Interaction of Positive and Negative Pions withAl27andSi28

The interaction of low-energy positive and negative pions with ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ and ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ was studied by the detection of $\gamma$ radiation emitted from residual nuclei formed in the reaction. Large cross sections for the removal of $\alpha$ clusters were found for both nuclei and both pion charges. A large asymmetry is observed between the cross sections for the (${\pi }^{-},{\pi }^{-}p$) and (${\pi }^{+},{\pi }^{+}n$) on ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ leading to mirror states in ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ and ${}_{}{}^{27}{}_{}{}^{}\mathrm{Si}$. The ratio of ${\pi }^{+}$ to ${\pi }^{-}$ cross sections for proton knockout is about a third of the value predicted by a simple impulse approximation.