He ions emitted in the interaction of 70-MeV pions with carbon, aluminum, nickel, silver, and gold were measured directly at $\theta =90\mathrm{}°$. Cross sections and energy spectra are presented. Calculations of intranuclear cascades followed by evaporation reproduced the experimental data reasonably well. The results of the calculations imply that: (1) pion absorption in flight plays a major role in reactions in which $\alpha$ particles are emitted; and (2) one-"$\alpha$"-removal lines, observed in deexcitation $\gamma$-ray experiments, do not imply large probability for real $\alpha$ emission. The ${\pi }^{-}$ to ${\pi }^{+}$ ratio of cross sections was found to be about 1.4.

NUCLEAR REACTIONS C, Al, Ni, Ag, Au (${\pi }^{-}, \alpha +\mathrm{anything}$), $E_{\pi }=70\mathrm{}$ MeV; measured He energy spectra; $\frac{d\sigma }{d\Omega }\left(90\mathrm{}°\right)$. Natural targets; evaporation analysis.

• Received 19 July 1977

DOI:https://doi.org/10.1103/PhysRevC.18.961