Time Variations of Natural Gamma Radiation

Abstract

Measurement of terrain-emanated natural uranium gamma radiation is difficult because of the inclusion by the measuring device of radiation from the decay of atmospheric radon. This paper presents the results of a 7-year study of time variations of the components of the natural gamma radiation. Accurate measurement and removal of the atmospheric equivalent uranium (eU) decay fraction provides the terrain-emanated eU radiation. Variations in equivalent thorium (eTh), eU, and potassium-40 (40K) natural radiation have been measured over this 7-year period at one location at 5-min intervals. Atmospheric diurnal changes in the concentrations of radon gamma-emitting daughters are observed to exceed surface-emanated eU radiation by more than 400% in the absence of rainfall and up to 3000% in rainfall. Gamma radiation from rainfall-deposited thoron daughters has been seen to increase the surface-emanated eTh radiation by more than 70%. Two different field vehicles currently contain the measurement systems. Data reduction requires the use of environmentally measured gamma ray standard spectra for eTh, eU, and 40K decay and cosmic radiation, plus measurement and use of vehicular system radiation backgrounds. Using these measurement methods, gamma radiation emitted from the earth's surface can be accurately determined day or night, except during and for up to1.5 days following rainfall. Monthly variations of the components of the natural gamma radiation are presented together with daily variations for six selected months of the measurement period.