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Alpha-particle-emission probabilities in the decay of 234U and photon-emission probabilities in the decays of 234U, 239Np and 243Am
Int. J. Appl. Radiat. Isot. Vol. 35. No. I2. pp. 1081-1087. 1984
0020-708X/84 53.00+0.00 Copyright ~ 1984 Pergamon Press Ltd
Printed in.Great Britain.All rigJats reserved
Alpha-Particle-Emission Probabilities in the Decay of 234U and Photon-Emission Probabilities in the Decays of 234U, 239Np and 243Am R. V A N I N B R O U K X ,
G. B O R T E L S a n d B. D E N E C K E
CEC-JRC, Geel Establishment, Central Bureau for Nuclear Measurements, B-2440, Geel, Belgium (Received 17 May 1984; in revised form 5 June 1984)
Alpha-particle-emissionprobabilities in the decay of ~ U and photon-emission probabilities in the decays of 23aU,239Npand :4~Amhave been measured. The ",-particle-emissionprobabilities were measured using an ion-implanted silicon detector. The results were obtained from measurements in various solid angles and extrapolations to solid angle zero. The photon-emission probabilities were deduced from the photon-emission rates, measured with a calibrated high-purity-Ge detector, and the disintegration rates, determined by ~-particle counting in well-defined solid angles. The present results improve appreciably the accuracy of the pertinent decay data.
I. Introduction
coincident photons are much lower for 243Am-239Np than for ~33Ba, a nuclide which is frequently used for the calibration of photon detectors in the same energy range. In the present work, new measurements of :c-particle-emission probabilities in the decay of 234U and of photon-emission probabilities in the decays of 234U, :39Np and 243Am are described.
An accurate assay of a suitable sample for actinides can be performed by ~-particle or y-ray spectrometry provided that the :t-particle and photon-emission probabilities are well known. For the nuclide -'3~U, which is the major contributor to the disintegration rate of most enriched uranium samples, detailed ~-emission-probability data were lacking up to now, the estimated accuracy being only about 4%. °) Also the accuracy of the 2. Materials and Sources photon-emission probabilities was only about I0~o and hence accurate new measurements were required. For the measurements on 234U, all sources were Such measurements increase confidence in the com- prepared from the uranium material AG 462 which pleteness and consistency of the decay scheme, m The contains 94.4% 234U. This material was used in 1970 required accuracies are I and 2% for the a-particle at CBNM for a 234U half-life determination.~6) From and photon-emission probabilities, respectively.(~) the uranium isotopic composition of the sample, Photon-emission probabilities for 239Np are often measured by mass spectrometry, and from careful needed for decay heat calculations~3) and for the :t-particle and 7-ray spectrometric measurements, the determination of the '39pu accumulation in reactors. (4) :3aU contribution to the ~-particle disintegration rate The most practical way to determine the photon- was determined to be 0.99985 4- 0.00005. The remainemission probabilities is to perform measurements on ing part of the disintegration rate is mainly due to the radioactive pair :43Am-:39Np. The nuclide 243Am :3°Th which had grown into the sample. In the 7-ray is long-lived and can be accurately standardized by measurements, special attention was paid to the ~-particle counting. Generally, the 2.4-day 239Np is in detection of 232U and z33U. Careful analyses of the secular equilibrium with 243Am. This pair could also spectra showed that within limits of 0.5 ppb and be used as a long-lived reference material for 1 ppm for 232U and 2~3U, respectively, these nuclides efficiency calibration of photon detectors (4'5) in the were not present in the sample. Eleven sources, all energy range 40-340 keV, if the photon-emission on stainless-steel backings, were prepared, either by probabilities were accurately known. It should be electrodeposition or by sublimation in vacuum. The noted that, for the same solid angles of the detection latter technique was used to obtain thin homogenesystems, the corrections for summing effects between ous sources. The diameter of the active spot was 1081 A.R,I. 35,12--A
1082
R.V.,,N~YBaO~_KXet al.
either 8 or 10 mm. The amount of uranium on the sources varied between 2 and 780 #g corresponding to "~aU activities from 0.4 to 180 kBq. Three of the sources, with uranium masses between 2 and 9#g, were used for the measurements of the :~-particleemission probabilities. For the photon-emissionprobability determinations, eight sources with uranium amounts between 20 and 780 gg were used. For the measurements on >3Am-:39Np, material obtained in 1979 from ORNL. Oak Ridge. was used. The radionuclidic purity of the sample was determined by :~-particle and 7-ray spectrometry. The only impurity which could be detected was :UAm The activity ratio :a~Am/:a3Am was found to be 0.0027 + 0.0001. From these measurements it could also be estimated that any contribution to the :~-particle disintegration rate by other impurities was smaller than 0. l°o of the :4-~Amactivity. Seven sources were prepared by drop deposition on stainless-steel backings. The diameter of the active spot varied between 6 and 12 mm and, the amount of 243Am between 1 and 6 #g, corresponding to :43Am activities from 7 to 43 kBq.
