Spectra of Prompt Fission Neutrons in the Reactions 235U(n, F) and 239Pu(n, F)

The influence of exclusive prefission neutron spectra, (n, xnf)1,…x
, on the observed spectra of prompt fission neutrons (PFN), the total kinetic energy (TKE) of fission fragments (products), and the average number of prompt fission neutrons is studied. The (n, xnf)1,…x  exclusive neutron spectra correspond to a self-consistent description of the fission reactions (neutron emission) 
235U(n, F)(235U(n, xn))) and 239Pu(n, F)(239Pu(n, xn))) for neutrons of energy extending up to 20 MeV. An extensive experimental database of PFN spectra made it possible to study in detail (confirm) an intricate dependence of the shape of observed PFN spectra on the fissility of 236U  and 240Pu compound nuclei. This effect is found to be correlated with the contributions of (n, xnf)) emissive fission reactions and with the competition of (n,nγ) and  (n, xn)  reactions. The exclusive prefission neutron spectra of (n, xnf)1,…x  reactions and the exclusive neutron spectra of (n, nγ) and (n, xn)1,…x reactions were calculated by means of the Hauser–Feshbach formalism simultaneously with the (n, F)- and  (n, хn)- cross sections. It is shown that the angular anisotropy of exclusive neutron spectra of (n, xnf) reactions has a substantial effect on the observed spectra of prompt fission neutrons and their average energies . The ratio of the average PFN energies
 for the forward and backward emission of prefission neutrons in the reactions235U(n, xnf) and 239Pu(n, xnf)  agrees with experimental data. Partial components associated with(n, f) and (n, xnf). reactions are singled out in the observed PFN spectra. The input values of model parameters are fixed in describing PFN spectra of thermal-neutron-induced fission reactions. The potential of the approach being considered for predictions of PFN spectra and their average energies in the reactions 238U(n, F) and 240Pu(n, F)  is demonstrated.