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Determination of gamma-ray emission probabilities in the decay of 235U and 231Th
Nuclear Instruments and Methods 193 (1982) 191-196 North-Holland Publishing Company
191
D E T E R M I N A T I O N OF G A M M A - R A Y EMISSION PROBABILITIES IN THE D E C A Y O F 23SU AND 231Th R. V A N I N B R O U K X
a n d B. D E N E C K E
Commission of the European Communities, Joint Research Centre, Geel Establishment, Central Bureau for Nuclear Measurements, B.2440, Geel, Belgium
The emission probabilities for the most prominent y-ray transitions in the energy range from 25 to 206 keV accompanying the decay of 235U and its radioactive daughter 231Th were determined using calibrated Si(Li) and high purity Ge detectors. The results are: P~(25.65) = (0.145±0.003), P~(143.8) = (0.109± 0.002), Pv(185.7) = (0.575 +_0.009),
Pv(84.16)= (0.066 + 0.003), P~(163.4)= (0.050±0.001), Py(205.3)= (0.050 ± 0.002).
The quoted uncertainties, corresponding to a 1o- confidence level, take into account random and systematic uncertainties.
1. Introduction T h e p r e s e n t k n o w l e d g e of t h e y - r a y e m i s s i o n p r o b a b i l i t i e s , 3' r a y s e m i t t e d p e r d i s i n t e g r a t i o n , in t h e d e c a y of 235U a n d its r a d i o a c t i v e d a u g h t e r 23~Th is u n s a t i s f a c t o r y . T h e a c c u r a c y r e q u i r e d for n o n - d e s t r u c t i v e analysis of fuel m a t e r i a l s is ___1% while t h e a c c u r a c y a c h i e v e d is ---10% [1]. T h e r e a r e o n l y a few a b s o l u t e m e a s u r e m e n t s r e p o r t e d [2-5]. A l l r e c e n t e v a l u a t i o n s [ 1 , 6 - 8 ] a r e still b a s e d o n t h e v a l u e of Pv = 0.54, w i t h o u t any u n c e r t a i n t y q u o t a t i o n , for t h e m o s t p r e d o m i n a n t y t r a n s i t i o n of 185.7 k e V , e v a l u a t e d by H y d e et al. [4] f r o m m e a s u r e m e n t s p e r f o r m e d b e f o r e 1960, a n d on m o r e r e c e n t r e l a t i v e m e a s u r e m e n t s [9-11]. In an a t t e m p t to i m p r o v e on t h e latter, t h e e m i s s i o n p r o b a b i l i t i e s for t h e m o s t p r o m -
inent 3' t r a n s i t i o n s in t h e e n e r g y r a n g e b e t w e e n 25 a n d 206 k e V a c c o m p a n y i n g t h e d e c a y of 235U a n d 231Th w e r e r e d e t e r m i n e d , z31Th is g e n e r a l l y in s e c u l a r e q u i l i b r i u m with its p a r e n t 235U.
2. Materials and sources T w e n t y s o u r c e s in all w e r e p r e p a r e d f r o m t h r e e different u r a n i u m m a t e r i a l s . S o m e d e t a i l s a b o u t t h e m a t e r i a l s u s e d a n d t h e definition of t h e 235U c o n t e n t of t h e s o u r c e s (section 3) a r e given in t a b l e 1. T h e s o u r c e s f r o m m a t e r i a l 1, a U - A I alloy c o n t a i n i n g 20% ( w t % ) of u r a n i u m , w e r e m a d e b y rolling t h e alloy t o t h i c k n e s s e s of 0.03, 0.2 a n d 0.5 m m , a n d b y cutting c i r c u l a r foils of 10 m m d i a m e t e r . T h e s e foils w e r e p a c k e d into A I
Table 1 Materials used and 235U determination Material No.
1 2 3
Chemical form
U-AI alloy uranium fluoride uranyl acetate
Number of sources
23SUcontent
Activity ratio R
at.%
mg per Method of source determination
/,(2asU) Iv(total)
6
93.22
isotope 1.5--25 dilution
-
3
99.47
0.1-0.6 t~ counting
0.1690±0.0017
11
99.97
0.1-1.0 ot counting
0.9716 _ 0.0010
0029-554X/82/0000--0000/$02.75 © 1982 N o r t h - H o l l a n d
Method of determination
isotope analysis ~t-particle spectrometry
IV. y RAY SPECTROSCOPY
192
R. Vaninbroukx, B, Denecke / Gamma-ray emission probabilities
boxes of 15 m m diameter and 3 m m height. After the measurements in the A1 boxes, the foils were removed and glued onto Pt-coated glass discs. The sources from material 2 were prepared by evaporation under vacuum [12] onto stainless steel discs. The diameter of the active area of these sources was 12.7 mm. For material 3, the sources were made by electrospraying [13] onto stainless steel discs. The diameter of the active area was varied between 6 and 12mm. The amount of 235U per source varied between approximately 0.1 and 25mg. Since the specific activity of 235U is 8 0 B q m g -~, the 235U disintegration rates of the sources varied between about 10 and 2000 Bq.
