Detection of Pion Single Charge Exchange in Zirconium

The reaction ${}_{}{}^{91}{}_{}{}^{}\mathrm{Zr}({\pi }^{+},{\pi }^0){}_{}{}^{91}{}_{}{}^{}\mathrm{Nb}_{}^{*}{}_{}{}^{}$ was observed by detecting the decay proton $\tilde{p}$ from the isobaric analog state ${}_{}{}^{91}{}_{}{}^{}\mathrm{Nb}_{}^{*}{}_{}{}^{}$ in the sequential process ${}_{}{}^{91}{}_{}{}^{}\mathrm{Zr}({\pi }^{+},{\pi }^0\tilde{p}){}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$. The observed cross section for an incident pion energy of 30 MeV is 1.1 ± 0.8 mb, and is in reasonable agreement with theoretical calculations.