The Close-in Exposure Field From Point Isotropic Gamma-Ray Source Located at an Air-Ground Interface

On the basis of a simplified theoretical model and formulas, a method is presented for calculating the exposure field in the air near an air-ground interface on (or very near) which a point isotropic source of monoenergetic gamma rays has been placed. Necessary functions have been calculated and are presented for cobalt-60 (1.25 MeV) and cesium-137 (0.662 MeV). Within the limitations of the model, calculated results have an uncertainty on the order of 1%, for source-detector distances out to about 15 m. Comparison with previous experimental and theoretical results is made. At detector locations very near the source (on the order of a meter or less), the exposure field is somewhat affected by the exact height of the source above the interface, even for variations on the order of a centimeter.