Backscattering factors, as fractions of the direct dose rate, are obtained for point sources of gamma radiation, specifically Cs137 and Co60, placed near a plane interface between vacuum and concrete. The method is based on the application of albedo principles, using the Chilton-Huddleston formulation for albedo. The results are considered practically applicable to air-concrete or air-ground interface situations, provided the source-detector, source-interface, and detector-interface distances are within certain limits. The lower limit is in theory the order of a mean free path of the source radiation in concrete, although under certain circumstances the present results are valid for distances even less. The upper limit appears to be on the order of a few dozen feet, but further precise experimental work is needed to establish this.