Isospin Dependence of Pion Absorption by Light Nuclei

The spectra of protons and the cross section of the ($\pi$,$\mathrm{pp}$) and ($\pi$,$\mathrm{pn}$) reactions induced by 65-MeV ${\pi }^{+}$ and ${\pi }^{-}$ on C and Al were measured. Ratios $R$, the pion absorption cross section for a $\mathrm{pn}$ nucleon pair to that for a $\mathrm{pp}$ or $\mathrm{nn}$ nucleon pair, were estimated to be 3.9±1.0 for C, and 3.8±0.9 for Al. These $R$ values are well explained by recent microscopic theories, which suggest that pion absorption on ${}_{}{}^3{}_{}{}^{}P$ nucleon pairs is important in the ($\pi$,$\mathrm{pn}$) process for these nuclei.