Angular Momentum Effects in the Gamma-Ray De-Excitation of Fission Fragments

The energy dissipated as $\gamma$ rays during the de-excitation of fission fragments (7.2±0.8 MeV for ${\mathrm{U}}^{235}$ thermal fission) is somewhat higher than has been estimated theoretically (4.9 MeV). It has been suggested that this discrepancy arises from the angular momentum of the fission fragments. We have made a quantitative evaluation of this possibility, taking into account the angular momentum dependence of the level density, the nonexistence of levels of a given angular momentum below some minimum energy (yrast energy), and the competition between neutron and $\gamma$-ray emission. The initial angular momentum distribution is that derived from measurements of isomers produced in fission. Initial excitation energies are based on the known de-excitation properties of fission fragments. All other parameters were derived from sources having no direct connection with the fission process. The calculations for an average pair of fragments (${\mathrm{Sr}}^{96}$ and ${\mathrm{Xe}}^{140}$) indicate that 7.1 MeV should appear as $\gamma$ rays and that the average photon energy is 0.9 MeV. The calculated average neutron energy and number of neutrons are also in agreement with experiment.