Systematics of Fragment Mass and Energy Distributions for Proton-Induced Fission ofU233,U235, andU238

Abstract

Thin targets of $^{233}\mathrm{U}$, $^{235}\mathrm{U}$, and $^{238}\mathrm{U}$ were bombarded with protons of several energies between 7 and 13 MeV. Correlated energies of fission-fragment pairs were measured with silicon surface-barrier detectors. Fragment mass vs total kinetic energy distributions were deduced, and their systematic variation with bombarding energy was studied. It was found that the yield in the valley of the mass distributions for symmetric divisions increases with increasing excitation energy, and that the peaks move slightly toward symmetry. The average total kinetic energy for mass divisions with one fragment mass \ensuremath{\simeq} 132 amu was found to decrease with increasing excitation energy, while for other mass divisions little change was observed. The average over-all total kinetic energy was found to decrease with increasing bombarding energy, and widths of both the kinetic energy distributions and the mass distributions were found to increase somewhat. The results are discussed in terms of fragment shell effects which appear to persist to our highest excitation energy (\ensuremath{\sim}18 MeV), and also in terms of the two-component fission hypothesis.