The cross sections of ${}_{}{}^{3,4}{}_{}{}^{}\mathrm{He}$(${\pi }^{-}$,$n$)${}_{}{}^{2,3}{}_{}{}^{}\mathrm{H}$ have been measured at 285, 428, 525, and 575 MeV, extending the information on the energy dependence beyond the region previously known (50-300 MeV). The cross sections beyond the region of the $\Delta$ resonance are found to decrease less rapidly than that of the elementary $\pi d\to \mathrm{pp}$ process. This energy dependence suggests that pion absorption in a nucleus is associated with $\pi +N$ (off-shell) scattering which includes $I=\frac{1}{2} \pi N$ resonances that are strongly suppressed in $\pi d\to \mathrm{pp}$.

• Received 5 January 1981

DOI:https://doi.org/10.1103/PhysRevLett.46.1562