Measurement of the Fission Thresholds of ${\mathrm{Pu}}^{239}$, ${\mathrm{U}}^{233}$, ${\mathrm{U}}^{235}$, and ${\mathrm{U}}^{238}$ Using the ($d, p$) Reaction

An experiment is described which measures the fission thresholds of thermally fissionable nuclides. In contrast to fission induced by neutron capture, use of the ($d, p$) reaction allows compound nuclei to be produced with excitations less than the binding energy of the last neutron. Measurements of the energy spectra of protons in coincidence with fission, normalized by the energy spectra of all emitted protons, have allowed the thresholds of ${\mathrm{Pu}}^{239}$, ${\mathrm{U}}^{233}$, and ${\mathrm{U}}^{235}$ to be observed. It should be emphasized that the compound nucleus is stable against neutron emission in the region of these fission thresholds, thus avoiding distortions by this competing reaction. The threshold of ${\mathrm{U}}^{238}$ was also obtained for comparison with neutron data. In the case of the even-even compound nuclei, the observed step structure of the fission threshold is a clear indication of a multiple fission barrier. This multiple barrier is predicted by the collective model of fission and is correlated with the experimental data. The equivalent neutron energies at the center of the first threshold are as follows: for ${\mathrm{Pu}}^{239}$, -1.61 Mev; for ${\mathrm{U}}^{233}$, -1.47 Mev; and for ${\mathrm{U}}^{235}$, -0.60 Mev. The second thresholds are observable only for the first two: for ${\mathrm{Pu}}^{239}$, -0.72 Mev, and for ${\mathrm{U}}^{233}$, -0.72 Mev. The threshold of ${\mathrm{U}}^{238}$ agrees well with the data from neutron excitation. In each case the fission probability decreases in the region of positive neutron energies. This would result either if neutron emission were to compete with fission or if there were a significant contribution to the proton spectrum from the electric breakup of the deuteron.