Nuclear-Structure Effects in Simple High-Energy Spallation Reactions

Cross sections for the ($p, \mathrm{pn}$), ($p, p2n$), ($p, 2pn$), and ($p, 2p2n$) reactions at 370 MeV with ${\mathrm{Cr}}^{50}$, ${\mathrm{Ni}}^{58}$, and ${\mathrm{Zn}}^{64}$ were determined in an investigation of the "anomalously low" ${\mathrm{Ni}}^{58}(p, pn){\mathrm{Ni}}^{57}$ cross section of ∼30 mb as compared to ∼60 mb for other medium mass nuclei. All of the cross sections for these corresponding reactions were found to be lower with ${\mathrm{Ni}}^{58}$ than with ${\mathrm{Zn}}^{64}$. These results rule out the explanation that the cross section for the ${\mathrm{Ni}}^{58}(p, pn){\mathrm{Ni}}^{57}$ reaction is low because of an anomalous excitation-energy spectrum following a ($p, \mathrm{pn}$) knockout reaction. Detailed consideration is given to the roles played by the one-step and two-step mechanisms and nuclear structure in determining the high-energy ($p, \mathrm{pn}$) cross section. It is concluded that the current theoretical models for simple spallation reactions are able to explain the anomalous ($p, \mathrm{pn}$) cross section, provided that previously ignored details of nuclear structure are taken into proper account. In the specific case of ${\mathrm{Ni}}^{58}$, a small nuclear skin thickness relative to neighboring nuclei can account for the low ($p, \mathrm{pn}$) cross section owing to the "surface localization" of the reaction.