Air-ground Interface Correction Factors for Gamma Emitters in Air

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Abstract
For isotropic point photon sources above an air-ground interface, the spectral energy fluences at the ground were calculated by the Monte Carlo method for source energies in the range 50 keV-10 MeV. The source heights and the horizontal source-detector distances varied from 0.05-5 mean free paths in air. Air-ground interface correction factors were calculated and then analytically approximated. The greatest deviations from unity were found for 100-keV sources located on the ground. Near the source, kerma free in air is enhanced by 30%, and far from the source it is reduced by a factor of 3.