3. Determination of the 2aSU disintegration rates
Two different methods were applied for the determination of the 235U disintegration rates of the sources: mass spectrometric isotope dilution and a counting. The 23~Th disintegration rates are equal to the 235U disintegration rates because of the secular equilibrium conditions. For material 1 the n u m b e r of 235U atoms N for each of the sources was determined by mass spectrometric isotope dilution [14]. For that purpose, the sources, after the m e a s u r e m e n t of the y-emission rates (section 4), were spiked with a well known amount of 233U and dissolved. The uranium was then separated from the aluminium and the ratio 235U/233U was measured. The accuracy of the 235U determination was estimated to be _+0.5%. The 235U disintegration rate is equal to AN, where )t is the decay constant of 235U. The latter quantity has been calculated from the half-life value r e c o m m e n d e d in ref. 15. The overall uncertainty in the disintegration rates is estimated to be _+0.6%. All uncertainties correspond to a lo" confidence level and take into account random and systematic uncertainties. For samples 2 and 3 the 235U disintegration rates were determined by a counting under a well defined low-geometry solid angle [16]. For sample 2, the corrections for the contribution from other uranium isotopes to the a count rates were calculated from the isotopic composition as determined by mass spectrometry [14] and the decay constants derived from the half-lives
r e c o m m e n d e d in I N D C ( N D S ) - I 2 1 ( N E ) [15]. For sample 3 these corrections were determined by a-particle spectrometry [17]. The values of these corrections for both materials, expressed as the activity ratio R of the 235U a activity over the total a activity are given in table 1. The uncertainties as a result of these corrections are estimated to be +1.0% for sample 2 and _+0.l% for sample 3, where nearly the complete a activity is due to 235U. The optimum accuracy obtainable for a counting under a defined lowgeometry solid angle is +-0.1% [16]. This accuracy could be attained for sample 2, but for sample 3, due to the low specific activity of the material and consequently the higher uncertainties on the corrections for self-absorption and background variations, only accuracies varying between _+0.3 and _+1.5% for the different sources were reached. The mean uncertainty for the 11 sources was _+0.6%. Finally, the overall uncertainty on the 235U disintegration rates becomes _+1.1% for sample 2 and _+0.7% for sample 3. The 235U content in mg per source, as given in table 1, can easily be derived from the n u m b e r of 235U atoms or the 235U disintegration rates.
4. Determination of the y - r a y emission rates
The y-ray emission rates of all the sources were determined using three calibrated photon detectors: a Si(Li) detector with an area of 3 cm 2 and a thickness of 3 m m and two high purity G e detectors with an area of i cm 2 and a thickness of 8 m m (detector GE-I), respectively, an area of 3 cm: and a thickness of 10 m m (detector GE-II). The Si(Li) detector was used for the y rays of 25.65 keV and 84.16 keV following the decay of 23~Th, while the two G e detectors were used additionally for the 3' rays of 143.8, 163.4, 185.7, and 205.3 keV accompanying the decay of 235U. The solid angles subtended by the detector systems were about 0.2 sr for the Si(Li) detector and 0.1 and l s r for the detectors G E - I , G E - I I respectively. A typical 235U-23~Th y-ray spectrum in the energy range concerned here is shown in fig. 1. Several corrections had to be applied. One was the result of the self-absorption of the photons. The values of this correction for the various
R. Vaninbroukx, B. Denecke / Gamma-ray emission probabilities
~
z
q
5 ~:>~
0
512
256
102/.
768
CHANNEL NUMBER
Fig. 1.
235U-231Thy-ray
spectrum (detector GE-I).
y rays and the different materials are given in table 2. These values have been derived from experimental determinations by extrapolation of the observed count rates as a function of the source thicknesses to thickness zero and independently by calculations using absorption coefficients obtained by interpolation from the values tabulated by Veigele [18]. In earlier work [19], it was shown that for small solid angles up to 0.2 sr, as is the case for the Si(Li) and GE-I detectors, the simple formula (1-e-~a)/tzd, where # is the absorption coefficient and d the source thickness in the corresponding units, gives reliable results. The uncertainty on this correction has been estimated to be about 20%. Except for the y rays of 25.65 keV from some sources of material 1, the influence on the accuracy of the y-ray emission rates is smaller than 1% and in most cases less than 0.1%. A second correction is that for differences of Table 2 Correction for self-absorption Correction (%) Energy Ev (keV)
Material 1
Material 2
Material 3
25.65 84.16 143.8 163.4 185.7 205.3
6--60 0.4-5.7 0.3-5.2 0.3-4.1 0.2-3.2 0.2-2.6
0.2-1.5 0--0.1 0--0.1 --0 --0 -0
0.4--2.5 0-4).1 6-0.1 0--0.1 -0 ~0
193
the diameter of the 235U sources from the diameter of about 6ram for the calibration sources. The differences in solid angles for sources of 6, 10, 12 and 12.7mm diameter as used here have been determined experimentally by varying the diameters of the sources from material 3 between 6 and 12 mm. They have also been calculated as a function of the diameters of the detectors and sources and the source to detector distances according to Masket and Rodgers [20]. The mean corrections are (1.9___ 0.2)% for the Si(Li) detector, (1.0_+0.1)% for detector GE-I and ( 4 . 2 - 0 . 4 ) % for GE-II. A third correction is due to summing effects, mainly by coincidences, between the y rays and L X-rays. The magnitude of this correction was estimated from an analysis of the spectra. It was found to vary for the different y rays from 0 to 1% for the Si(Li) detector, from 0 to 0.4% for GE-I and from 0 to 6% for GE-II. The mean uncertainty on the y-ray emission rates caused by this correction was _-_0.4% for detector GE-II and less than 0.1% for the two other detectors. Further corrections such as those for dead time losses and background could easily be determined with sufficient accuracy and do not introduce any substantial uncertainty. Before the 235U-231Th measurements the detectors were calibrated for all the types of sources used; reference sources were used, prepared by the deposition of known amounts of accurately standardized solutions of suitable radionuclides. The photon emission probabilities for the efficiency calibration of the detectors were adopted from a survey of literature data [21-37]. The nuclides used, the photon energies and the emission probabilities with their uncertainties are given in table 3. For all the measurements the peak areas were determined in two ways: (1) by "manual" extrapolation of the "background" from the low- and high-energy side of the peaks to the axis of the peak centroids, and by evaluating the remaining peak contents; (2) using an adapted version of the programme Cutipie [38]. The reproducibilities of the peak area determinations were found to be better than _+0.3%. For monoenergetic y-rays the agreement between both peak determination methods is better than 1%; the "manual" method yields peaks areas (0.7_+0.2)% higher than the Cutipie method. For X-ray peaks IV. y RAY SPECTROSCOPY
R. Vaninbroukx, B. Denecke / Gamma-ray "emission probabilities
194
Table 3 Nuclides and photon emission probabilities used for the calibration of the detectors Nuclides
Radiation
Energy E~ (keV)
Emission probability (Py)
Refs.
