Mesure de {\bar \nu _p} et Ēγ Pour la Fission de 232Th, 235U et 237Np Induite par des Neutrons D’Energie Comprise Entre 1 et 15 MeV

The average number of prompts neutrons, \({\bar \nu _p}\), and the average prompt gamma-ray energy, Ēγ, have been measured for the neutron induced fission of 232Th, 235U, and 237Np in the energy range from 1 up to 15 MeV. Data were obtained relative to the corresponding quantities \({\bar \nu _{Cf}}\) and ĒγCf for the 252Cf spontaneous fission. For 235U and 237Np, \({\bar \nu _p}\) is approximately a linear function of energy with small changes in slope around 4 MeV and around 6 MeV and 12 MeV (second and third chance fission). For 232Th the second chance fission results in a strong increase of \({\bar \nu _p}\), due to the increase of the fragment total kinetic energy with excitation energy for that nucleus. A linear correlation observed between Ēγ and \({\bar \nu _p}\) is interpreted in terms of a competition between neutron and gamma-ray emission, connected with an increase of the average angular momentum of the fission fragments with excitation energy. The variance of the fission neutron multiplicity distributions is observed to be proportional to the excitation energy of the fissioning nucleus.