Novel methodology for the quantitative assay of fissile materials using temporal and spectral β-delayed γ-ray signatures

An analytical model for the generation of β-delayed γ-ray spectra following thermal-neutron-induced fission of mixed samples of 235U235U and 239Pu239Pu is presented. Using an energy-dependent figure-of-merit to designate the spectral regions employed in the assay, the unique temporal β-delayed γ-ray signatures are utilized to determine the fraction of 239Pu239Pu in a mixed U–Pu sample. By evaluating the β-delayed γ-ray temporal signatures of both 235U235U and 239Pu239Pu within a 3 keV energy bin, traditional sources of systematic uncertainty in quantitative assay using β-delayed γ-ray signals, such as self-attenuation of the sample and energy-dependent γ-ray detection efficiency, are significantly reduced. The effects of the time-dependent Compton-continuum and growth of longer-lived nuclides on the quantitative assessment are explored. This methodology represents a promising extension of the conventional means of analysis for quantitative assay of fissile materials using β-delayed γ-ray signatures.