Aerial gamma-ray surveys of the natural environment measure the flux of gamma rays produced by the radioactive decay of 40K, 214Bi, and 208Tl in the upper 10-20 cm of surface materials. 40K is a radioactive potassium isotope which can be used to estimate the total amount of potassium in the soils and rocks. 214Bi is a decay product of the 238U radioactive decay series and is used to estimate the uranium concentrations, and 208Tl, a decay product of the 232Th radioactive decay series, is used to estimate thorium concentrations. Aerial gamma-ray data covering the 48 contiguous states of the United States have been compiled to produce maps showing the distributions of equivalent uranium, equivalent thorium, and potassium. This compilation involved processing the aerial survey data from about 470 1?? ?? 2?? quadrangle maps. ?? 1990.
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Modern aerial gamma-ray spectrometry and regional potassium map of the conterminous United States