Fission Mass Yield Dependence on Angular Momentum

Fission mass yields from the compound nucleus ${\mathrm{U}}^{236}$ have been measured for 19-29-MeV alpha-particleinduced fission of ${\mathrm{Th}}^{232}$ and 9-18-MeV neutron-induced fission of ${\mathrm{U}}^{235}$ with particular attention to the yields of symmetric fission. Decreases in the symmetric fission yield are seen at the 25-MeV onset of ($\ensuremath{\alpha}, 2nf$) fission and the corresponding 14-MeV onset of ($n, 2{n}^{\ensuremath{'}}f$) fission. Above these energies, the symmetric fission yields are approximately 13% greater for alpha-particle-induced fission than for neutron-induced fission at the same excitation energies. This increase is partially explained by the fission-chance ratio of higher energy fissions relative to fissions of the most de-excited nuclei being greater for alpha particles than for neutrons. Angular-momentum-dependent cross sections for these fissions were calculated, and the use of reasonable values of level densities with a rigid moment of inertia resulted in a calculated effect of about one-half this observed effect. The difference remaining is possibly explained by fissions following direct interactions. Mass yields of 9.1-18.1-MeV neutron-induced fission of ${\mathrm{Th}}^{232}$ and 4.7-18.1-MeV neutron-induced fission of ${\mathrm{U}}^{238}$ were also measured. Some indication of a small central peak in the mass distribution was observed in the yields from ${\mathrm{U}}^{236}$ compound-nucleus fission, but not from ${\mathrm{Th}}^{233}$ compound-nucleus fission.