Joseph S. Duval, et al., (U.S. Geological Survey) report that naturally occurring radioactive isotopes can be found in most rocks and soils. The most abundant of these are potassium (K40), uranium (U238), and thorium (Th232). The radioactive decay of these elements results in the emission of gamma-rays of sufficient intensity that they can be measured using instruments in aircraft flying close to the ground. The measurement technique is known as airborne gamma-ray spectrometry (AGRS) and quantitative methods were developed 30 years ago that allow data to be expressed in terms of element concentrations and dose rate.
In the 1970s, following the 1973 OPEC oil embargo, the governments of Canada and the U.S. took steps to stimulate uranium exploration. The Uranium Reconnaissance Program (URP) in Canada, and the National Uranium Resource Evaluation (NURE) program in the U.S., included AGRS surveys with similar technical specifications.
The NURE program covered almost all the U.S. but the Canadian program ended before much of Canada could be surveyed. Small areas were added during the 1980s and 90s, bringing coverage of Canada to about 35%. The data from each country were published in a variety of formats at different scales. The work presented here is a unified compilation of the U.S. and Canadian datasets and is intended to facilitate regional comparisons on a continental scale.
Maps of the data show many-fold variations in the surface abundances of potassium, thorium and uranium, and also in their relative abundances. A dose rate map illustrates the resulting variation of background radiation with location. These data provide a regional picture of the natural radioactivity across 60% of North America and demonstrate the importance of standardized measurement procedures.
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