Prompt fission neutron spectra of U above emissive fission threshold

Statistical model calculations of prompt fission neutron spectra (PFNS) from ²³⁵U(n,F) reaction were performed for incident neutron energies up to E_n20 MeV. Exclusive spectra of the pre-fission (pre-saddle) (n,xnf) reaction neutrons, were calculated with Hauser–Feshbach statistical model, fission and (n,xn) 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.