E. L., Ibid., MDDC-393 (October 1946). (5) Koshland, D. E., Ibid., CN-2041 (January 1945). (6) Kraus, K. A., Proc. Intern. Conf. Peaceful Uses Atomic Energy, Geneva, 1955, 7, 245-57 (Pub. 1956). ( 7 ) Kraus, K. A., U.S. Atomic Energy Comm. Declassified Rept. CN3399 (June 1945).
(4) King,
(8) Kraus, K. A., Howland, J. J., U. S.
Atomic Energy Comm. Secret Rept. CN-1764 (July 1944). (9) Moore, F. L., ANAL. CHEM.28, 997
(1956). (10) Ibid., 29, 941 (1957). (11) Thomas, J. R., Crandall, H. W., U.S.
Atomic Energy Comm. Secret Rept. CN-3733 (December 1946). (12) Werner, L. B., Perlman, I., U. S.
Atomic Energy Comm. Declassified Rept. BC-1 (April 1946). (13) Wolter, F. J., Ibid., ISC-14 (May 1946). (14) Rolter, F. J., Brown, H. D.,
U.S. Atomic Energy Comm. Confidential Rept. CN-2719 (June 1945).
RECEIVEDfor review May 31, 1957. Accepted August 20, 1957.
Determination of Uranium-235 by Gamma Scintill ation Spectrometry GEORGE H. MORRISON and JAMES F. COSGROVE Chemisfry laborafory, Sylvania Elecfric Producfs Inc., Flushing, N. Y.
b A method for determination of uranium-235 i s based on the use of gamma scintillation spectrometry to measure the gamma photons emitted during natural radioactive decay. The photopeak energy of the gammaemitting radionuclide provides qualitative identification, and the uranium-235 i s estimated quantitatively b y measuring the area under the photopeak and comparing it with the area under the photopeak obtained from a standard sample of known concentration. The method is rapid and nondestructive; accuracy and precision are within 1% over the concentration range of 0.72 to 100% uranium-235. The limit of sensitivity of the method is approximately 0.05% uranium-235.
A
exist for determination of the isotopic abundance of uranium-235 in uranium-containing materials. Methods for determination of uranium-235 concentration relative to that of uranium-234 and uranium-238 have been based on differences in mass (13-16) and on isotopic line shifts in emission spectroscopy (1, 4, 6, 8, 17, 18). The natural radioactivity of uranium samples has been employed, based on alpha particle disintegration rates (2, 3, 7 , IO), measurement of the growth of beta-emitting daughters from the chemically separated uranium (5),and gamma ray emission of uranium435 based on a discriminating counting technique (19). Several methods involve neutron activation followed by radioactive measurement. Thus, neutron-induced fissions have been detected by using a fission counter (11, 12) and barium fission has been isolated and counted after activation (16). Another method involves the proportionality between the VARIETY OF METHODS
1770
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ANALYTICAL CHEMISTRY
uranium-235 concentration and the specific gamma ray disintegration rates caused by irradiating the sample with thermal neutrons of constant intensities (9). Many of these methods require knowledge of the age of the sample or a sample that can be consumed in the analysis. The method described is rapid and nondestructive and has accuracy and precision within 1% over the concentration range of 0.72 to 100% uranium235, although it can be extended to approximately 0.057, uranium-235. It is based on the use of gamma scintillation spectrometry to measure the photopeaks resulting from the 0.143- and 0.184-m.e.v. gamma photons emitted by uranium-235 during its natural radioactive decay. The area under the photopeaks of an unknonn sample is compared to that of the standard sample. EXPERIMENTAL
Samples. STANDARDS. S a t u r a l uranium oxide (U308), which contains 0.7201, uranium-235, was used as a standard sample for comparison with unknown samples of low uranium-235 content. A standard sample of uranium oxide containing 80.248% uranium-235 was compared trith more highly enriched samples. It was prepared b y diluting a n enriched sample containing 93.17770 uranium-235 with natural uranium oxide. L-SKNOWNS. Samples of enriched uranium oxide powder containing 2.9687, 21.506, 65.516, and 93.177% uranium-235 were obtained from the U.S. Atomic Energy Commission for use as unknowns. These samples are supplied as standard samples and the uranium-235 content was determined by mass spectrometry. Measurement of Activity. Activity was measured with a thallium-activated sodium iodide scintillation crys-
