Dose rate conversion factors, soil thickness and their influence on natural background dose rate in air above carbonate terrains

Abstract

The natural background dose rate in air above ground surface is the sum of doses arising from cosmic radiation and radionuclides in air, soil and bedrock. The resulting dose rate arising from radionuclides in soil and bedrock (ground component) is the product of characteristic radionuclides specific activity in soil or bedrock with the accompanying dose rate conversion factors (DRCF). The sum of products between unit dose rates from each emitted photon energy and the emission intensity of all accompanying radionuclides is presented with DRCF for characteristic radionuclide. DRCF are calculated for unit 40K activity in soil and bedrock as well as for unit head radionuclides activity of 232Th decay series, 235U decay series, 238U- 230Th subseries and 226Ra- 210Pb subseries, in relation to soil thickness. Presented DRCF values are generally in good agreement with those published previously, and strongly depend on soil thickness. Recalculation of ground component of natural background dose rate data collected by airborne or carborne surveys above carbonate terrains into radionuclides concentration in the ground is possible in two cases. Recalculated values present radionuclide concentrations in bedrock in the case where soil has been completely missing. In the case of well-developed soil (thickness of 25 cm or more) it represents concentrations in soil relatively well. The laboratory gamma-spectrometry of 40K, 226Ra, 228Ac or 208Tl and 238U in samples collected during conventional geochemical sampling provides a good database of characteristic radionuclides. Such a database is applicable for assessment of ground components of natural background dose rate, as well as for geochemical map constructions.