3. Alpha-Particle-Emission Probabilities in the Decay of n4U A. Measurements
The probabilities per decay P~o, P~5~ and P~:~ for emission of :~ particles to the ground level and the 53- and 174-keV levels in :~°Th, respectively, were measured. The corresponding :~-particle energies are about 4776, 4724 and 4605 keV. I:) Figure I shows the corresponding peaks in the :~-particle pulse-height distribution. All other :~-emission probabilities, the sum of which is less than 10-s, Iv) can be neglected. The P=~ values were obtained by measurement of I=~= N ] Z N , in various solid angles f~ and by extrapolation to f~ equals zero. N~ is the number of events recorded in the :q peak (Fig. 1). Only vacuum evaporated sources were used for these measurements. The sources were measured in 105 ~0
)o 4 ..J LU Z z
10 3
m
102
solid angles ranging from about 0.04 to 0.3 sr, using a passivated ion-implanted silicon-junction detector ~' of 20 ram: active area. For a few measurements a premium grade Au-Si surface-barrier detector of 100 mm 2 with a diaphragm of 8-mm diameter was used. Part of the measurements were carried out using a small permanent magnet in between the source and the detector 9 to deflect most of the conversion electrons away from the detector. In this manner the rates of true and accidental coincidences between :~-particles and conversion electrons were reduced by a factor of 12. Measurement cycles were run for 2 h duration with counting rates from 5 to 37s -~. Subsequently the pulse-height distributions were added to each other to obtain 16 summed spectra with t.vpically 8 × 105 counts for N~0. Prior to the spectrum addition, the positions of :~0 were fitted and, if required, matched by shifting spectra over an integer number of channels. This procedure allowed for adequate peak-drift correction and hence, improved resolution. The :q) peak-to-valley ratio for the summed spectra was typically 120 for measurements with a solid angle v 0 . 0 4 s r and 55 for fl~-,0.3sr. For the energy range considered in the peak analysis, the correction for :e,>tailing at the :~53peak amounted to typically 4°~i; for ~ ~-, 0.04 sr. and 7~" for f~ ~-, 0.3 sr. The energy o scale in the spectra was 0.69 keV per channel. The background pulse rate in the energy range of interest was 5 × 10 -a s -t, from randomly distributed events. Sum pulses from ~-particle and conversion electron (e-) true coincident events are lost out of the :~5_~ and :~74 peaks and add to the adjacent ~ peak of higher energy. Consequently, the number of events N~ recorded in the peaks (Fig. 1) changes accordingly. Moreover, the sum pulses from :~_~3or :~:a L-shellconversion electrons of the .... / ~a.2 transition may affect the tail extrapolation. Similar effects, which are however negligibly small, exist as a result of :~-eaccidental pile-up. The true :~-e- coincidences will, in the case of measurements in a low geometry, give rise to a linear relationship between the relative :q peak intensities I~, = N,/£N~ and the solid angle fL H°) They vanish for fa--,0 and, hence, the =-particle-emission probabilities become equal to the extrapolated values. The P,~ values were obtained from a linear fit to the data of Fig. 2 for D..~0. The slopes of the lines were found to be 2.5 times larger than those obtained from calculations.(m) The solid angles were readily calculated using the '-3aU disintegration rates, which had been measured in a counter with a well-defined small solid angle. B. Results and discussion
0 101
7oo 2oo
l i ~oo CHANNEL
8oo
8oo
NUMBER
Fig. 1. Typical summed ~-particle spectrum ofZ3~U.
The results for P~ are presented in the Table 1. The quoted random uncertainties (68°/, confidence interval, n = 16) are obtained from linear regression analyses. The systematic uncertainties are mainly due to the :3°Th grown into the :3~U samples and to the peak-tailing corrections. The uncertainties due to
1083
Alpha-particle-emission probabilities o
0.730-
OY2S-
0220-
0215P~.O
0.710-
o oi~
o;s
0285-
/ ' - P •S3
0.280-
~~-~,-~-~ ~a ~ ~
i
llS
~
is ,Io-2
o SOURCE ~ 8 ram., 1830 Bq a SOURCE ~l 10 ram., 2100 Bq ~ SOURC.~_~E 3~17 Bmm, 8_Bq
e.1
0275-
0.270-
0.265
0'5
0 o'.I
0.0021- o ~
i
,'s
~
zs ~io'2
Pa 17t.
0.00 0. --
0.0019,
IG
c,
O.O0~B- ~E
{)
0.0017-
o
o
o o'1
o's
i
t's
~
2's ,10"2
SOLID ANGLE //. TE
Fig. 2. Determination of the :~-particle-emission probabilities. Open marks correspond to data points measured with an ion-implanted detector and solid triangles to those with a surface barrier detector. Error bars indicate _+ l SD from counting statistics. interferences by the other uranium isotopes and by background pulses were found to be negligible. The overall uncertainties in the P,~ values, corresponding to the 68% confidence level, were obtained by summing quadratically the individual uncertainties, These results, for which a substantial effort has been devoted to reach a high accuracy, are about 30 times more accurate than the values given by various evaluators/LTJ*) 4. 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
A. Measurement of the disintegration rates The disintegration rates, No, of the sources were measured by ~-particle counting in well-defined low
solid angles. °2) The solid angle varied between 0.002 and 0.75 sr. The typical uncertainties are summarized in Table 2. They all correspond to a 68% confidence level. Summing the individual uncertainties quadratically, we obtain the overall uncertainties in N 0 for 234Uand -'~3Am of + 0.09 and + 0.13°,o respectively.
B. Determination of the photon-emission rates The photon-emission rates, Nph, for the most important 7 and x rays were measured using an efficiency-calibrated high-purity-Ge detector. The area of the detector was about 1 cm 2 and its thickness 8 mm. The solid angle subtended by the detection system was about 0.1 sr, the source-to-detector distance being 28 mm. The nuclides used for the call-
Table 1. Alpha-particle-emissionprobabilities in the decay of ZJ4U Corresponding Typical uncertainty components .particle Emission Level in energy probability :3~Th Tail :3°Th (keV) (P~ Random influence correction 174 keV 4605 0.00206 _- 0.00004 0.00002 0.00001 0.00003 53 keV 4724 0.2842 = 0.0005 0.0003 0.0001 0.0003 0 4776 0.7138 = 0.0005 0.0003 0.0001 0.0003
R. VAN~,~ROt.'KX et al.
1084
Table 2. Uncertainties in the measurements of the disintegration rates :~aU :~Am
(?;)
(%)
Random Solid angle Background Tail extrapolation Impurities
0.03 0.06 0.02
0.05 0.05 0,05
0.04
0.02
0.05
0. l 0
Total (68% confidence level)
0,09
0.13
bration, the photon energies, the emission rates with their uncertainties and the mode of peak evaluation have been reported elsewhere. ~.t~ The uncertainties in the detection efficiencies given by the calibration curve were estimated to be I - 2 ~ for energies up to 120 keV, and 0.5-1~ o for higher energies, depending on the accuracies in the emission probabilities of the calibration photons which were used in the energy regions concerned.
