-emission probability is nearly an order of m a g n i t u d e more accurate t h a n the published d a t a a n d it agrees excellently with the evaluations of Halbert, ~4°)Lagoutine ~61a n d Helmer. °91 To our knowledge, the K-conversion coefficient has not been m e a s u r e d before. By using K / L + ~ 10 and the only relative conversion electron meas u r e m e n U 181 an estimate of ctK = 0.0033 is o b t a i n e d which is 2 0 % lower t h a n o u r measurement. The half life value agrees excellently with t h a t given by Halbert ~4°) and Lagoutine. ~61 The value, however, given in these evaluations is based entirely o n a private c o m m u n i c a t i o n of Walz et al. ~4° to the evaluators. O n the other h a n d there is disagreement with nearly all other previous measurements, even with the most recent one of M o m m s e n . ~3s) Acknowledgements--The authors express their thanks to E.

Celen for the 4nil-y-coincidence measurements, B. M. Coursey (NBS) for liquid scintillation measurements and helpful discussions, and W. Zehner for the source preparation.

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