The Elastic Scattering of 1.33 MeV and 2.76 MeV Gamma Rays by Lead

A report is given of measurements of the elastic-scattering cross section of lead for gamma-rays of 1.33 MeV and 2.76 MeV at scattering angles between 40° and 120°. With 1.33 MeV γ-rays good agreement was obtained with the theoretical predictions on the basis of Rayleigh and Thomson scattering alone. The Rayleigh scattering cross section was calculated from Franz's formulae which are non-relativistic and based on the Thomas-Fermi approximation to the electron distribution in the atom. With 2.76 MeV γ-rays the experimental cross sections were compared with an unpublished curve given by Bethe calculated for the K electrons alone, in addition to the values given by Franz's formulae. These theoretical curves are calculated only for Rayleigh and nuclear Thomson scattering, and at all angles in the range investigated the measured cross sections were found to be greater than those predicted. The discrepancy is possibly due either to secondary effects, or the occurrence of Delbruck scattering.