Alpha-Particle Emission During Nuclear Fission at Moderate Excitation Energies

Abstract

Protons, deuterons, and helium ions of various energies up to 42 MeV were used to induce fission in thin targets of ${\mathrm{Th}}^{232}$, ${\mathrm{U}}^{235}$, ${\mathrm{U}}^{238}$, and N${\mathrm{P}}^{237}$. $\ensuremath{\alpha}$-particle emission during fission was detected by a system of solid-state detectors coupled to a coincidence-gated pulse-height analysis system. $\ensuremath{\alpha}$ particles emitted prior to fission were distinguished from fission-accompanied $\ensuremath{\alpha}$ particles by the tight angular correlation between $\ensuremath{\alpha}$-particle and fission fragments which prevails in the latter case. It was found that the probability of $\ensuremath{\alpha}$ emission during fission is insensitive to excitation energy over the energy interval from about 5 to 38 MeV for a variety of heavy nuclei from thorium to plutonium. There is a suggestion that $\ensuremath{\alpha}$-particle emission is fractionally more probable for spontaneous fission than for fission of the same nuclei excited to energies \ensuremath{\ge}5 MeV. $\ensuremath{\alpha}$-particle emission in the fission of ${\mathrm{At}}^{213}$ (${\mathrm{Bi}}^{209}$+42 MeV $\ensuremath{\alpha}$) was detected, the probability of $\ensuremath{\alpha}$-accompanied fission being about $\frac{1}{3}$ of that for a typical thorium or uranium nuclide. A possible effect of angular momentum on the probability of $\ensuremath{\alpha}$ emission during fission was looked for by forming ${\mathrm{Pu}}^{239}$ in two different ways, the excitation energy being the same but the angular momentum being different. No significant effect was observed. A correlation between the energy of the $\ensuremath{\alpha}$ particles and the sharpness of their angular distribution relative to the fission fragments, which had been seen at lower energies, was observed here to hold as well at higher excitation energies. In an experiment to determlne whether a correlation exists between the probability of $\ensuremath{\alpha}$-particle emission and the mass ratio of the two heavy fission fragments, there is a suggestion that near-symmetric fragments have slightly less probability of emitting an $\ensuremath{\alpha}$ particle than asymmetric heavy fragments, although the effect is not very pronounced.