57Co
y 3, Y
14.41 122.06 136.47
0.095 -+0.003 0.856 -+0.004 0.106-+ 0.001
[7, 2 1-25]
~ogCd
Ag K~ Ag Ka y
22.1 25.(/ 88.0
0.865 _+0.030 (t. 185 _+0.0(17 0.037 -+0.0() 1
[7, 22, 25]
133Ba
Cs K,, Cs Ko 3' y(79.6 + 81) y 3, 3, 3,
30.8 35.2 53.2 80.9 276.4 302.9 356.0 383.8
(I.995 _+0.010 0.234 -+0.005 0.022 _+0.001 0.360 _+0.005 0.072 +_0.001 0.183 _+0.001 0.620 _+0.004 0.089 _+0.001
[7, 22, 25-29]
139Ce
La K, La KB 3'
33.3 38.0 165.85
0.648 _+0.015 0.152 _+0.005 0.799 _+0.003
[7, 22, 24, 25, 29-31]
14ICe
Pr K~ Pr Kt~ y
35.9 411.9 145.43
0.136 _+0.002 0.032 _+0.001 0.482 _+0.003
[7, 22, 32, 33]
152Eu
Sm K~ Sm KB y y 3'
39.9 45.7 121.8 244.7 344.3
0.591 _+0.012 0.149_+ 0.003 0.284 _+0.0(13 0.074 _+0.002 0.265 _+0.003
[7,22,25,26,34,35]
Z41Am
Np L~ Np L, Np Lr y y
0.132 _+0.004 0.195 _+0.004 0.048 _ 0.001 0.024 _+0.001 0.360 _+0.003
[7, 22, 23, 25, 32, 36, 37]
13.9 17.7 20.9 26.35 59.54
d i f f e r e n c e s of u p t o 3 % w e r e o b s e r v e d . F o r t h e c a l i b r a t i o n s a n d f o r t h e 235U-231Thm e a s u r e m e n t s t h e m e a n v a l u e s of b o t h d e t e r m i n a t i o n s w e r e u s e d . A t y p i c a l p e a k e f f i c i e n c y c u r v e is s h o w n in fig. 2. F r o m t h e u n c e r t a i n t i e s in t h e p h o t o n e m i s s i o n p r o b a b i l i t i e s of t h e r e f e r e n c e n u c l i d e s , t h e a c c u r a c y of t h e s t a n d a r d i z a t i o n of t h e s o u r c e s a n d t h e s m o o t h n e s s of t h e e f f i c i e n c y curve through the experimental points, the o v e r a l l u n c e r t a i n t y in t h e d e t e c t i o n e f f i c i e n c y is e s t i m a t e d t o b e + 2 . 5 % f o r t h e y r a y s of 25.65 a n d 8 4 . 1 6 k e V , -+0.6% f o r t h e y r a y s of 143.8, 163.4 a n d 185.7keV, a n d -+1.0% f o r t h e 205.3 k e V 3' ray.
5. Results The y-ray emission probabilities are obtained by d i v i d i n g t h e y e m i s s i o n r a t e s by t h e 235U o r 231Th d i s i n t e g r a t i o n r a t e s . T h e m e a n results, o b t a i n e d f o r t h e m e a s u r e m e n t s o n all t h e s o u r c e s using the three detectors for the different m a t e r i a l s , a r e g i v e n in t a b l e 4. T h e last c o l u m n of this t a b l e g i v e s t h e w e i g h t e d m e a n v a l u e s ; t h e y h a v e t o b e c o n s i d e r e d as o u r final v a l u e s . T h e q u o t e d u n c e r t a i n t i e s , c o r r e s p o n d i n g to a loconfidence level, take into account random and systematic uncertainties. According to Grinberg et al. [39], t h e r a n d o m a n d s y s t e m a t i c un-
R. Vaninbroukx, B. Denecke / Gamma-ray emission probabilities
195
Table 4 Results of the y-emission probability determinations Energy
Emission probability Pv
(keY) 25.65 84.16 143.8 163.4 185.7 205.3
Material 1
Material 2
Material 3
Final value
0.1445 - 0.0035 0.0666 ±- 0.0020 0.1090 ---0.0021 0.0502 ± 0.0011 0.5780 ± 0.0085 0.0494 ± 0.0015
0.1444 ± 0.0030 0.0652 +__0.0026 0.1086 _ 0.0024 0.0488 ± 0.0015 0.5707 +--0.0098 0.0501 ___0.0020
0.1453 ± 0.0028 0.0626 ± 0.0034 0.1084 ± 0.0023 0.0496 ---0.0012 0.5755 ± 0.0088 0.0515 ± 0.0015
0.145 - 0.003 0.066 ---0.003 0.109 ± 0.002 0.050 ± 0.001 0.575 ± 0.009 0.050 ± 0.002
abilities of the other ~/ rays to that of the "reference" ray are in fairly g o o d agreement with recent evaluated values [6-8].
10-2 86-
z _m
The authors would like to thank Messrs. H. Mast, J. Pauwels and F. Peetermans for the preparation of the sources, Messrs. M. Gallet, A. Loopmans and E. Sattler for the mass spectrometric determinations and M.G. Bortels for the a-particle spectrometry measurements.
2-
~_ 10-3
~
a6-
References
i
DETECTOR roE-[
I
--SOURCES
ON Pt COATED GLASS DISCS. . . .
