Nuclear Scattering of 50- to 130-MevγRays

Using targets of H, D, Li, Be, C, Cu, and Sn, we have measured the energy distribution of scattered $\gamma$ rays at 45°, 90°, and 135°. In first approximation one would expect a scattering cross section given by the coherent addition of Thomson scattering amplitudes for $Z$ free protons. Important deviations, probably of mesonic origin, are observed above 90 Mev. Below 70 Mev in complex nuclei there is an increase in cross section which probably results from nuclear resonance scattering. At small angles in the forward direction, we observe large numbers of inelastically scattered $\gamma$ rays, which apparently result from inner bremsstrahlung associated with electron pair production.