Typical photon spectra for z3aU and 2~3Am-239Np are shown in Figs 3 and 4, respectively. To obtain the final photon-emission rates, several corrections had to be applied. A first was due to photon selfabsorption. The values of this correction for the various photons have been calculated from the source thicknesses and the photon attenuation coefficients obtained by interpolation between the values tabulated by Veigele. °~ For z34U this correction, especially for the L x rays, was also determined experimentally by extrapolation of the specific count rates to zero source thickness. The uncertainty introduced by this correction was smaller than 0. lVo for the "3~UL x rays and negligible in all other cases. A second correction was due to summing effects for coincident photons. These corrections were calculated from the decay schemes of ~ U , (7~ 23~Np(~6)and 2~3Am{~?~and the peak and total detection efficiencies. For the various photons the correction factor varied
M
..z
t~
#-.
.J
t.d
z
g g
E bJ Z L,J
-
e 12S PHOTON E N E R G Y
I keV]
Fig. 3. Photon spectrum of :34U,
o~ uJ
~c L.U Z LU
>= ..,¢
z
t~
0
'I SO
lO0
150
250
200
PHOTON
ENERGY
[keV]
Fig. 4. Photon spectrum of :~3Am-23~Np.
. _ A _ _ 300
350
/
I1.1 ThL I 13.0(E) ThE,
0.0604 0.6850 0.0035 0.0013
+ 0.0013 __.0.0150 + 0.0001 ± 0.0001
0.0077 ± 0.0002 0.0160 ± 0.003 0.0208 + 0.0003
285.41 315.81 334.30
43.53 74.67 86.79 142.18
0.0129 -I- 0.0002 0.1360 + 0.0030 0.2220 ± 0.0050 0.2750 + 0.0040 0.0007 + 0.0001 0.0346 + 0.0005 0.0028 -I- 0.0002 0. I 121 + 0.0018 0.0012 + 0.0001 0.0008 ± 0.0001 0.1438 ± 0.0021
0.1035 ::!:0.0014 (I.24 + 0.02)10 -1 (2.53 + 0.07)10 -5 (4.15 _ 0.10)10 -5 (3.41 + 0.04)10 -4
0.0123 + 0.0003
0.0022+0.0001 0.0360 + 0.0007 0.0530 + 0.0012
61.48 99.55 PuKe2 103.76 PuK~ 106.13 181.71 209.75 226.42 228.19 254.41 272.84 277.60 277.60
Total ThLx 53.20 89.95 Thk,2 93.35 ThK,i 120.88
16.2(I~) ThL~ 19.0(Ii) ThL r
This work
_+ 0.0130 + 0.0001 + 0.0024 + 0.0004 + 0.0064 ± 0.0002 + 0.0001 ± 0.0040
0.0504 0.6000 0.0031 0.0011
+ ± + ±
0.0060 0.0600 0.0004 0.0002
0.0078 + 0.0007 0.0159 + 0.0011 0.0203 ± 0.0018
0.2270 0.0011 0.0324 0.0034 0.1072 0.0010 0.0008 0.1410
0.0096 ± 0.0013
t
(4.10 .+ 0.40)10 '~
(1.19 + 0.10)10 -3
Nuclear Data Sheets
Re£ 17 (1981)
Ref. 16 (1977)
t (1977) Ref.7
Photon-emission probability, Pph
0.67110 + 0.0100 0.0033 ± 0.0001 0.0012 + 0.00t)1
jg
0060+o006t
-1 0.0163 ± 0.0007 0.0210 + 0.0010
]
0.0093 ± 0.0006
0.1500 + 0.(1050 0.1430 -I- 0.0024
0.0336 + 0.0014 0.0024 + 0.0003 0.1178 ± 0.0044
0.1970 + 0.0080 0.2660 + 0.0100
0.0487 ± 0.0010 0.0105 + 0.0004 0.0981 + 0.0013
0.0366 + 0.0008
0.0022+ 0.0001
Recent measurements (not included in NDS)
Ref. 20 (1983)
Ref. 18
Ref. 19 (1979)
Ref. 18 (1977)
Ref. 21 (1977)
+ 0.0030 ± 0.0001 + 0.000l _+0.0040
+ 0.006(I -I- 0.008(I ± 0.0090 + 0.00008 + 0.0010
0.6600 ± 0.0300
00,,0+0000
0.0152 ± 0.01X)5 0.0195 ± 0.0007
0.0076 i 0.0002
0.1140 0.0011 0.0008 0.1450
0.1450 0.2220 0.2780 0.00075 0.0342
Ahmad and Walflgren Ref. 5 (1972)*
*Note added in proof--More recently, 1. Ahmad (Natl. lnstrum. Methods 193, 9 (1982)] published new results; they agree, within tile quoted uncertainties, with the results of tile present work.
243AIn
239Np
2~U
Nuclide
Photon energy "(keV)
