Air-ground correction factor for gamma radioactive clouds

A Monte Carlo study has been done to calculate the spectral energy fluence above the ground due to an isotropical gamma emitter at a height h above the ground. The kerma was calculated for 10 source energies in the range 0.01 - 10 MeV. The results are compared with the kerma due to gamma emitters in infinite air. The greatest deviations are found for the case that the source is near the found. In the calculation the lowest height was 0.05 mfp. The presence of the ground was two effects (i) for great lateral distances (in the calculation around 5 mfp) and source energies in the range 80 - 150 keV it reduces the kerma by a factor 3; (ii) near the source and for source energies in the range 80 - 500 keV it enhances the kerma by 10%. The airground correction factor is analytically approximated so that it may be easily used in programs that calculate the dose due to gamma emitting clouds. (Author)