Prompt Fission Neutron Spectra of 235U(n, F), 239Pu(n, F), and 240Pu(n, F)

Prompt fission neutron spectra (PFNS) of 235U(n, F), 239Pu(n, F) and 240Pu(n, F) for the incident neutron energy range Еn ≲ 20 MeV are suitable for investigating how fissility of N-odd/N-even, Z-even target nuclides A and residual nuclides A + 1 − x affect the emission x of pre-fission and post-fission neutrons. Pre-fission neutrons may amount to ~15% of the average number of prompt fission neutrons, but they produce a stronger effect on the share of the fission reaction energy between the excitation and kinetic energies of the fission fragments. The PFNS shape appreciably depends on the contribution of pre-fission neutrons to the PFNS, (n, xnf)1,…,x and (n, xn)1,…,x reaction competition, and fissilities of the N-odd and N-even compound and residual nuclides. Exclusive neutron spectra of 235U(n, xn)1,..., x reactions are calculated simultaneously with the cross sections for the 235U(n, F), 234U(n, F), 233U(n, F), and 232U(n, F) reactions and the 235U(n, 2n) reaction, 235U(n, nX) neutron emission spectra at Еn ~14 MeV, and inelastic neutron scattering cross sections. Exclusive pre-fission neutron spectra of the 239Pu(n, xnf)1,…,x and 240Pu(n, xnf)1,…,x reactions are calculated simultaneously with the cross sections for the 240Pu(n, F), 239Pu(n, F), 238Pu(n, F), 237Pu(n, F), and 236Pu(n, F) reactions and the 239Pu(n, nX) neutron emission spectra at Еn ~14 MeV. Initial model parameters are defined by fitting the 235U(nth, f) and 239Pu(nth, f) PFNS data. Initial model parameters for the 240Pu(n, F) PFNS are fixed by description of spontaneous fission neutron spectra (PSFNS) of 240Pu(sf). Considering the 240Pu (n, xnf)1,..., x exclusive pre-fission neutron spectra, the prompt fission neutron spectra, total kinetic energy of fission fragments TKE (total kinetic energy), and average number of prompt fission neutrons (PFNs) are predicted and interpreted for 240Pu(n, F). The measured 239Pu(n, F)/235U(n, F), 239Pu(n, F)/238U(n, F), and 235U(n, F)/238U(n, F) PFNS ratios for Еth < Еn < 20 MeV are reproduced using the calculated 235U(n, F), 238U(n, F) and 239Pu(n, F) PFNS. Predicted PFNS of 240Pu(n, F) at Еn ≲ 20 MeV are supported by measurements at the LANSCE neutron source.