Table 3. Photon-emission probabilities in the decay of 2~U, ~39Np and 243Am
t~
a
g
o
W
>
1086
R. VANINBROUKXet al.
Level in :3UTh 174 keV 53 keV
Table 4. Internal-conversioncoefficientsin the decay of -'~U Transition and photon-emission Total internal conversion Corresponding probabilities coe~ciencs x photon energy (keV) P:, P~ x 10: Experimental Theoretical 120.88 0.00206 = 0.00004 0.341 : 0.004 5.04 = o. 11 5 53.20 0.2862 - 0.0005 1.24 -.-0.02 231 -_ 4 233
between 1.000 and 1.003 for -"~"U and >3Am and between 1.000 and 1.006 for '39Np. The uncertainty due to this correction was in all cases smaller than O.O5°0. A third correction was due to the difference between the diameter of the sources and that of the used calibration sources. The correction was calculated from the diameters of the detector and the sources and the source-to-detector distance. Typical correction factors are (1.012 + 0.001) and (l.018 + 0.001) for sources with diameters of 10 and 12mm, respectively. 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. The overall uncertainties on the Nph values were obtained by summing quadratically the individual uncertainties. C. Results and discussion
The photon-emission probabilites, Pph, are obtained by dividing the emission rates .~'v'ph by the disintegration rates No. The results are given in Table 3. The quoted uncertainties, corresponding to a 68% confidence level, take into account random uncertainties, estimated from the various individual results obtained from the different sources, and the uncertainties in the disintegration rates N O and the photon-emission rates Nph. For comparison, Table 3 gives also the values recommended by the Nuclear Data Sheets (NDS) :'t6~7 and the results of recent measurements tS-'-° not included in NDS. The results obtained by A h m a d and Wahlgren ~s~ for 239Np and 2~3Am, although included in the N D S , are also given. They are quoted with considerably lower uncertainties than the N D S values and have perhaps been somewhat underweighted in the evaluations. The present results for the Th-L x-ray-emission probabilities in the decay of 234U, especially those for L~, L.. and total L x, are significantly higher than the results of Bemis and Tubbs. ('-t~Our value for the total L x-ray emission probability agrees better with the theoretical value of (0.109_ 0.007), calculated using the formula Pox = Y V,c% where V~ are the numbers of vacancies in the L, subshells, including those created by Coster-Kronig transitions, and c,)~ are the related fluorescence yields. The values o f V, were calculated from the theoretical L-subshell internal conversion coefficients for the 53.20 and 120.88-keV transitions and the Coster-Kronig transition probabilities. The conversion coefficients were obtained by interpolation between the values tabulated by R6sel et al. ~22~ The Coster-Kronig transition probabilities
and the L subshell fluorescence yields were taken from Krause) TM For the other photon-emission probabilities of:3~U and for those of 239Np and "-~3Am the present m e a s u r e m e n t s certainly improve the accuracy status. In the frame of the I A E A Coordinated Research Programme on the Measurement and Evaluation of Transactinium Isotope Nuclear Data, r:~ evaluations of all experimental data will be performed and recommended values given. These evaluations are expected to be available at the end of 1984.
5. Consistency of the P~- and Pph Data for 234U; Internal Conversion Coefficients The total transition probability Ptr for deexcitation of a given level by 7 rays and conversion electrons is equal to the sum of all transitions populating that level. Since for the 121-keV and the weak higher levels no cross-over 7 rays are observed, the probabilities for depopulating the 121-keV and 53-keV levels are equal to the sum of all :~-particle transitions to the respective level and to all higher levels. The quantities P~r and Pph are related by the equation 7 = Ptr/Pph - 1, where = is the total internal conversion coefficient for the 7 transition considered. The results for the total conversion coefficients deduced from our measurements are given in Table 4 together with theoretical values obtained from R6sel et al. ~2"-~The excellent agreement between the experimental and theoretical values supports the reliability of our results and improves appreciably the confidence in the completeness and consistency of the z3~U decay scheme. Acknowledgements--The authors are grateful to W. Olden-
hof for the special efforts spent on the preparation of the 234U sources.
References l. Nichols A. L. In Transactiniurn Isotope Nuclear Data-1979, IAEA-TECDOC-232, p. 67 (IAEA, Vienna, 1980). 2. Lorenz A. Fifth Research Coordination Meeting on the Measurement and Evaluation o f Transactinium Isotope Nuclear Data, INDC(NDS)-I38/GE (1982). 3. Lorenz A. Second L4EA Advisory Group Meeting on Transactinium Isotope Nuclear Data, Cadarache 1979,
Summary Report (IAEA. Vienna, 1979). 4. Mozhaev V. K., Dulin V. A. and Kazanskii Yu. A. Sot. At. Energy, 47, 566 (1979). 5. Ahmad I. and Wahlgren M. Nucl. lnstrum. Methods 99, 333 (1972). 6. De Bi6vre P., Lauer K. F., Le Duigou Y., Motet H., M/ischenborn G., Spaepen J., Spernol A., Vaninbroukx R. and Verdingh V. In Chemical Nuclear Data (Ed.
Alpha-particle-emission probabilities Hurre{1 M. L.) p. 221 (The British Nuclear Energy Society, London. 1971). 7. Ellis Y. A. Nucl. Data Sheets 21, 493 (1977). 8. Kemmer J.. Burger P., Henck R. and Heyn e E. IEEE Trans. ,Vucl. Sci. Vol. NS-29, 733 (1982). 9. Bortels G. In Annual Progress Report on ,Vuclear Data 1983, NEANDC(E) 252, p. 81 (1984). I0. Bortels G., Denecke B. and Vaninbroukx R. Nucl. lnstrum. Methods 223, 329 (1984). 11. Lederer C. M. and Shirley V. A. Table of Isotopes, 7th edn, (John Wiley, New York, 1978). 12. Spernol A. and Denecke B. In Chemical Nuclear Data (Ed. Hurell M. L.) p. 199 (The British Nuclear Energy Society, London, 1971). 13. Vaninbroukx R. and Denecke B. Nucl. lnstrum. Methods 193, 191 (1982). 14. Vaninbroukx R. and Hansen H. H. Int. J. Appl. Radiat. lsot. 34, 1395 (1983). 15. Veigele W. M. J. At. Data Tables 5, 5i (1973).
1087
16. Schmorak M. R. Nucl. Data Sheets 21, 91 (1977). 17. Ellis-Akovali Y. A. Nucl. Data Sheets 33, 79 (1981). 18. Starozhukov D. [., Popov Yu. S. and Privalova P. A. Soy. At. Energy 42, 355 (1977). 19. Mozhaev V. K., Dulin V. A. and Kazanskii Yu.A. Soc. At. Energy 47, 566 (t979). 20. Holloway S. P., Olomo J. B. and MacMahon T. D. In Nuclear Data for Science and Technology (Ed. B6ckhoff K. H.) p. 287 (D. Reidel, Dordrecht, 1983). 21. Bemis C. E. Jr and Tubbs L. Absolute L-Series X-Ray and Low-Energy Gamma-Ray Yields for Most Transuranium Nuclides, U.S. ERDA Report ORNL-5297, p. 93 (1977). 22. R6sel F.. Fries H. M., Alder K. and PauIi H. C. At. Data NucL Data Tables 21, Nos 4,5 (1978). 23. Krause M. 0. J. Phys. Chem. Ref Data 8, 307 (1979). 24. Lorenz A. Si.,:th Research Coordination .Meeting on the Measurement and Et'aluation of Transactinh~m Isotope Nuclear Data, INDC (NDS)-I47/GE (1983).