I 10-~
I
I 10
20
I 40
60
80 100
200
400
PHOTON ENERGY {keV)
Fi~. 2. Peak detection efficiencies.
certainties are combined by using the formula tsSE+~,Y, Si. Here, ts is the student t-factor depending on the degrees of freedom, which are based on the number of single determinations, and ,~81 is the linear sum of the individual systematic uncertainties 8i. All the sources were measured at least two times with each of the detectors. Thereby, the number of determinations was large and the mean value of ts was 1.05. Our result for the 1 8 5 . 7 k e V ray, the most intense o n e and often considered as a "reference" 3' ray, is 6.5% higher than the previously adopted value, based on a few very old measurements. The ratios of the emission prob-
[1] A.L. Nichols, Transactinium Nuclear D a t a - 1979, IAEA-TECDOC-232 (IAEA, Vienna, 1980) p. 67. [2] C.W. Malich, Bull. Am. Phys. Soc., Ser. II, 1 (1956) 43. [3] R.C. Pilger, F.S. Stephens, F. Asaro and I. Perlman, Bull. Am. Phys. Soc., Ser. II 2 (1957) 394. [4] E.K. Hyde, I. Perlman and G.T. Seaborg, The nuclear properties of the heavy elements, vol. II: detailed radioactive properties (Prentice Hall, Englewood ClifIs, S J 1964). [5] R. Gaeta and M.A. Vignon, Nucl. Phys. 76 (1966) 353. [6] M.R. Schmorak, Nucl. Data Sheets 21 (1977) 91. [7] C.M. Lederer and V.S. Shirley, eds., Table of isotopes, 7th Ed. (John Wiley, New York, 1978). [8] W.B. Ewbank, Transactinium Isotope Nuclear D a t a 1979, IAEA-TECDOC-232 (IAEA, Vienna, 1980) p. 109. [9] W. Teoh and R.D. Connor, Nucl. Phys. A228 (1974) 432. [10] E. Vano, R. Gaeta, L. Gonzalez and C.F. Liang, Nucl. Phys. A251 (1975) 225. [11] S.A. Baranov, V.M. Shatinsky, A.C. Zelenkov and V.A. Pchelin, Sov. J. Nucl. Phys. 26 (1977) 921. [12] J. Van Audenhove, P. De Bi~vre, J. Pauwels, F. Peetermans, M. Gallet and A. Verbruggen, Nucl. Instr. and Meth. 167 (1979) 61. [13] V. Verdingh, Nucl. Instr. and Meth. 102 (1972) 497. [14] P. D e BiEvre, Advances in Mass Spectrometry, Vol. VII, ed., N.R. Daly (Heyden, London, 1976) p. 395. IV. 3' R A Y S P E C T R O S C O P Y
196
R. Vaninbroukx, B. Denecke / Gamma-ray emission probabilities
[15] A. Lorenz, ed, INDC(NDS)-121/NE (1980). [16] A. Spernol and B. Denecke, Chemical nuclear data. (The Institution of Civil Engineers, London, The British Nuclear Energy Society, 1971) p. 199. [17] G. Bortels, KFK 2862-EUR 6402e, Part Ili (1979) p. 24. [18] W.M.J. Veigele, Atomic Data Tables 5 (1973) 51. [19] A. Paulsen, R. Widera, R. Vaninbroukx and H. Liskien, Nucl. Sci. Eng. 76 (1980) 331. [20] A.V.H. Masket and W.C. Rodgers, TID-14975 (1962). [21] H.E. Freund and J.C. McGeorge, Z. Phys. 238 (1970) 6. [22] Nuclear Data Sheets, ed., Nuclear Data Project, ORNL, Oak Ridge. [23] J.S. Hansen, J.C. McGeorge, D. Nix, W.D. Schmidt-Ott. I. Unus and R.W. Fink, Nucl. Instr. and Meth. 106 (1973) 365. [24] U. Sch/)tzig, K. Debertin and K.F. Walz, Nucl. Instr. and Meth. 169 (1980) 43. [25] R.G. Helmer and C.W. Reich, Third coordinated research meeting on the measurement and evaluation of transactinium isotope nuclear data, ed.. A. Lorenz, INDC(NDS)-118/NE (1980). [26] R.J. Gehrke, R.G. Helmer and R.C. Greenwood, Nucl. Instr. and Meth. (1977) 405. [27] U. Sch6tzig, K. Debertin and K.F. Walz, Int. J. Appl. Rad. Isot. 28 (1977) 503.
[28] B. Chauvenet, J. Morel and J. Legrand, ICRM-S-6 (1980). [29] Y. Yoshizawa, H. Inoue, M. Hoshi, K. Shizuma and Y. fwata, JAERI-M 8811 (1980). [30] H.H. Hansen and D. Mouchel, Z. Phys. A274 (1975) 335. [31] H.H. Hansen and D. Mouchel, Z. Phys. A276 (1976) 303. [32] J. Legrand, J.P. Perolak C. Bac and J. Gorry, Int. J. Appl. Rad. Isot. 26 (1975) 179. [33] H.H. Hansen, E. Celen, G. Grosse, D. Mouchel, A. Nylansted Larsen and R. Vaninbroukx, Z. Phys. A290 (1979) 113. [34] J. Legrand, J. Morel and A. Traverse, Bulletin BNM no. 19 (1975) 23. [35] K. Debertin, Nucl. Instr. and Meth. 165 (1979) 165. [36] J.L. Campbell and L.A. McNelles, Nucl. Instr. and Meth. 117 (1974) 519. ]37] J. Plch, J. Zderadicka and L. Kokta, Czech. J. Phys. B26 (1976) 1344. [38] W. Teoh, Nucl. Instr. and Meth. 109 (1973) 509. [39] B. Grinberg, J.P. Brethon, F. Lagoutine, Y. Le Gallic, J. Legrand, A.H. Wapstra, H.M. Weiss, W. Bambynek, E. De Roost, H.H. Hansen and A. Spernol, Atomic Energy Rev. 11 (1973) 516.
