Prompt fission neutron spectra of ²³⁸ U(n,f) above emissive fission threshold

Abstract.

Model calculations were performed to interpret prompt fission neutron spectra (PFNS) of the ²³⁸ U(n, f) reaction for incident neutron energies $E_{\rm n} = 6$-18 MeV. Pre-fission (pre-saddle) ${}^{238}U(n, {\it x} nf)$ reaction neutron spectra were calculated with Hauser-Feshbach statistical model, ²³⁸ U fission and $(n, {\it x}n)$ reaction cross-section data being described consistently. The increase of the cut-off energy of (n, nf) reaction neutron spectra with excitation energy of fissioning nucleus is described. For $E_{\rm n} = 6$-9 MeV the low-energy PFNS component, which is due to the contribution of pre-fission (n, nf) neutrons, is compatible with measured data. Average energy of prefission (n, nf) neutrons is shown to be rather dependent on $E_{\rm n}$. For $E_{\rm n} = 13$-18 MeV, a decrease of measured PFNS average neutron energies is interpreted. Spectra of neutrons, evaporated from fission fragments, were approximated as a sum of two Watt distributions. The reduced fission fragment velocity is assumed for the neutron emission during fragment acceleration. Several interpretations of observed soft neutron excess are investigated, i.e., possible uncertainties of emissive fission contributions and additional neutron source. We claim the soft neutron excess cannot be attributed to the ${}^{238}U(n, {\it x} nf)$ pre-saddle neutrons contribution.