Implementation of photon elastic scattering in GEANT4

A complete theoretical framework for photon elastic scattering has been implemented in GEANT4 and is validated against experimental results. The implementation covers all atomic numbers and energy range between 0.3 MeV and 20 MeV. The total elastic scattering cross section is calculated based on scattering amplitudes to account for interferences between coherent processes. New scattering amplitudes are calculated for Rayleigh scattering to extend the cross section data table and avoid interpolation and extrapolation. Validation on a significant number of test cases showed good agreement between our implementation and experimental data.