Radiative Pion Capture inHe4

The spectrum of high-energy $\gamma$ rays following the absorption of negative pions in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ has been investigated. The observed shape of the spectrum and the measured total capture rate [(0.704±0.066) ×${10}^{15}$ ${\sec }^{-1\mathrm{}}$] agree well with theoretical predictions, assuming that the pions proceed through excitation of states in ${}_{}{}^4{}_{}{}^{}\mathrm{H}$ with $J^{\pi }=2^{-}$ at 3.4, $J^{\pi }=1^{-}$ at 5.1, and $J^{\pi }=1^{-}$ at 7.4 MeV relative to the ${}_{}{}^3{}_{}{}^{}\mathrm{H}+n$ threshold, and taking an operator $\approx {\tau }^{\mathrm{}(-)\mathrm{}}\sigma e^{i\overrightarrow{\mathrm{k}}·\overrightarrow{\mathrm{r}}}$ for the calculation of the nuclear-matrix elements.