Isobaric Yields in the Reactions of 2.9-Bev Protons with Arsenic-75

Cross sections for the formation of four isobars of $A=72\mathrm{}$ from bombardment of ${\mathrm{As}}^{75}$ with 2.9-Bev protons are reported. The cross sections as a function of $Z$ fit a Gaussian curve with a peak at $Z=32.3\mathrm{}$ and a width at half-maximum of 1.4 charge units. Cross sections for six other nuclides in the mass range $66\le A\le 74$ were also measured, and their cross sections lie close to the Gaussian when plotted against $Z-Z_A$, with the exception of ${\mathrm{As}}^{74}$. An analysis of the yields of ${\mathrm{Ga}}^{72}$ and ${\mathrm{Zn}}^{72}$ indicates that most of the ${\mathrm{Zn}}^{72}$ is not formed in a ($p, 4p$) reaction but in a ($p$, $3pn{\pi }^{+}$) reaction, or similar process in which one or more ${\pi }^{+}$ mesons are emitted, with a cross section of >0.018 mb.