0020-708X/84 53.00+0.00 Copyright ~ 1984 Pergamon Press Ltd
Printed in.Great Britain.All rigJats reserved
Alpha-Particle-Emission Probabilities in the Decay of 234U and Photon-Emission Probabilities in the Decays of 234U, 239Np and 243Am R. V A N I N B R O U K X ,
G. B O R T E L S a n d B. D E N E C K E
CEC-JRC, Geel Establishment, Central Bureau for Nuclear Measurements, B-2440, Geel, Belgium (Received 17 May 1984; in revised form 5 June 1984)
Alpha-particle-emissionprobabilities in the decay of ~ U and photon-emission probabilities in the decays of 23aU,239Npand :4~Amhave been measured. The ",-particle-emissionprobabilities were measured using an ion-implanted silicon detector. The results were obtained from measurements in various solid angles and extrapolations to solid angle zero. The photon-emission probabilities were deduced from the photon-emission rates, measured with a calibrated high-purity-Ge detector, and the disintegration rates, determined by ~-particle counting in well-defined solid angles. The present results improve appreciably the accuracy of the pertinent decay data.
I. Introduction
coincident photons are much lower for 243Am-239Np than for ~33Ba, a nuclide which is frequently used for the calibration of photon detectors in the same energy range. In the present work, new measurements of :c-particle-emission probabilities in the decay of 234U and of photon-emission probabilities in the decays of 234U, :39Np and 243Am are described.
An accurate assay of a suitable sample for actinides can be performed by ~-particle or y-ray spectrometry provided that the :t-particle and photon-emission probabilities are well known. For the nuclide -'3~U, which is the major contributor to the disintegration rate of most enriched uranium samples, detailed ~-emission-probability data were lacking up to now, the estimated accuracy being only about 4%. °) Also the accuracy of the 2. Materials and Sources photon-emission probabilities was only about I0~o and hence accurate new measurements were required. For the measurements on 234U, all sources were Such measurements increase confidence in the com- prepared from the uranium material AG 462 which pleteness and consistency of the decay scheme, m The contains 94.4% 234U. This material was used in 1970 required accuracies are I and 2% for the a-particle at CBNM for a 234U half-life determination.~6) From and photon-emission probabilities, respectively.(~) the uranium isotopic composition of the sample, Photon-emission probabilities for 239Np are often measured by mass spectrometry, and from careful needed for decay heat calculations~3) and for the :t-particle and 7-ray spectrometric measurements, the determination of the '39pu accumulation in reactors. (4) :3aU contribution to the ~-particle disintegration rate The most practical way to determine the photon- was determined to be 0.99985 4- 0.00005. The remainemission probabilities is to perform measurements on ing part of the disintegration rate is mainly due to the radioactive pair :43Am-:39Np. The nuclide 243Am :3°Th which had grown into the sample. In the 7-ray is long-lived and can be accurately standardized by measurements, special attention was paid to the ~-particle counting. Generally, the 2.4-day 239Np is in detection of 232U and z33U. Careful analyses of the secular equilibrium with 243Am. This pair could also spectra showed that within limits of 0.5 ppb and be used as a long-lived reference material for 1 ppm for 232U and 2~3U, respectively, these nuclides efficiency calibration of photon detectors (4'5) in the were not present in the sample. Eleven sources, all energy range 40-340 keV, if the photon-emission on stainless-steel backings, were prepared, either by probabilities were accurately known. It should be electrodeposition or by sublimation in vacuum. The noted that, for the same solid angles of the detection latter technique was used to obtain thin homogenesystems, the corrections for summing effects between ous sources. The diameter of the active spot was 1081 A.R,I. 35,12--A
1082
R.V.,,N~YBaO~_KXet al.
either 8 or 10 mm. The amount of uranium on the sources varied between 2 and 780 #g corresponding to "~aU activities from 0.4 to 180 kBq. Three of the sources, with uranium masses between 2 and 9#g, were used for the measurements of the :~-particleemission probabilities. For the photon-emissionprobability determinations, eight sources with uranium amounts between 20 and 780 gg were used. For the measurements on >3Am-:39Np, material obtained in 1979 from ORNL. Oak Ridge. was used. The radionuclidic purity of the sample was determined by :~-particle and 7-ray spectrometry. The only impurity which could be detected was :UAm The activity ratio :a~Am/:a3Am was found to be 0.0027 + 0.0001. From these measurements it could also be estimated that any contribution to the :~-particle disintegration rate by other impurities was smaller than 0. l°o of the :4-~Amactivity. Seven sources were prepared by drop deposition on stainless-steel backings. The diameter of the active spot varied between 6 and 12 mm and, the amount of 243Am between 1 and 6 #g, corresponding to :43Am activities from 7 to 43 kBq.
