Energy deposition kernels at low photon energy including coherent scatter

Abstract

The Low-Energy Photon-Scattering (LSCAT) expansion to the EGS4 Monte Carlo code has been applied to the sphere EGS4 code scasph.mortran to compute energy deposition kernels at photon energies below 100 keV. The LSCAT routines were used to replace the default atomic form factor and cross-section data used by EGS4 to model coherent scatter with measured molecular form factor and cross-section data. Energy deposition kernels were calculated with: (1) coherent scatter effects neglected, (2) coherent scatter effects included using the default EGS4 atomic form factor, and (3) coherent scatter effects included using the molecular form factor. These energy deposition kernels were utilized in a convolution/superposition code developed for mega-voltage therapeutic X-rays modified to compute dose at low photon energy. Depth dose calculations were made for 30 keV photons, where the coherent scatter contribution is maximum, using kernels which model coherent scatter in each of the three ways described. These calculations show a difference between the kernel that neglects coherent scatter compared to the kernels that include coherent scatter, but show no distinction between the atomic and molecular form factor kernels. A separation of the kernels into their primary and scatter components reveals that the depth dose calculation is dominated by the primary component. An examination of the kernels separated into their primary and scatter components suggest that the sphere EGS4 code includes the coherent scatter contribution in the primary component of the kernel.