Quantification of Trace-Level Fissile Samples via Short-Lived Delayed Gamma Spectroscopy

We describe a generalized means of isotopic assay and quantification of multicomponent fissile mixtures based on the observation of short-lived fission-product gamma signatures. Given that the characteristic mass yield curves differ for individual fissile species, this can be used to construct an overdetermined system of equations corresponding to the observed photopeak yields, the solution of which yields information on the individual isotopic masses of parent fissile isotopes within the system. This method is then demonstrated for single-isotope samples of ²³³U, ²³⁵U, and ²³⁹Pu; binary fissile mixtures of U and Pu; and trace fissile samples contained within intact typical environmental sampling media, each irradiated and measured using the High Flux Isotope Reactor at the Oak Ridge National Laboratory. Our findings indicate that this method can reliably be used to quantify nanogram-level fissile masses in binary mixtures with uncertainties on the order of 2%-7%. Finally, observed discrepancies with individual fission-product indicators and likely sources of bias are discussed.