tal (Harshan- Chemical Co.) 1.5 inches in diameter and 1.5 inches deep, viewed by a Du Mont 6292 photomultiplier tube. The powdered samples in plastic planchet5 1 inch in diameter were placed directly on the crystal. The output of the photomultiplier tube was analyzed with an Atomic Instruments Model 520 20-channel pulse height analyzer employing a fixed slit width of 0.5 volt. The area under the photoelectric peaks resulting from the 0.143and 0.184-m.e.v. gamma photons was measured with a planimeter. The resulting area was compared with that of a n equal weight of standard sample t o determine the concentration of uranium235. RESULTS AND DISCUSSION
An examination of the gamma spectra of uranium samples containing different amounts of uranium-235 (Figure 1) reveals the presence of a photoelectric peak resulting from the 0.093m.e.r. thorium-234 gamma photon, and those from the 0.094-, 0.143-, 0.184-, 0.289-, and 0.386-m.e.v. uranium-235 gamma photons. The 0.289and 0.386-m.e.v. photopeaks could not be used for the determination of uranium-23.5 because of their small size. They barely show up in the spectrum of natural uranium and hence their use would limit the concentration range over nhich the method is applicable. The 0.094-m.e.v. photopeak could not be used because of the inability to resolve this peak from the 0.093-n1.e.v. photopeak of thorium-234. Therefore the 0.184-m.e.v. uranium-235 photopeak, M hich results in the mx.;imum area, was the most suitable for determination of uranium-235. However, as a result of the inability of the spectrometer to resolve the 0.184-m.e.v. photopeak and the smaller 0.143-m.e.v. photopeak completely, it was necessary
to measure the combined areas for comparison with the corresponding areas resulting from a standard sample. Precision and Accuracy. T e n accurately m eighed portions of each sample were counted with t h e 20channel analyzer (Table I ) . For each point used in obtaining t h e photopeaks, a t least 10,000 counts were taken and t h e area under t h e photopeaks was compared with that obtained from a n approvimately equal weight of standard. I n view of the large difference in activity, it was not possible to obtain an accurate value for samples containing more than approximately 10% uranium-235 whrn an equal weight of natural uranium was used as the standard. When a larger weight of natural uranium standard n as used to compensate for the large difference in activity, self-absorption effects resulted. Correction for absorption effects decreased accuracy. Consequently, a n equal weight of a standard of comparable uranium-235 content was used for comparison with the unknown sample. Thus, a standard containing 80.248% uranium-235 served adequately for samples over the range of 20 to 1007c uranium-235. CONCLUSIONS
The scintillation spectrometric method is rapid and nondestructive, with accuracy and precision within 1% over the concentration range of 0.72 t o 100% uranium-235. The limit of sensitivity of the method is approximately 0.05% uranium-235 and is determined b y the minimum area that can be accurately measured. rllthough it is
Table 1.
Precision and Accuracy of Gamma Scintillation Spectrometric Method 1
desirable to compare an unknown sample with a n equal weight of a standard of comparable uranium-235 content, samples of different weight can be compared if self-absorption corrections are made, but with a decrease in accuracy. The method can be applied to the determination of the uranium-235 content of uranium samples in a variety of physical and chemical forms. Similarly, determination of the uranium-235 content of matrix materials other than pure uranium is possible, if absorption effects are taken into consideration, and no gamma-emitting radioactive impurities are present in the sample, that contribute to the areas being measured. Although no study was made here of the determination of uranium-235 in the presence of fission product gamma emitters, it should be possible to apply
Figure 1. Gamma spectra uranium samples of varying uranium-235 concentration
of
b.
