Study of (α,αn) Reactions with 900-MeV Helium Ions

Cross sections for ($\alpha ,\alpha n$) reactions on ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$, ${}_{}{}^{65}{}_{}{}^{}\mathrm{Cu}$, and ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ at 900 MeV have been determined relative to the ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}(\alpha ,\alpha 2pn){}_{}{}^{24}{}_{}{}^{}\mathrm{Na}$ monitor reaction. They are 49.3 ± 3.5, 64.6 ± 2.8, and 95.8 ± 1.4 mb, respectively. The cross section used for the monitor reaction at this energy was 22.0 mb; this was determined through a comparison of the ${}_{}{}^{24}{}_{}{}^{}\mathrm{Na}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ activities produced in A1 foil by 920-MeV helium ions. The general trends observed in ($\alpha ,\alpha n$) cross sections as a function of target composition are discussed. The direct population of the (12-) to (2-) states of ${}_{}{}^{196}{}_{}{}^{}\mathrm{Au}$ is 0.07 and this suggests a mechanism of direct emission of the neutron from the target surface. The observed trend of $\sigma (\alpha ,\alpha n)$ with the neutron skin thickness of the target also supports this mechanism. These reactions are compared with the high-energy ($p,pn$) reactions of the same targets.