3. Alpha-Particle-Emission Probabilities in the Decay of n4U A. Measurements
The probabilities per decay P~o, P~5~ and P~:~ for emission of :~ particles to the ground level and the 53- and 174-keV levels in :~°Th, respectively, were measured. The corresponding :~-particle energies are about 4776, 4724 and 4605 keV. I:) Figure I shows the corresponding peaks in the :~-particle pulse-height distribution. All other :~-emission probabilities, the sum of which is less than 10-s, Iv) can be neglected. The P=~ values were obtained by measurement of I=~= N ] Z N , in various solid angles f~ and by extrapolation to f~ equals zero. N~ is the number of events recorded in the :q peak (Fig. 1). Only vacuum evaporated sources were used for these measurements. The sources were measured in 105 ~0
)o 4 ..J LU Z z
10 3
m
102
solid angles ranging from about 0.04 to 0.3 sr, using a passivated ion-implanted silicon-junction detector ~' of 20 ram: active area. For a few measurements a premium grade Au-Si surface-barrier detector of 100 mm 2 with a diaphragm of 8-mm diameter was used. Part of the measurements were carried out using a small permanent magnet in between the source and the detector 9 to deflect most of the conversion electrons away from the detector. In this manner the rates of true and accidental coincidences between :~-particles and conversion electrons were reduced by a factor of 12. Measurement cycles were run for 2 h duration with counting rates from 5 to 37s -~. Subsequently the pulse-height distributions were added to each other to obtain 16 summed spectra with t.vpically 8 × 105 counts for N~0. Prior to the spectrum addition, the positions of :~0 were fitted and, if required, matched by shifting spectra over an integer number of channels. This procedure allowed for adequate peak-drift correction and hence, improved resolution. The :q) peak-to-valley ratio for the summed spectra was typically 120 for measurements with a solid angle v 0 . 0 4 s r and 55 for fl~-,0.3sr. For the energy range considered in the peak analysis, the correction for :e,>tailing at the :~53peak amounted to typically 4°~i; for ~ ~-, 0.04 sr. and 7~" for f~ ~-, 0.3 sr. The energy o scale in the spectra was 0.69 keV per channel. The background pulse rate in the energy range of interest was 5 × 10 -a s -t, from randomly distributed events. Sum pulses from ~-particle and conversion electron (e-) true coincident events are lost out of the :~5_~ and :~74 peaks and add to the adjacent ~ peak of higher energy. Consequently, the number of events N~ recorded in the peaks (Fig. 1) changes accordingly. Moreover, the sum pulses from :~_~3or :~:a L-shellconversion electrons of the .... / ~a.2 transition may affect the tail extrapolation. Similar effects, which are however negligibly small, exist as a result of :~-eaccidental pile-up. The true :~-e- coincidences will, in the case of measurements in a low geometry, give rise to a linear relationship between the relative :q peak intensities I~, = N,/£N~ and the solid angle fL H°) They vanish for fa--,0 and, hence, the =-particle-emission probabilities become equal to the extrapolated values. The P,~ values were obtained from a linear fit to the data of Fig. 2 for D..~0. The slopes of the lines were found to be 2.5 times larger than those obtained from calculations.(m) The solid angles were readily calculated using the '-3aU disintegration rates, which had been measured in a counter with a well-defined small solid angle. B. Results and discussion
0 101
7oo 2oo
l i ~oo CHANNEL
8oo
8oo
NUMBER
Fig. 1. Typical summed ~-particle spectrum ofZ3~U.
The results for P~ are presented in the Table 1. The quoted random uncertainties (68°/, confidence interval, n = 16) are obtained from linear regression analyses. The systematic uncertainties are mainly due to the :3°Th grown into the :3~U samples and to the peak-tailing corrections. The uncertainties due to
1083
Alpha-particle-emission probabilities o
0.730-
OY2S-
0220-
0215P~.O
0.710-
o oi~
o;s
0285-
/ ' - P •S3
0.280-
~~-~,-~-~ ~a ~ ~
i
llS
~
is ,Io-2
o SOURCE ~ 8 ram., 1830 Bq a SOURCE ~l 10 ram., 2100 Bq ~ SOURC.~_~E 3~17 Bmm, 8_Bq
e.1
0275-
0.270-
0.265
0'5
0 o'.I
0.0021- o ~
i
,'s
~
zs ~io'2
Pa 17t.
0.00 0. --
0.0019,
IG
c,
O.O0~B- ~E
{)
0.0017-
o
o
o o'1
o's
i
t's
~
2's ,10"2
SOLID ANGLE //. TE
Fig. 2. Determination of the :~-particle-emission probabilities. Open marks correspond to data points measured with an ion-implanted detector and solid triangles to those with a surface barrier detector. Error bars indicate _+ l SD from counting statistics. interferences by the other uranium isotopes and by background pulses were found to be negligible. The overall uncertainties in the P,~ values, corresponding to the 68% confidence level, were obtained by summing quadratically the individual uncertainties, These results, for which a substantial effort has been devoted to reach a high accuracy, are about 30 times more accurate than the values given by various evaluators/LTJ*) 4. 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
A. Measurement of the disintegration rates The disintegration rates, No, of the sources were measured by ~-particle counting in well-defined low
solid angles. °2) The solid angle varied between 0.002 and 0.75 sr. The typical uncertainties are summarized in Table 2. They all correspond to a 68% confidence level. Summing the individual uncertainties quadratically, we obtain the overall uncertainties in N 0 for 234Uand -'~3Am of + 0.09 and + 0.13°,o respectively.
B. Determination of the photon-emission rates The photon-emission rates, Nph, for the most important 7 and x rays were measured using an efficiency-calibrated high-purity-Ge detector. The area of the detector was about 1 cm 2 and its thickness 8 mm. The solid angle subtended by the detection system was about 0.1 sr, the source-to-detector distance being 28 mm. The nuclides used for the call-
Table 1. Alpha-particle-emissionprobabilities in the decay of ZJ4U Corresponding Typical uncertainty components .particle Emission Level in energy probability :3~Th Tail :3°Th (keV) (P~ Random influence correction 174 keV 4605 0.00206 _- 0.00004 0.00002 0.00001 0.00003 53 keV 4724 0.2842 = 0.0005 0.0003 0.0001 0.0003 0 4776 0.7138 = 0.0005 0.0003 0.0001 0.0003
R. VAN~,~ROt.'KX et al.
1084
Table 2. Uncertainties in the measurements of the disintegration rates :~aU :~Am
(?;)
(%)
Random Solid angle Background Tail extrapolation Impurities
0.03 0.06 0.02
0.05 0.05 0,05
0.04
0.02
0.05
0. l 0
Total (68% confidence level)
0,09
0.13
bration, the photon energies, the emission rates with their uncertainties and the mode of peak evaluation have been reported elsewhere. ~.t~ The uncertainties in the detection efficiencies given by the calibration curve were estimated to be I - 2 ~ for energies up to 120 keV, and 0.5-1~ o for higher energies, depending on the accuracies in the emission probabilities of the calibration photons which were used in the energy regions concerned.
