Nuclear Structure Effects in High-Energy (p, pn) Reactions

Cross sections for the ($p, \mathrm{pn}$) reaction of ${\mathrm{Cu}}^{65}$, ${\mathrm{Zn}}^{66}$, ${\mathrm{Ge}}^{70}$, ${\mathrm{Ge}}^{72}$, ${\mathrm{Se}}^{76}$, ${\mathrm{Br}}^{79}$, and ${\mathrm{Br}}^{81}$ have been measured at 2.9 GeV. The following variation with target neutron number is indicated by these results, as well as by recent measurements for ${\mathrm{Ga}}^{69}$, ${\mathrm{Ga}}^{71}$, and ${\mathrm{As}}^{75}$: The ($p, \mathrm{pn}$) cross sections increase from 50 mb at $N=36\mathrm{} \mathrm{to} 64$ mb at $N=40\mathrm{}$, then decrease to 48 mb at $N=42\mathrm{}$, and finally increase to 59 mb at $N=46\mathrm{}$. The results are compared with Benioff's calculation of ($p, \mathrm{pn}$) cross sections and good agreement is obtained on the assumption that the $1f_{\frac{7}{2}}$ shell is available for the ($p, \mathrm{pn}$) reaction up to $N=40\mathrm{}$ and unavailable thereafter. The effect of nuclear deformation on the availability of this shell is considered in detail and is found to be small.