THE MEASUREMENT OF PU-240/PU-239 AND PU-238/PU-239 ISOTOPIC-RATIOS BY ALPHA-PARTICLE SPECTROMETRY

The measurement of the alpha-activity ratio of Pu-238/Pu-(239+Pu-240) is a routine practice in the determination of the isotopic composition of plutonium. However, measurement of the atomic ratio of Pu-240/Pu-239 by alpha-particle spectrometry is hampered due to insufficient energy resolution for the set of closely spaced peaks of these two isotopes. Passivated and implanted, planar silicon (PIPS) detectors have recently become available with an energy resolution of 10 keV or better, which significantly improves the deconvolution of spectra from plutonium samples. A set of alpha sources was prepared on porcelain disks by ignition, and the spectra were accumulated at a gain of approximately 1 keV per channel. The GRPANL computer program as developed by Lawrence Livermore National Laboratory was used to analyze the spectra. The isotopic ratios were measured in parallel by mass spectrometry. It was found that the agreement on the ratios of Pu-240/Pu-239 and Pu-238/Pu-239 between mass spectrometry and measurements by PIPS detectors was within +/-2%. Half-life values were obtained from the literature (M. Lammer and O. Schwerer, Handbook of Nuclear Data for Safeguards, Rep. INDC(NDS)-248, IAEA, Vienna, 1991; ref. [5]) Other factors were also studied to improve the accuracy of the data. The alpha-particle emission probabilities of highly enriched Pu-239 and Pu-240 have been measured. The alpha-particle energies obtained in the fitting were in agreement with those in ref. [5]. The fitted energy values were used throughout this work.