Proton spectra of ${}_{}{}^{3,4}{}_{}{}^{}\mathrm{He}({\pi }^{\pm }, \mathrm{p})$ and ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}({\pi }^{\pm }, \mathrm{p})$ have been measured at ${\theta }_{\mathrm{p}}=20\mathrm{}°$ for $T_{\pi }=50, 100, 150, 200, \mathrm{and}, 250$ MeV, and at ${\theta }_{\mathrm{p}}=20\mathrm{}° \mathrm{and} 40°$ for $T_{\pi }=295\mathrm{}$ MeV. A small contributor to these spectra is the exclusive pion absorption reaction for which the cross section of ${}_{}{}^4{}_{}{}^{}\mathrm{He}({\pi }^{+}, \mathrm{p}){}_{}{}^3{}_{}{}^{}\mathrm{He}$ is reported. Major contributions are identified to come from quasifree $\pi \mathrm{N}\to \pi \mathrm{p}$ scattering and $\pi \mathrm{NN}\to \mathrm{Np}$ absorption for which cross sections are also reported. The quasifree cross sections are compared with those of the free reactions $\pi \mathrm{N}\to \pi \mathrm{p}$ and $\pi \mathrm{d}\to \mathrm{pp}$. We use these results to assess the contribution of quasifree reactions to the total reaction cross section. The comparison of the quasifree $\pi \mathrm{NN}\to \mathrm{Np}$ reaction relative to $\pi \mathrm{d}\to \mathrm{pp}$ is discussed in terms of differences in the attenuation and the dynamics of the reactions. This information is of interest for the phenomenological interpretation of complex reactions such as the two-nucleon absorption model of ${}_{}{}^3{}_{}{}^{}\mathrm{He}({\pi }^{-}, \mathrm{n}){}_{}{}^2{}_{}{}^{}\mathrm{H}$, ${}_{}{}^4{}_{}{}^{}\mathrm{He}({\pi }^{-}, \mathrm{n}){}_{}{}^3{}_{}{}^{}\mathrm{H}$, and ${}_{}{}^4{}_{}{}^{}\mathrm{He}({\pi }^{+}, \mathrm{p}){}_{}{}^3{}_{}{}^{}\mathrm{He}$ in which $\pi \mathrm{d}\to \mathrm{pp}$ is assumed to be an elementary process. This experiment also provides information on the cross section of ${\pi }^{-}\mathrm{pp}\to \mathrm{pn}$ from which we determine the dependence of pion absorption on the isospin of the initial NN state.

NUCLEAR REACTIONS ${}_{}{}^{3,4}{}_{}{}^{}\mathrm{He}({\pi }^{\pm }, \mathrm{p})$, ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}({\pi }^{\pm }, \mathrm{p})$ $E=50-295\mathrm{}$ MeV; measured $\sigma (E; E_{\mathrm{p}})$; $\theta =20, 40°$.

• Received 17 December 1982

DOI:https://doi.org/10.1103/PhysRevC.28.304