Terrestrial-origin skyshine at sea level

Abstract

Low-energy continuous gamma radiation with a maximum energy distribution at ∼70 keV reaches the Earth's surface from the upper hemisphere. In addition to components resulting from cosmic-origin low-energy gamma radiation, there is also a prominent contribution arising from gamma photons emitted by environmental radionuclides, which are backscattered by air above ground (commonly referred as “skyshine” radiation). Since both components are covering the same energy region of gamma radiation (mainly 30 keV–350 keV), it is not simple to determine the separate contributions of each radiation component to the total gamma flux. The efficient way to solve this long-standing problem is to study the backscattering of gamma radiation on the atmospheric air by Monte Carlo simulations. In this work, the simulations were performed in order to obtain air-backscattered spectra, as well as gamma photon fluxes which can be expected for specified activity concentrations of natural radionuclides (40K, 226Ra, 232Th) distributed in the ground. The simulation results were compared with experimental measurements of low-energy photon flux in the open area from the upper hemisphere. Furthermore, the influence of height above ground and distance from the shore on the skyshine intensity reduction is explored.