Numerical evaluation of the natural gamma radiation field at aerial survey heights

In computational studies of the count rates to be expected in airborne, radiometric surveys of geological formations, knowledge is required of the aerial gamma radiation field produced by the radioactive minerals in the ground. The data presented in this work permits calculation of the energy and angular distribution of the gamma-ray flux at distances of between 0 and 200 m from a plane, homogeneous ground with known abundances of thorium, uranium, and potassium, A tabulation permitting calculation of the gamma dose rate in the air is also given. The data are applicable to any normal ground material (rock or soil) in which uranium and thorium are in secular equilibrium with their respective daughters. Besides, the air denaity may have an arbitrary variation with the distance to the ground. In calculating the flux of gamma rajs that strike an airborne Nal(Tl) detector it is suggested to represent the bottom of the aircraft by an equivalent layer of air. For use in the numerical evaluation of aerial total count rates the angular gamma-ray flux at the point of detection is approximated in accordance with the double-P. polynomial expansion method. A detailed tabulation of flux expansion coefficients, calculated for 0.05-MeV wide energy intervals, is presented. To evaluate the differential count rates of high-energy Th-U-K gamma rays, it is convenient to make use of the wellknown formulas for a flux of unscattered gamma rays. With this application in view data for insertion in these formulas are also given.