Radiochemical Studies of Neutron-Induced Fission of U235 and U238 and the Two-Mode Fission Hypothesis

It has been suggested that the effect of changing excitation energy on the shape of the fission product mass-yield curve from fission of a single nuclear species is due to the change in the relative amounts of two energy-independent modes of fission, each giving rise to its own characteristic mass-yield curve. It is shown here that this hypothesis predicts a linear relationship between the fission yields of any pair of fission products measured at a set of excitation energies. Linear relationships are also predicted between pairs of fission yields measured relative to the yield of some reference fission product. Fission product yields relative to the fission yield of ${\mathrm{Mo}}^{99}$ were measured for fission of ${\mathrm{U}}^{235}$ and of ${\mathrm{U}}^{238}$ with neutron beams of mean energies ranging from 2 to 10 Mev. The predicted linear relationships were observed in all cases. However results for fission yields from ${\mathrm{U}}^{235}$ with thermal neutrons do not fall on the corresponding observed lines. The two-mode fission hypothesis is a possible explanation for the linear relationships observed but does not explain all of the data.