Typical photon spectra for z3aU and 2~3Am-239Np are shown in Figs 3 and 4, respectively. To obtain the final photon-emission rates, several corrections had to be applied. A first was due to photon selfabsorption. The values of this correction for the various photons have been calculated from the source thicknesses and the photon attenuation coefficients obtained by interpolation between the values tabulated by Veigele. °~ For z34U this correction, especially for the L x rays, was also determined experimentally by extrapolation of the specific count rates to zero source thickness. The uncertainty introduced by this correction was smaller than 0. lVo for the "3~UL x rays and negligible in all other cases. A second correction was due to summing effects for coincident photons. These corrections were calculated from the decay schemes of ~ U , (7~ 23~Np(~6)and 2~3Am{~?~and the peak and total detection efficiencies. For the various photons the correction factor varied
M
..z
t~
#-.
.J
t.d
z
g g
E bJ Z L,J
-
e 12S PHOTON E N E R G Y
I keV]
Fig. 3. Photon spectrum of :34U,
o~ uJ
~c L.U Z LU
>= ..,¢
z
t~
0
'I SO
lO0
150
250
200
PHOTON
ENERGY
[keV]
Fig. 4. Photon spectrum of :~3Am-23~Np.
. _ A _ _ 300
350
/
I1.1 ThL I 13.0(E) ThE,
0.0604 0.6850 0.0035 0.0013
+ 0.0013 __.0.0150 + 0.0001 ± 0.0001
0.0077 ± 0.0002 0.0160 ± 0.003 0.0208 + 0.0003
285.41 315.81 334.30
43.53 74.67 86.79 142.18
0.0129 -I- 0.0002 0.1360 + 0.0030 0.2220 ± 0.0050 0.2750 + 0.0040 0.0007 + 0.0001 0.0346 + 0.0005 0.0028 -I- 0.0002 0. I 121 + 0.0018 0.0012 + 0.0001 0.0008 ± 0.0001 0.1438 ± 0.0021
0.1035 ::!:0.0014 (I.24 + 0.02)10 -1 (2.53 + 0.07)10 -5 (4.15 _ 0.10)10 -5 (3.41 + 0.04)10 -4
0.0123 + 0.0003
0.0022+0.0001 0.0360 + 0.0007 0.0530 + 0.0012
61.48 99.55 PuKe2 103.76 PuK~ 106.13 181.71 209.75 226.42 228.19 254.41 272.84 277.60 277.60
Total ThLx 53.20 89.95 Thk,2 93.35 ThK,i 120.88
16.2(I~) ThL~ 19.0(Ii) ThL r
This work
_+ 0.0130 + 0.0001 + 0.0024 + 0.0004 + 0.0064 ± 0.0002 + 0.0001 ± 0.0040
0.0504 0.6000 0.0031 0.0011
+ ± + ±
0.0060 0.0600 0.0004 0.0002
0.0078 + 0.0007 0.0159 + 0.0011 0.0203 ± 0.0018
0.2270 0.0011 0.0324 0.0034 0.1072 0.0010 0.0008 0.1410
0.0096 ± 0.0013
t
(4.10 .+ 0.40)10 '~
(1.19 + 0.10)10 -3
Nuclear Data Sheets
Re£ 17 (1981)
Ref. 16 (1977)
t (1977) Ref.7
Photon-emission probability, Pph
0.67110 + 0.0100 0.0033 ± 0.0001 0.0012 + 0.00t)1
jg
0060+o006t
-1 0.0163 ± 0.0007 0.0210 + 0.0010
]
0.0093 ± 0.0006
0.1500 + 0.(1050 0.1430 -I- 0.0024
0.0336 + 0.0014 0.0024 + 0.0003 0.1178 ± 0.0044
0.1970 + 0.0080 0.2660 + 0.0100
0.0487 ± 0.0010 0.0105 + 0.0004 0.0981 + 0.0013
0.0366 + 0.0008
0.0022+ 0.0001
Recent measurements (not included in NDS)
Ref. 20 (1983)
Ref. 18
Ref. 19 (1979)
Ref. 18 (1977)
Ref. 21 (1977)
+ 0.0030 ± 0.0001 + 0.000l _+0.0040
+ 0.006(I -I- 0.008(I ± 0.0090 + 0.00008 + 0.0010
0.6600 ± 0.0300
00,,0+0000
0.0152 ± 0.01X)5 0.0195 ± 0.0007
0.0076 i 0.0002
0.1140 0.0011 0.0008 0.1450
0.1450 0.2220 0.2780 0.00075 0.0342
Ahmad and Walflgren Ref. 5 (1972)*
*Note added in proof--More recently, 1. Ahmad (Natl. lnstrum. Methods 193, 9 (1982)] published new results; they agree, within tile quoted uncertainties, with the results of tile present work.
243AIn
239Np
2~U
Nuclide
Photon energy "(keV)
