The total cross section for the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(\gamma ,{\pi }^{-}){}_{}{}^{12}{}_{}{}^{}\mathrm{N}$ has been determined by observation of the residual ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$ radioactivity. The cross section was extracted from the bremsstrahlung excitation function which was measured in the region between 3.6 and 12.6 MeV above the threshold, with one point 33.5 MeV above the threshold. The variation of the measured cross section with energy is far more rapid than is predicted by calculations using the $\overrightarrow{\epsilon }·\overrightarrow{\sigma }$ interaction. Even when the full interaction Hamiltonian is used, the experimental cross section rises somewhat more rapidly than predicted.

• Received 15 March 1976

DOI:https://doi.org/10.1103/PhysRevLett.37.819