Angular and Kinetic Properties of Scission Neutrons within Time-dependent Density Functional Theory

Abstract

Scission-neutron emission is investigated in $^{235}\mathrm{U}(\mathrm{n}_{\mathrm{th}},\mathrm{f})$, $^{239}\mathrm{Pu}(\mathrm{n}_{\mathrm{th}},\mathrm{f})$ and $^{252}\mathrm{Cf}(\mathrm{sf})$ within time-dependent density functional theory. Using a substantially larger simulation domain than in previous studies, the angular and energy distributions of emitted scission neutrons are extracted over a specific range of emission angles. At these angles, scission neutrons are absent below a threshold energy of roughly $1.5$--$2\,\mathrm{MeV}$, and instead contribute predominantly to the higher energy part of the prompt fission neutron spectrum. Combining the calculated scission-neutron spectrum with a Maxwellian model for the evaporated component, constrained by low-energy experimental data, reproduces the measured high-energy prompt-fission-neutron yield in both $^{239}\mathrm{Pu}(\mathrm{n}_{\mathrm{th}},\mathrm{f})$ and $^{252}\mathrm{Cf}(\mathrm{sf})$, whereas the evaporation-only model systematically underestimates it. This identifies a signature of scission neutrons already present in existing high-energy prompt fission neutron spectra and constitutes direct evidence for a non-negligible scission-neutron component in prompt fission neutron emission.