Delayed Phase Neutron Energy (DAPHNE) analysis for discrimination of fissile and fertile uranium

Active neutron interrogation (ANI) offers a potential approach for verifying nuclear disarmament by detecting special nuclear material in treaty-accountable items. This work introduces the Delayed Phase Neutron Energy (DAPHNE) concept, which exploits the distinct energy spectra of prompt and delayed neutrons to discriminate between highly enriched uranium (HEU) and depleted uranium (DU) in subcritical assemblies. Using a generalized point kinetics model and Monte Carlo N-Particle (MCNP) simulations including realistic detector response, we demonstrate that during the delayed phase following pulsed neutron interrogation, high-multiplication objects containing HEU exhibit a pronounced high-energy neutron component (above 0.5 MeV) that is largely absent in low-multiplication DU objects. This signature arises because delayed neutrons induce secondary fissions in fissile material, producing additional prompt neutrons that are magnified by the high subcritical multiplication factor. Simulations of 25 kg uranium spheres with 14 MeV, 2.5 MeV, and 0.1 MeV neutron sources confirm that the high-energy component remains detectable even through substantial DU shielding, owing to the low absorption cross-sections of fast neutrons. Detector response simulations using a liquid scintillator show that the WGU and DU signatures differ by two to three orders of magnitude in count rate, confirming practical detectability with commercially available equipment. The DAPHNE approach addresses a critical verification challenge by enabling fissile material detection without requiring detailed knowledge of the inspected item’s geometry, which is typically classified information in disarmament contexts.