Identification and Quantification of Plutonium and Uranium from Fission Product Gamma-Ray Spectra.

A technique has been developed to distinguish between ^{239}Pu and ^{235}U by observing fission product delayed gamma-rays produced by fissions induced by an external neutron source. If the number of induced fissions per source neutron per unit mass can be determined from Monte Carlo simulation, the material can also be quantified. Trials were performed with yellowcake, HEU-metal, and Pu-metal samples using a TRIGA reactor and a large ^{252}Cf source as neutron sources. Fission product gamma-ray spectra were collected using a high-resolution hpGe detector over time intervals ranging from 60 s to 3000 s following the end of irradiation. By virtue of being greatly overdetermined, the identity of the Special Nuclear Material (SNM) can be unambiguously determined with a high degree of confidence in all cases by applying a set of Figure of Merit functions. Identification can be made without regard to the properties of the matrix provided a sufficient number of fissions can be induced within the sample to permit observation of the fission product gamma-rays. Once identified, the SNM can be quantified with an accuracy determined mainly by the ability to accurately model the fission response of the system using Monte Carlo simulation, within 3.8 percent in this study.