191
D E T E R M I N A T I O N OF G A M M A - R A Y EMISSION PROBABILITIES IN THE D E C A Y O F 23SU AND 231Th R. V A N I N B R O U K X
a n d B. D E N E C K E
Commission of the European Communities, Joint Research Centre, Geel Establishment, Central Bureau for Nuclear Measurements, B.2440, Geel, Belgium
The emission probabilities for the most prominent y-ray transitions in the energy range from 25 to 206 keV accompanying the decay of 235U and its radioactive daughter 231Th were determined using calibrated Si(Li) and high purity Ge detectors. The results are: P~(25.65) = (0.145±0.003), P~(143.8) = (0.109± 0.002), Pv(185.7) = (0.575 +_0.009),
Pv(84.16)= (0.066 + 0.003), P~(163.4)= (0.050±0.001), Py(205.3)= (0.050 ± 0.002).
The quoted uncertainties, corresponding to a 1o- confidence level, take into account random and systematic uncertainties.
1. Introduction T h e p r e s e n t k n o w l e d g e of t h e y - r a y e m i s s i o n p r o b a b i l i t i e s , 3' r a y s e m i t t e d p e r d i s i n t e g r a t i o n , in t h e d e c a y of 235U a n d its r a d i o a c t i v e d a u g h t e r 23~Th is u n s a t i s f a c t o r y . T h e a c c u r a c y r e q u i r e d for n o n - d e s t r u c t i v e analysis of fuel m a t e r i a l s is ___1% while t h e a c c u r a c y a c h i e v e d is ---10% [1]. T h e r e a r e o n l y a few a b s o l u t e m e a s u r e m e n t s r e p o r t e d [2-5]. A l l r e c e n t e v a l u a t i o n s [ 1 , 6 - 8 ] a r e still b a s e d o n t h e v a l u e of Pv = 0.54, w i t h o u t any u n c e r t a i n t y q u o t a t i o n , for t h e m o s t p r e d o m i n a n t y t r a n s i t i o n of 185.7 k e V , e v a l u a t e d by H y d e et al. [4] f r o m m e a s u r e m e n t s p e r f o r m e d b e f o r e 1960, a n d on m o r e r e c e n t r e l a t i v e m e a s u r e m e n t s [9-11]. In an a t t e m p t to i m p r o v e on t h e latter, t h e e m i s s i o n p r o b a b i l i t i e s for t h e m o s t p r o m -
inent 3' t r a n s i t i o n s in t h e e n e r g y r a n g e b e t w e e n 25 a n d 206 k e V a c c o m p a n y i n g t h e d e c a y of 235U a n d 231Th w e r e r e d e t e r m i n e d , z31Th is g e n e r a l l y in s e c u l a r e q u i l i b r i u m with its p a r e n t 235U.
2. Materials and sources T w e n t y s o u r c e s in all w e r e p r e p a r e d f r o m t h r e e different u r a n i u m m a t e r i a l s . S o m e d e t a i l s a b o u t t h e m a t e r i a l s u s e d a n d t h e definition of t h e 235U c o n t e n t of t h e s o u r c e s (section 3) a r e given in t a b l e 1. T h e s o u r c e s f r o m m a t e r i a l 1, a U - A I alloy c o n t a i n i n g 20% ( w t % ) of u r a n i u m , w e r e m a d e b y rolling t h e alloy t o t h i c k n e s s e s of 0.03, 0.2 a n d 0.5 m m , a n d b y cutting c i r c u l a r foils of 10 m m d i a m e t e r . T h e s e foils w e r e p a c k e d into A I
Table 1 Materials used and 235U determination Material No.
1 2 3
Chemical form
U-AI alloy uranium fluoride uranyl acetate
Number of sources
23SUcontent
Activity ratio R
at.%
mg per Method of source determination
/,(2asU) Iv(total)
6
93.22
isotope 1.5--25 dilution
-
3
99.47
0.1-0.6 t~ counting
0.1690±0.0017
11
99.97
0.1-1.0 ot counting
0.9716 _ 0.0010
0029-554X/82/0000--0000/$02.75 © 1982 N o r t h - H o l l a n d
Method of determination
isotope analysis ~t-particle spectrometry
IV. y RAY SPECTROSCOPY
192
R. Vaninbroukx, B, Denecke / Gamma-ray emission probabilities
boxes of 15 m m diameter and 3 m m height. After the measurements in the A1 boxes, the foils were removed and glued onto Pt-coated glass discs. The sources from material 2 were prepared by evaporation under vacuum [12] onto stainless steel discs. The diameter of the active area of these sources was 12.7 mm. For material 3, the sources were made by electrospraying [13] onto stainless steel discs. The diameter of the active area was varied between 6 and 12mm. The amount of 235U per source varied between approximately 0.1 and 25mg. Since the specific activity of 235U is 8 0 B q m g -~, the 235U disintegration rates of the sources varied between about 10 and 2000 Bq.
