Prompt fission neutron spectra of 235U above emissive fission threshold

Abstract

Statistical model calculations of prompt fission neutron spectra (PFNS) from 235U( n , F ) reaction were performed for incident neutron energies up to E n ∼ 20 MeV . Exclusive spectra of the pre-fission (pre-saddle) ( n , x n f ) reaction neutrons, were calculated with Hauser–Feshbach statistical model, fission and ( n , x n ) reaction cross section data being described consistently. Spectra of neutrons, evaporated from the fission fragments, were approximated as a sum of two Watt distributions. The reduced neutron velocity in the center-of-mass system due to the neutron emission during fragment acceleration was assumed. PFNS component due to pre-saddle neutrons is evidenced in the shape of the measured PFNS data. We show it to be strongly correlated with the emissive fission contributions to the observed fission cross sections.