0.9709 g. of natural U30s containing 0.727, U235 0.0257 g. of U308 containing 93.1777, U235
1. 2. 3. 4. 5.
0.093 m.e.v. y-UZs-Th234 0.143 m.e.v. y-UZ36 0.184 m.e.v. y-UZ36 0.289 m.e.v. -,-U235 0.386 m.e.v. -f-U235
a. lo3
$ V‘
\
102
IO
0.3
4
”,
3 002 21 46 65 24 92 43 2 986 21 71 65 79 94 17 2 989 21 51 64 30 92.75 65.00 94,28 2.937 21.78 65.65 93.35 2.964 21.59 92.10 21.41 65.86 3,001 21.46 65.97 93.42 2.952 21.19 65.51 94.34 2.983 21.70 65.37 93,69 2.997 21.82 64.61 93.29 2.933 Av. 2.9744, 21.563, 65.330, 93.382, iz0. 734 iz0. 195 & O . 551 iz0.0262 0.84 0.79 Coefficient of variation, 7, 0.88 0.90 Uranium-235 by mass spectrometry, % ’ 2.9687, 21.506, 65.516, 93.177, &0.003 iz0.022 zk0.066 izO.014 Error, 70 0.17 0.27 0.28 0.22 Results obtained by comparison of sample with equal weight of standard containing 80.248% U236. Standard used for comparison wit,h sample 1 was an equal weight of natural U308(0.727, U235).
Io4
0.2 ENERGY (M.EY.)
Samples” 2 3 Uranium-235, Weight
0.4
the method to the analysis of spent or partially spent fuel elements after chemical separation of the uranium by uranyl nitrate extraction. LITERATURE CITED
Brody, J. K., J . O p t . SOC.Amer. 42, 408 (1952).
Clark, F. L., Spencer-Palmer, H. J., Woodward, R. N., Brit. Rept. BR 431, H. hI. Stationery Office, May 1944. Cohen, B., Hull, D.. E., U. S.Atomic Energy Commisslon Rept. A-1235, Part 2 (Aug. 28, 1944). Daniel. J. L.. Ibid.. HW-31911 (May .~ 21, i954). ’
Derham, J., Fenning, F. IT., Brit. Atomic Energy Rept. AERE R/R834 (1951). Ibid., AERE R/R 1149 (hlarch 24, 1953). Dunning, J. R., Booth,. E. R., Grosse, A. V., U.S.Atomic Energy Commission Rept. A-62 (July 1940). Gordon, N. E,, Brightsen, R. A., Cook, H. D., Wilson, C. R., Ibid., WAPD-81 ( J u P-, 1~ -14.57’1 ---I. Hudgens, J. E., 7\leyer, R. C., Ibid., NBL-1 26 (April 1956). Hull, D. E., Ibid., A-1235, Part 1 (Jan. 18, 1944). Kennedv, J. W., Segre, E., Zbid., A158 (April 20, 1942). Kennedy, J. IT., Segre, E., UCRLSpec. 9 (March 29, 1943). 1gier, A. O., Phys. Rev. 55, 150 (1939). Nier, A. O., Inghram, M. G., Ney, E. P., U. S.Atomic Enernv Commission Rept. A313 70ctober 1942). (15) &$I, L. L., in Rodden, C. J. (ed.), Analytical Chemistry of the Manhattan Project,’’ p. 727, McGraw-Hill, New York, 1950. (16) Sevfang, A. P., Smales, A. A., Anniyst 78, 394 (1953). (17) Smith, D. D., Burkhart, L. E., E.S. Atomic Energy Commission Rept. Y-226 (hlarch 4, 1948). (18) Stukenbroeker, G. L., Smith, D. D., Burkhart, L. E., hIcNalley, J. R., Jr., Ibid., Y-436 (May 12, 1949). (19) Kright, W. B., Jr., McIlhenny, R. C., Wachter, J. W.,I b d , Y1087 (Sept. 1, 1954). RECEIVED for review May 4, 1957. Accepted August 21, 1957. A-
VOL. 2 9 , NO. 1 2 , DECEMBER 1957
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