Plutonium Isotopic Measurements by Gamma-Ray Spectroscopy

The nondestructive assay of plutonium is important as a safeguard tool in accounting for stategic nuclear material. Several nondestructive assay techniques, e.g., calorimetry and spontaneous fission assay detectors, require a knowledge of plutonium and americium isotopic ratios to convert their raw data to total grams of plutonium. This paper describes a nondestructive technique for calculating plutonium-238, plutonium-240, plutonium-241 and americium-241 relative to plutonium-239 from measured peak areas in the high resolution gamma-ray spectra of solid plutonium samples. Gamma-ray attenuation effects have been minimized by selecting sets of neighboring peaks in the spectrum whose components are due to the different isotopes. Since the detector efficiencies are approximately the same for adjacent peaks, the accuracy of the isotopic ratios are dependent on the half-lives, branching intensities and measured peak areas. The data presented describes the results obtained by analyzing gamma-ray spectra in the energy region from 120 to 700 keV. The majority of the data analyzed was obtained from plutonium material containing 6% plutonium-240. Sample weights varied from 0.25 g to approximately 1.2 kg. The methods have also been applied to plutonium samples containing up to 23% plutonium-240 with weights of 0.25 to 200g. Results obtained by gamma-ray spectroscopy are compared to chemical analyses of aliquots taken from the bulk samples