The cross section of ${\pi }^{-}+{}_{}{}^3{}_{}{}^{}\mathrm{He}\to d+n$ and ${\pi }^{-}+{}_{}{}^4{}_{}{}^{}\mathrm{He}\to t+n$ has been measured at energies between $T=50\mathrm{} \mathrm{and} 295$ MeV and angles between ${\theta }_n\approx 20° \mathrm{and} 150°$ covering the momentum transfer range $q\approx 0.4-1.0$ GeV/c. A phenomenological analysis of the data is attempted to identify the characteristic $T$, $\theta$, and $q$ dependences of the ($\pi$,$n$) reaction dynamics. Most conspicuous is the angular dependence of the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(${\pi }^{-}$,$n$)${}_{}{}^3{}_{}{}^{}\mathrm{He}$ which mirrors characteristic features of $\pi$ + He elastic scattering. The $T$ dependence of ($\pi$,$n$) in ${}_{}{}^{3,4}{}_{}{}^{}\mathrm{He}$ (for $\theta <70\mathrm{}°$) is similar to $\pi +N$ or $\pi$ + He elastic scattering. The clear dependence of $T$ and $\theta$ is indicative of $\pi +N$ (off-shell) scattering in the basic pion-absorption process. The nuclear form factor $q$ dependence is most pronounced for forward angles, and the largely exponential falloff with $q$ is similar to that of analogous nuclear reactions. Comparisons with theoretical calculations show that crucial ingredients are missing in currently used models and general directions of needed refinements are indicated.

NUCLEAR REACTIONS ${\pi }^{-}+{}_{}{}^3{}_{}{}^{}\mathrm{He}\to d+n$ and ${\pi }^{-}+{}_{}{}^4{}_{}{}^{}\mathrm{He}\to t+n$, $T_{\pi }=50-295\mathrm{}$ MeV, ${\theta }_n=20\mathrm{}°-150\mathrm{}°$; measured $\sigma ({\theta }_d,T_{\pi })$ and $\sigma ({\theta }_t,T_{\pi })$, discussed reaction mechanisms; the energy and nuclear form factor dependence of the reaction dynamics.

• Received 26 November 1980

DOI:https://doi.org/10.1103/PhysRevC.24.1102