Prompt fission neutron spectra of ²³⁸U and ²³²Th above emissive fission threshold

Statistical model calculations were performed to interpret the prompt fission neutron spectra (PFNS) from ²³⁸U(n,f) and ²³²Th(n,f) reactions for the incident neutron energies E_n ∼ 6-18 MeV. Spectra of the prefission (presaddle) (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 presaddle 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. The dependence of these contributions on the target nuclide fissility and incident neutron energy is shown to be pronounced in the PFNS shapes for ²³²Th(n,f) and ²³⁸U(n,f) reactions. High-energy tails of the first neutrons of (n,nf) and (n,2nf) reactions are shown to be evidenced in the PFNS measured data trends.