3. Determination of the 2aSU disintegration rates
Two different methods were applied for the determination of the 235U disintegration rates of the sources: mass spectrometric isotope dilution and a counting. The 23~Th disintegration rates are equal to the 235U disintegration rates because of the secular equilibrium conditions. For material 1 the n u m b e r of 235U atoms N for each of the sources was determined by mass spectrometric isotope dilution [14]. For that purpose, the sources, after the m e a s u r e m e n t of the y-emission rates (section 4), were spiked with a well known amount of 233U and dissolved. The uranium was then separated from the aluminium and the ratio 235U/233U was measured. The accuracy of the 235U determination was estimated to be _+0.5%. The 235U disintegration rate is equal to AN, where )t is the decay constant of 235U. The latter quantity has been calculated from the half-life value r e c o m m e n d e d in ref. 15. The overall uncertainty in the disintegration rates is estimated to be _+0.6%. All uncertainties correspond to a lo" confidence level and take into account random and systematic uncertainties. For samples 2 and 3 the 235U disintegration rates were determined by a counting under a well defined low-geometry solid angle [16]. For sample 2, the corrections for the contribution from other uranium isotopes to the a count rates were calculated from the isotopic composition as determined by mass spectrometry [14] and the decay constants derived from the half-lives
r e c o m m e n d e d in I N D C ( N D S ) - I 2 1 ( N E ) [15]. For sample 3 these corrections were determined by a-particle spectrometry [17]. The values of these corrections for both materials, expressed as the activity ratio R of the 235U a activity over the total a activity are given in table 1. The uncertainties as a result of these corrections are estimated to be +1.0% for sample 2 and _+0.l% for sample 3, where nearly the complete a activity is due to 235U. The optimum accuracy obtainable for a counting under a defined lowgeometry solid angle is +-0.1% [16]. This accuracy could be attained for sample 2, but for sample 3, due to the low specific activity of the material and consequently the higher uncertainties on the corrections for self-absorption and background variations, only accuracies varying between _+0.3 and _+1.5% for the different sources were reached. The mean uncertainty for the 11 sources was _+0.6%. Finally, the overall uncertainty on the 235U disintegration rates becomes _+1.1% for sample 2 and _+0.7% for sample 3. The 235U content in mg per source, as given in table 1, can easily be derived from the n u m b e r of 235U atoms or the 235U disintegration rates.
4. Determination of the y - r a y emission rates
The y-ray emission rates of all the sources were determined using three calibrated photon detectors: a Si(Li) detector with an area of 3 cm 2 and a thickness of 3 m m and two high purity G e detectors with an area of i cm 2 and a thickness of 8 m m (detector GE-I), respectively, an area of 3 cm: and a thickness of 10 m m (detector GE-II). The Si(Li) detector was used for the y rays of 25.65 keV and 84.16 keV following the decay of 23~Th, while the two G e detectors were used additionally for the 3' rays of 143.8, 163.4, 185.7, and 205.3 keV accompanying the decay of 235U. The solid angles subtended by the detector systems were about 0.2 sr for the Si(Li) detector and 0.1 and l s r for the detectors G E - I , G E - I I respectively. A typical 235U-23~Th y-ray spectrum in the energy range concerned here is shown in fig. 1. Several corrections had to be applied. One was the result of the self-absorption of the photons. The values of this correction for the various
R. Vaninbroukx, B. Denecke / Gamma-ray emission probabilities
~
z
q
5 ~:>~
0
512
256
102/.
768
CHANNEL NUMBER
Fig. 1.
235U-231Thy-ray
spectrum (detector GE-I).
y rays and the different materials are given in table 2. These values have been derived from experimental determinations by extrapolation of the observed count rates as a function of the source thicknesses to thickness zero and independently by calculations using absorption coefficients obtained by interpolation from the values tabulated by Veigele [18]. In earlier work [19], it was shown that for small solid angles up to 0.2 sr, as is the case for the Si(Li) and GE-I detectors, the simple formula (1-e-~a)/tzd, where # is the absorption coefficient and d the source thickness in the corresponding units, gives reliable results. The uncertainty on this correction has been estimated to be about 20%. Except for the y rays of 25.65 keV from some sources of material 1, the influence on the accuracy of the y-ray emission rates is smaller than 1% and in most cases less than 0.1%. A second correction is that for differences of Table 2 Correction for self-absorption Correction (%) Energy Ev (keV)
Material 1
Material 2
Material 3
25.65 84.16 143.8 163.4 185.7 205.3
6--60 0.4-5.7 0.3-5.2 0.3-4.1 0.2-3.2 0.2-2.6
0.2-1.5 0--0.1 0--0.1 --0 --0 -0
0.4--2.5 0-4).1 6-0.1 0--0.1 -0 ~0
193
the diameter of the 235U sources from the diameter of about 6ram for the calibration sources. The differences in solid angles for sources of 6, 10, 12 and 12.7mm diameter as used here have been determined experimentally by varying the diameters of the sources from material 3 between 6 and 12 mm. They have also been calculated as a function of the diameters of the detectors and sources and the source to detector distances according to Masket and Rodgers [20]. The mean corrections are (1.9___ 0.2)% for the Si(Li) detector, (1.0_+0.1)% for detector GE-I and ( 4 . 2 - 0 . 4 ) % for GE-II. A third correction is due to summing effects, mainly by coincidences, between the y rays and L X-rays. The magnitude of this correction was estimated from an analysis of the spectra. It was found to vary for the different y rays from 0 to 1% for the Si(Li) detector, from 0 to 0.4% for GE-I and from 0 to 6% for GE-II. The mean uncertainty on the y-ray emission rates caused by this correction was _-_0.4% for detector GE-II and less than 0.1% for the two other detectors. Further corrections such as those for dead time losses and background could easily be determined with sufficient accuracy and do not introduce any substantial uncertainty. Before the 235U-231Th measurements the detectors were calibrated for all the types of sources used; reference sources were used, prepared by the deposition of known amounts of accurately standardized solutions of suitable radionuclides. The photon emission probabilities for the efficiency calibration of the detectors were adopted from a survey of literature data [21-37]. The nuclides used, the photon energies and the emission probabilities with their uncertainties are given in table 3. For all the measurements the peak areas were determined in two ways: (1) by "manual" extrapolation of the "background" from the low- and high-energy side of the peaks to the axis of the peak centroids, and by evaluating the remaining peak contents; (2) using an adapted version of the programme Cutipie [38]. The reproducibilities of the peak area determinations were found to be better than _+0.3%. For monoenergetic y-rays the agreement between both peak determination methods is better than 1%; the "manual" method yields peaks areas (0.7_+0.2)% higher than the Cutipie method. For X-ray peaks IV. y RAY SPECTROSCOPY
R. Vaninbroukx, B. Denecke / Gamma-ray "emission probabilities
194
Table 3 Nuclides and photon emission probabilities used for the calibration of the detectors Nuclides
Radiation
Energy E~ (keV)
Emission probability (Py)
Refs.
