Long-Range Particle Emission in Coincidence with Fission at Moderate Excitation Energies

Abstract

The production rate of long-range particles in coincidence with fission has been studied in bombardments of ${\mathrm{Th}}^{232}$ and ${\mathrm{U}}^{238}$ with 10.5-MeV protons and with helium ions of 29.5 and 42.0 MeV. The number of coincident charged particles per fission observed in the proton bombardments is consistent with the numbers of such particles observed in earlier measurements (spontaneous fission, neutron-induced fission). These results indicate that the production rate of charged particles in fission decreases smoothly with increasing excitation energy of the fissioning compound nucleus. For a nucleus excited to 15 MeV, the rate is about half that for a nucleus in its ground state. The charged particle yields obtained in the helium ion bombardments are, however, much larger than one would expect on the basis of the excitation energy dependence indicated by the other (lower energy) measurements. Some possible explanations for these observations are briefly and qualitatively considered.