Shape of spectra and mean energies of prompt fission neutrons from 237Np fission induced by primary neutrons of energy in the range En < 20 MeV

An analysis of the spectrum of prompt neutrons originating from 237Np fission induced by primary neutrons of energy En = 14.7 MeV confirms the results obtained previously for 232Th (En = 14.7, 17.7 MeV), 235U (14.7 MeV), and 238U (13.2, 14.7, 16, and 17.7 MeV) nuclei. In the experimental spectrum measured in the emissive fission of 237Np, there is also an excess of soft neutrons in the energy range E < 2 MeV in relation to what follows from a traditional theoretical description that considers two sources of prompt fission neutrons: fully accelerated fission fragments and excited nuclei prior to their separation. In just the same way as in the cases of 232Th and 235,238U, the shape of the prompt-fission-neutron spectrum for 237Np is reproduced theoretically over the entire measured range of secondary-neutron energies upon including nonaccelerated fragments as a third source of neutrons in the computational scheme. A description of the spectra and mean energies of prompt fission neutrons versus the bombarding-neutron energy is obtained on the basis of experimental data and their analysis. The results on mean energies are compared with data on the proton-induced fission of 236,238U nuclei.