Fission-Fragment Mass and Charge Distribution for the Moderately ExcitedU236Compound Nucleus

The helium-ion-induced fission of ${\mathrm{Th}}^{232}$ (${\mathrm{U}}^{236*}$) has been investigated by using high-sensitivity mass-spectrometric techniques. The relative cumulative yields for the majority of isotopes in the mass ranges 83 to 90 and 131 to 153 have been measured. The xenon cumulative yields show no structure at these excitations (20-57 MeV). The independent yields of 15 isotopes produced in the fission of ${\mathrm{U}}^{236*}$ have been measured for various excitation energies ranging from 20 to 57 MeV. In addition, a number of independent and cumulative yields have been measured for the helium-ion-induced fission of ${\mathrm{U}}^{235}$. The nuclear-charge distribution function was measured directly for one isobaric sequence and indirectly in another; the distribution was found to be Gaussian $y_i\propto \exp [-\frac{{(Z-Z_p)}^2}{0.95}]$, and independent of the excitation in ${\mathrm{U}}^{236*}$ (to 39 MeV). The empirical $Z_p$ function obtained from our measured independent yields shows none of the trends observed for low-excitation fission of ${\mathrm{U}}^{236*}$ in the 82-neutron-shell region. The total number of neutrons emitted per fission as a function of mass ratio of the fragments was derived from the $Z_p$ function; the neutron yields were found to depend strongly on the asymmetry of the fission mode. Fragment neutron yields were derived from an analysis of the empirical $Z_p$ function and the theoretically predicted $Z_p$ function.