Total Cross Section for (γ,π−) Reactions in theΔRegion

Total cross sections for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($\gamma$,${\pi }^{-}$)${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$($\gamma$,${\pi }^{-}$)${}_{}{}^7{}_{}{}^{}\mathrm{Be}$ have been measured from threshold to 360 MeV photon energy by detecting the radioactivity of the residual nuclei, thereby singling out the ground state of ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$ and the ground and first excited states of ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$. The cross sections are found to peak at about 40 MeV pion energy and then to fall gradually. In contrast to pion charge exchange and other photopion experiments, these results are well reproduced both in shape and in magnitude by distorted-wave impulse-approximation calculations.