He4andHe3Particles from Au, Bi, and Th Nuclides Bombarded by 157-MeV Protons

Energy distribution measurements have been carried out for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at different angles when Au, Bi, and Th targets are bombarded by 157-MeV protons. The telescope method was applied to solid-state detectors. The angular distribution has two components. The first, due to an isotropic process, is attributed to the evaporation mechanism. The second, preferentially in the forward direction, is taken as evidence for a knock-out mechanism. Prompt cascade nucleons are believed to be scattered by $\alpha$ clusters on the surface of the nucleus and to eject $\alpha$ particles. About 40 nucleons may be organized in $\alpha$ clusters in heavy nuclei.