57Co
y 3, Y
14.41 122.06 136.47
0.095 -+0.003 0.856 -+0.004 0.106-+ 0.001
[7, 2 1-25]
~ogCd
Ag K~ Ag Ka y
22.1 25.(/ 88.0
0.865 _+0.030 (t. 185 _+0.0(17 0.037 -+0.0() 1
[7, 22, 25]
133Ba
Cs K,, Cs Ko 3' y(79.6 + 81) y 3, 3, 3,
30.8 35.2 53.2 80.9 276.4 302.9 356.0 383.8
(I.995 _+0.010 0.234 -+0.005 0.022 _+0.001 0.360 _+0.005 0.072 +_0.001 0.183 _+0.001 0.620 _+0.004 0.089 _+0.001
[7, 22, 25-29]
139Ce
La K, La KB 3'
33.3 38.0 165.85
0.648 _+0.015 0.152 _+0.005 0.799 _+0.003
[7, 22, 24, 25, 29-31]
14ICe
Pr K~ Pr Kt~ y
35.9 411.9 145.43
0.136 _+0.002 0.032 _+0.001 0.482 _+0.003
[7, 22, 32, 33]
152Eu
Sm K~ Sm KB y y 3'
39.9 45.7 121.8 244.7 344.3
0.591 _+0.012 0.149_+ 0.003 0.284 _+0.0(13 0.074 _+0.002 0.265 _+0.003
[7,22,25,26,34,35]
Z41Am
Np L~ Np L, Np Lr y y
0.132 _+0.004 0.195 _+0.004 0.048 _ 0.001 0.024 _+0.001 0.360 _+0.003
[7, 22, 23, 25, 32, 36, 37]
13.9 17.7 20.9 26.35 59.54
d i f f e r e n c e s of u p t o 3 % w e r e o b s e r v e d . F o r t h e c a l i b r a t i o n s a n d f o r t h e 235U-231Thm e a s u r e m e n t s t h e m e a n v a l u e s of b o t h d e t e r m i n a t i o n s w e r e u s e d . A t y p i c a l p e a k e f f i c i e n c y c u r v e is s h o w n in fig. 2. F r o m t h e u n c e r t a i n t i e s in t h e p h o t o n e m i s s i o n p r o b a b i l i t i e s of t h e r e f e r e n c e n u c l i d e s , t h e a c c u r a c y of t h e s t a n d a r d i z a t i o n of t h e s o u r c e s a n d t h e s m o o t h n e s s of t h e e f f i c i e n c y curve through the experimental points, the o v e r a l l u n c e r t a i n t y in t h e d e t e c t i o n e f f i c i e n c y is e s t i m a t e d t o b e + 2 . 5 % f o r t h e y r a y s of 25.65 a n d 8 4 . 1 6 k e V , -+0.6% f o r t h e y r a y s of 143.8, 163.4 a n d 185.7keV, a n d -+1.0% f o r t h e 205.3 k e V 3' ray.
5. Results The y-ray emission probabilities are obtained by d i v i d i n g t h e y e m i s s i o n r a t e s by t h e 235U o r 231Th d i s i n t e g r a t i o n r a t e s . T h e m e a n results, o b t a i n e d f o r t h e m e a s u r e m e n t s o n all t h e s o u r c e s using the three detectors for the different m a t e r i a l s , a r e g i v e n in t a b l e 4. T h e last c o l u m n of this t a b l e g i v e s t h e w e i g h t e d m e a n v a l u e s ; t h e y h a v e t o b e c o n s i d e r e d as o u r final v a l u e s . T h e q u o t e d u n c e r t a i n t i e s , c o r r e s p o n d i n g to a loconfidence level, take into account random and systematic uncertainties. According to Grinberg et al. [39], t h e r a n d o m a n d s y s t e m a t i c un-
R. Vaninbroukx, B. Denecke / Gamma-ray emission probabilities
195
Table 4 Results of the y-emission probability determinations Energy
Emission probability Pv
(keY) 25.65 84.16 143.8 163.4 185.7 205.3
Material 1
Material 2
Material 3
Final value
0.1445 - 0.0035 0.0666 ±- 0.0020 0.1090 ---0.0021 0.0502 ± 0.0011 0.5780 ± 0.0085 0.0494 ± 0.0015
0.1444 ± 0.0030 0.0652 +__0.0026 0.1086 _ 0.0024 0.0488 ± 0.0015 0.5707 +--0.0098 0.0501 ___0.0020
0.1453 ± 0.0028 0.0626 ± 0.0034 0.1084 ± 0.0023 0.0496 ---0.0012 0.5755 ± 0.0088 0.0515 ± 0.0015
0.145 - 0.003 0.066 ---0.003 0.109 ± 0.002 0.050 ± 0.001 0.575 ± 0.009 0.050 ± 0.002
abilities of the other ~/ rays to that of the "reference" ray are in fairly g o o d agreement with recent evaluated values [6-8].
10-2 86-
z _m
The authors would like to thank Messrs. H. Mast, J. Pauwels and F. Peetermans for the preparation of the sources, Messrs. M. Gallet, A. Loopmans and E. Sattler for the mass spectrometric determinations and M.G. Bortels for the a-particle spectrometry measurements.
2-
~_ 10-3
~
a6-
References
i
DETECTOR roE-[
I
--SOURCES
ON Pt COATED GLASS DISCS. . . .