Table 3. Photon-emission probabilities in the decay of 2~U, ~39Np and 243Am
t~
a
g
o
W
>
1086
R. VANINBROUKXet al.
Level in :3UTh 174 keV 53 keV
Table 4. Internal-conversioncoefficientsin the decay of -'~U Transition and photon-emission Total internal conversion Corresponding probabilities coe~ciencs x photon energy (keV) P:, P~ x 10: Experimental Theoretical 120.88 0.00206 = 0.00004 0.341 : 0.004 5.04 = o. 11 5 53.20 0.2862 - 0.0005 1.24 -.-0.02 231 -_ 4 233
between 1.000 and 1.003 for -"~"U and >3Am and between 1.000 and 1.006 for '39Np. The uncertainty due to this correction was in all cases smaller than O.O5°0. A third correction was due to the difference between the diameter of the sources and that of the used calibration sources. The correction was calculated from the diameters of the detector and the sources and the source-to-detector distance. Typical correction factors are (1.012 + 0.001) and (l.018 + 0.001) for sources with diameters of 10 and 12mm, respectively. 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. The overall uncertainties on the Nph values were obtained by summing quadratically the individual uncertainties. C. Results and discussion
The photon-emission probabilites, Pph, are obtained by dividing the emission rates .~'v'ph by the disintegration rates No. The results are given in Table 3. The quoted uncertainties, corresponding to a 68% confidence level, take into account random uncertainties, estimated from the various individual results obtained from the different sources, and the uncertainties in the disintegration rates N O and the photon-emission rates Nph. For comparison, Table 3 gives also the values recommended by the Nuclear Data Sheets (NDS) :'t6~7 and the results of recent measurements tS-'-° not included in NDS. The results obtained by A h m a d and Wahlgren ~s~ for 239Np and 2~3Am, although included in the N D S , are also given. They are quoted with considerably lower uncertainties than the N D S values and have perhaps been somewhat underweighted in the evaluations. The present results for the Th-L x-ray-emission probabilities in the decay of 234U, especially those for L~, L.. and total L x, are significantly higher than the results of Bemis and Tubbs. ('-t~Our value for the total L x-ray emission probability agrees better with the theoretical value of (0.109_ 0.007), calculated using the formula Pox = Y V,c% where V~ are the numbers of vacancies in the L, subshells, including those created by Coster-Kronig transitions, and c,)~ are the related fluorescence yields. The values o f V, were calculated from the theoretical L-subshell internal conversion coefficients for the 53.20 and 120.88-keV transitions and the Coster-Kronig transition probabilities. The conversion coefficients were obtained by interpolation between the values tabulated by R6sel et al. ~22~ The Coster-Kronig transition probabilities
and the L subshell fluorescence yields were taken from Krause) TM For the other photon-emission probabilities of:3~U and for those of 239Np and "-~3Am the present m e a s u r e m e n t s certainly improve the accuracy status. In the frame of the I A E A Coordinated Research Programme on the Measurement and Evaluation of Transactinium Isotope Nuclear Data, r:~ evaluations of all experimental data will be performed and recommended values given. These evaluations are expected to be available at the end of 1984.
5. Consistency of the P~- and Pph Data for 234U; Internal Conversion Coefficients The total transition probability Ptr for deexcitation of a given level by 7 rays and conversion electrons is equal to the sum of all transitions populating that level. Since for the 121-keV and the weak higher levels no cross-over 7 rays are observed, the probabilities for depopulating the 121-keV and 53-keV levels are equal to the sum of all :~-particle transitions to the respective level and to all higher levels. The quantities P~r and Pph are related by the equation 7 = Ptr/Pph - 1, where = is the total internal conversion coefficient for the 7 transition considered. The results for the total conversion coefficients deduced from our measurements are given in Table 4 together with theoretical values obtained from R6sel et al. ~2"-~The excellent agreement between the experimental and theoretical values supports the reliability of our results and improves appreciably the confidence in the completeness and consistency of the z3~U decay scheme. Acknowledgements--The authors are grateful to W. Olden-
hof for the special efforts spent on the preparation of the 234U sources.
References l. Nichols A. L. In Transactiniurn Isotope Nuclear Data-1979, IAEA-TECDOC-232, p. 67 (IAEA, Vienna, 1980). 2. Lorenz A. Fifth Research Coordination Meeting on the Measurement and Evaluation o f Transactinium Isotope Nuclear Data, INDC(NDS)-I38/GE (1982). 3. Lorenz A. Second L4EA Advisory Group Meeting on Transactinium Isotope Nuclear Data, Cadarache 1979,
Summary Report (IAEA. Vienna, 1979). 4. Mozhaev V. K., Dulin V. A. and Kazanskii Yu. A. Sot. At. Energy, 47, 566 (1979). 5. Ahmad I. and Wahlgren M. Nucl. lnstrum. Methods 99, 333 (1972). 6. De Bi6vre P., Lauer K. F., Le Duigou Y., Motet H., M/ischenborn G., Spaepen J., Spernol A., Vaninbroukx R. and Verdingh V. In Chemical Nuclear Data (Ed.
Alpha-particle-emission probabilities Hurre{1 M. L.) p. 221 (The British Nuclear Energy Society, London. 1971). 7. Ellis Y. A. Nucl. Data Sheets 21, 493 (1977). 8. Kemmer J.. Burger P., Henck R. and Heyn e E. IEEE Trans. ,Vucl. Sci. Vol. NS-29, 733 (1982). 9. Bortels G. In Annual Progress Report on ,Vuclear Data 1983, NEANDC(E) 252, p. 81 (1984). I0. Bortels G., Denecke B. and Vaninbroukx R. Nucl. lnstrum. Methods 223, 329 (1984). 11. Lederer C. M. and Shirley V. A. Table of Isotopes, 7th edn, (John Wiley, New York, 1978). 12. Spernol A. and Denecke B. In Chemical Nuclear Data (Ed. Hurell M. L.) p. 199 (The British Nuclear Energy Society, London, 1971). 13. Vaninbroukx R. and Denecke B. Nucl. lnstrum. Methods 193, 191 (1982). 14. Vaninbroukx R. and Hansen H. H. Int. J. Appl. Radiat. lsot. 34, 1395 (1983). 15. Veigele W. M. J. At. Data Tables 5, 5i (1973).
1087
16. Schmorak M. R. Nucl. Data Sheets 21, 91 (1977). 17. Ellis-Akovali Y. A. Nucl. Data Sheets 33, 79 (1981). 18. Starozhukov D. [., Popov Yu. S. and Privalova P. A. Soy. At. Energy 42, 355 (1977). 19. Mozhaev V. K., Dulin V. A. and Kazanskii Yu.A. Soc. At. Energy 47, 566 (t979). 20. Holloway S. P., Olomo J. B. and MacMahon T. D. In Nuclear Data for Science and Technology (Ed. B6ckhoff K. H.) p. 287 (D. Reidel, Dordrecht, 1983). 21. Bemis C. E. Jr and Tubbs L. Absolute L-Series X-Ray and Low-Energy Gamma-Ray Yields for Most Transuranium Nuclides, U.S. ERDA Report ORNL-5297, p. 93 (1977). 22. R6sel F.. Fries H. M., Alder K. and PauIi H. C. At. Data NucL Data Tables 21, Nos 4,5 (1978). 23. Krause M. 0. J. Phys. Chem. Ref Data 8, 307 (1979). 24. Lorenz A. Si.,:th Research Coordination .Meeting on the Measurement and Et'aluation of Transactinh~m Isotope Nuclear Data, INDC (NDS)-I47/GE (1983).