Validating Fission Models for Novel Safeguards Systems through Measurements of Correlated Prompt Neutron and Gamma-ray Signatures

Nuclear fission is a complex de-excitation whereby an unstable heavy nucleus splits into two, or more, smaller fragments, releasing radiation mostly in the form of neutrons and gamma rays. Accurate models for simulating fission neutron and gamma-ray emission are vital for designing new safeguards and nonproliferation systems. Present models are often limited in detail or incorrect for important isotopes of plutonium and uranium. New fission models based on theory, including CGMF and FREYA, are in development and improve our ability to accurately predict the fission emissions of key isotopes, but they have not been fully validated with correlated data. We have performed experiments at Los Alamos National Laboratory using the Chi-Nu array of 54 organic scintillator detectors of prompt neutrons and gamma rays from spontaneous (252Cf) and neutron-induced (235U) fission. These measurements are compared to simulations in which the fission event generator is varied between MCNPX-PoliMi, CGMF, and FREYA. Our analysis gives particular focus to correlated particle events.