Prompt fission neutron spectra of n+~(235)U above the(n,nf) fission threshold

Calculations of prompt fission neutron spectra(PFNS) from the235U(n, f) reaction were performed with a semi-empirical method for E n = 7.0 and 14.7 Me V neutron energies. The total PFNS were obtained as a superposition of(n,xnf) pre-fission neutron spectra and post-fission spectra of neutrons which were evaporated from fission fragments, and these two kinds of spectra were taken as an expression of the evaporation spectrum.The contributions of(n,xnf) fission neutron spectra on the calculated PFNS were discussed. The results show that emission of one or two neutrons in the(n,nf) or(n,2nf) reactions influences the PFNS shape, and the neutron spectra of the(n,xnf) fission-channel are soft compared with the neutron spectra of the(n,f) fission channel. In addition,analysis of the multiple-chance fission component showed that second-chance fission dominates the PFNS with an incident neutron energy of 14.7 Me V whereas first-chance fission dominates the 7 Me V case.