Evaporation of Coincident Protons inα+Ni58Reactions

A study has been made of nuclear reactions in which two or more protons are emitted, in the bombardment of nickel isotopes by alpha particles. The most extensive data were obtained for 32-Mev alpha particles, with a ${\mathrm{Ni}}^{58}$ target. A smaller amount of data were obtained at 27 and 42 Mev, and with ${\mathrm{Ni}}^{60}$ and ${\mathrm{Ni}}^{62}$ targets. The two protons were observed in a coincidence arrangement, using scintillation counter $\frac{\mathrm{dE}}{\mathrm{dx}}-E$ detectors. The energy correlations and distributions, the angular correlations and distributions and the yields were compared with predictions based on the statistical theory. There was extensive agreement, and therefore it is concluded that most of the observed events arise from evaporation from an excited compound nucleus. For ${\mathrm{Ni}}^{58}$ at 32 Mev, the cross section for compound nuclear events in which two or more protons are evaporated was found to be 560±50 mb which is over one third the total reaction cross section. This large cross section can be quantitatively understood in terms of the proton richness of the emitting compound nucleus. This conclusion follows from an analysis in which it is shown that the relative proton and neutron emission probabilities in other experiments (especially with 14-Mev incident neutrons) are strongly correlated with proton richness.