A Generalized Method for Fissile Material Characterization Using Short-Lived Fission Product Gamma Spectroscopy

Characterizing the fissile content of nuclear materials is of particular interest to the safeguards and nuclear forensics communities. Short-lived fission product gamma spectroscopy offers a significant reduction in analysis time and detection limits when compared to traditional non-destructive assay measurements. Through this work, a fully generalizable method that can be applied to variations in fissile compositions and neutron spectra was developed for the modeling and measurement of short-lived fission product gamma-rays. This method uses a 238-group neutron flux that was characterized for two pneumatic tube positions in the High Flux Isotope Reactor using flux monitor irradiations. This flux spectrum was then used in determining theoretical fission product photopeak emission rates per unit fissile mass during measurement. From these theoretical values, a mathematical method for characterizing the fissile material within a sample was established and demonstrated in several cases. The Oak Ridge National Laboratory's High Flux Isotope Reactor and Neutron Activation Analysis Laboratory provided optimal conditions to conduct high-flux irradiations with short decay periods. Experiments with samples containing nanogram quantities of Uranium 235, Uranium 233, and Plutonium 239 were accurately characterized through measurement of short-lived fission product photopeaks. These measured photopeaks were combined with the simulated theoretical production rates to construct an overdetermined system of linear equations. Once this overdetermined system was solved, it showed high accuracy in quantifying fissile content. These experiments resulted in errors less of than 10\% for quantification of fissile material in single element samples, two element mixed samples, and varying enrichments of uranium on IAEA swipes.