Buildup of electronic equilibrium below the surface of an irradiated medium.
This paper deals with the theoretical deduction of the buildup of electronic equilibrium near an air-water interface irradiated by a collimated photon beam. Anisotropy of secondary electron emission is taken account of by using isotropic electron point kernels suitable weighted with the appropriate distribution function and the contribution from the secondary electron flux in air is treated in the equivalent path length approximation. This method is readily applicable to practical situations where an electron filter is employed. For a typical 60Co beam the contributions from secondary electrons generated in the filter, the intervening air column and water are separately determined and analysed. Also presented are data on the variation of surface dose with filter-to-source distance for various field sizes.