Measurement of Fission Neutron Multiplicities for Thorium-232 and Uranium-235 with Incident Neutron Energies to 49 MeV

Fission neutron multiplicities have been measured for neutrons incident on ²³²Th with energies ranging from 1.1 to 49 MeV and for neutrons incident on ²³⁵U with energies from 17 to 49 MeV. The Lawrence Livermore National Laboratory 100-MeV electron Linac was used to produce a white source of neutrons. Incident neutron energies were measured using time-of-flight techniques. Fission neutrons were detected in a liquid scintillator using pulse-shape discrimination. All ²³²Th neutron multiplicities were measured relative to ²³⁵U at each incident neutron energy. Above 15 MeV the multiplicities were determined for ²³²Th and ²³⁵U by using lower energy data from the ²³⁵U sample to measure the neutron detector efficiency. Corrections for angular anisotropy and spectral temperatures of the fission neutrons were minimized through the use of a spherical shell of ²³⁵U surrounding the fission chamber. The present results for ²³²Th extend available multiplicity data into the previously unreported regions: 1.1 to 1.3 MeV and 17 to 49 MeV. The ²³⁵U results also extend significantly beyond previously reported data. For the ²³²Th case, previously observed deviations from linearity below 2 MeV and near the (n, n′f) threshold have been confirmed. In addition, this experiment suggests a continued rise in neutron multiplicity with decreasing incident neutron energy down to 1.1 MeV. A value for of ²³¹Th(n,f) is inferred from the ²³²Th results above the (n,n′f) threshold. The ²³²Th measurements reported here for neutron energies above 15 MeV show an average value of , which agrees with a value calculated from the binding energies of the pre-scission evaporated neutrons and the assumed mean kinetic energies. The ²³⁵U data do not exhibit such a close agreement, suggesting that shell effects may be disappearing more rapidly in this nucleus as the excitation energy increases.