I 10-~
I
I 10
20
I 40
60
80 100
200
400
PHOTON ENERGY {keV)
Fi~. 2. Peak detection efficiencies.
certainties are combined by using the formula tsSE+~,Y, Si. Here, ts is the student t-factor depending on the degrees of freedom, which are based on the number of single determinations, and ,~81 is the linear sum of the individual systematic uncertainties 8i. All the sources were measured at least two times with each of the detectors. Thereby, the number of determinations was large and the mean value of ts was 1.05. Our result for the 1 8 5 . 7 k e V ray, the most intense o n e and often considered as a "reference" 3' ray, is 6.5% higher than the previously adopted value, based on a few very old measurements. The ratios of the emission prob-
[1] A.L. Nichols, Transactinium Nuclear D a t a - 1979, IAEA-TECDOC-232 (IAEA, Vienna, 1980) p. 67. [2] C.W. Malich, Bull. Am. Phys. Soc., Ser. II, 1 (1956) 43. [3] R.C. Pilger, F.S. Stephens, F. Asaro and I. Perlman, Bull. Am. Phys. Soc., Ser. II 2 (1957) 394. [4] E.K. Hyde, I. Perlman and G.T. Seaborg, The nuclear properties of the heavy elements, vol. II: detailed radioactive properties (Prentice Hall, Englewood ClifIs, S J 1964). [5] R. Gaeta and M.A. Vignon, Nucl. Phys. 76 (1966) 353. [6] M.R. Schmorak, Nucl. Data Sheets 21 (1977) 91. [7] C.M. Lederer and V.S. Shirley, eds., Table of isotopes, 7th Ed. (John Wiley, New York, 1978). [8] W.B. Ewbank, Transactinium Isotope Nuclear D a t a 1979, IAEA-TECDOC-232 (IAEA, Vienna, 1980) p. 109. [9] W. Teoh and R.D. Connor, Nucl. Phys. A228 (1974) 432. [10] E. Vano, R. Gaeta, L. Gonzalez and C.F. Liang, Nucl. Phys. A251 (1975) 225. [11] S.A. Baranov, V.M. Shatinsky, A.C. Zelenkov and V.A. Pchelin, Sov. J. Nucl. Phys. 26 (1977) 921. [12] J. Van Audenhove, P. De Bi~vre, J. Pauwels, F. Peetermans, M. Gallet and A. Verbruggen, Nucl. Instr. and Meth. 167 (1979) 61. [13] V. Verdingh, Nucl. Instr. and Meth. 102 (1972) 497. [14] P. D e BiEvre, Advances in Mass Spectrometry, Vol. VII, ed., N.R. Daly (Heyden, London, 1976) p. 395. IV. 3' R A Y S P E C T R O S C O P Y
196
R. Vaninbroukx, B. Denecke / Gamma-ray emission probabilities
[15] A. Lorenz, ed, INDC(NDS)-121/NE (1980). [16] A. Spernol and B. Denecke, Chemical nuclear data. (The Institution of Civil Engineers, London, The British Nuclear Energy Society, 1971) p. 199. [17] G. Bortels, KFK 2862-EUR 6402e, Part Ili (1979) p. 24. [18] W.M.J. Veigele, Atomic Data Tables 5 (1973) 51. [19] A. Paulsen, R. Widera, R. Vaninbroukx and H. Liskien, Nucl. Sci. Eng. 76 (1980) 331. [20] A.V.H. Masket and W.C. Rodgers, TID-14975 (1962). [21] H.E. Freund and J.C. McGeorge, Z. Phys. 238 (1970) 6. [22] Nuclear Data Sheets, ed., Nuclear Data Project, ORNL, Oak Ridge. [23] J.S. Hansen, J.C. McGeorge, D. Nix, W.D. Schmidt-Ott. I. Unus and R.W. Fink, Nucl. Instr. and Meth. 106 (1973) 365. [24] U. Sch/)tzig, K. Debertin and K.F. Walz, Nucl. Instr. and Meth. 169 (1980) 43. [25] R.G. Helmer and C.W. Reich, Third coordinated research meeting on the measurement and evaluation of transactinium isotope nuclear data, ed.. A. Lorenz, INDC(NDS)-118/NE (1980). [26] R.J. Gehrke, R.G. Helmer and R.C. Greenwood, Nucl. Instr. and Meth. (1977) 405. [27] U. Sch6tzig, K. Debertin and K.F. Walz, Int. J. Appl. Rad. Isot. 28 (1977) 503.
[28] B. Chauvenet, J. Morel and J. Legrand, ICRM-S-6 (1980). [29] Y. Yoshizawa, H. Inoue, M. Hoshi, K. Shizuma and Y. fwata, JAERI-M 8811 (1980). [30] H.H. Hansen and D. Mouchel, Z. Phys. A274 (1975) 335. [31] H.H. Hansen and D. Mouchel, Z. Phys. A276 (1976) 303. [32] J. Legrand, J.P. Perolak C. Bac and J. Gorry, Int. J. Appl. Rad. Isot. 26 (1975) 179. [33] H.H. Hansen, E. Celen, G. Grosse, D. Mouchel, A. Nylansted Larsen and R. Vaninbroukx, Z. Phys. A290 (1979) 113. [34] J. Legrand, J. Morel and A. Traverse, Bulletin BNM no. 19 (1975) 23. [35] K. Debertin, Nucl. Instr. and Meth. 165 (1979) 165. [36] J.L. Campbell and L.A. McNelles, Nucl. Instr. and Meth. 117 (1974) 519. ]37] J. Plch, J. Zderadicka and L. Kokta, Czech. J. Phys. B26 (1976) 1344. [38] W. Teoh, Nucl. Instr. and Meth. 109 (1973) 509. [39] B. Grinberg, J.P. Brethon, F. Lagoutine, Y. Le Gallic, J. Legrand, A.H. Wapstra, H.M. Weiss, W. Bambynek, E. De Roost, H.H. Hansen and A. Spernol, Atomic Energy Rev. 11 (1973) 516.