Forward-angle differential cross sections at low energies (≤80 MeV) are reported for pion single charge exchange to the isobaric analog state for nuclei from ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ to ${}_{}{}^{120}{}_{}{}^{}\mathrm{Sn}$. A minimum in the 0° excitation function near 50 MeV persists for all of the measured nuclei and reflects the analogous minimum in the ${\pi }^{-}p\to {\pi }^{0}n$ cross section that is caused by a nearly perfect cancellation of the $s$- and $p$-wave $\pi$-nucleon amplitudes. The persistence of this feature in these nuclei indicates a strong suppression of the net effect of the medium and constrains the interplay of the medium effects.

• Received 29 July 1985

DOI:https://doi.org/10.1103/PhysRevLett.55.1862