DOSE-RATE CONVERSION FACTORS, SOIL THICKNESS AND THEIR INFLUENCE ON NATURAL BACKGROUND DOSE-RATE IN AIR ABOVE CARBONATE TERRAINS

The natural background dose rate in air above ground surface is the su m of doses arising from cosmic radiation and radionuclides in air, soi l and bedrock. The resulting dose rate arising from radionuclides in s oil and bedrock (ground component) is the product of characteristic ra dionuclides 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 characte ristic radionuclide. DRCF are calculated for unit K-40 activity in soi l and bedrock as well as for unit head radionuclides activity of Th-23 2 decay series, U-235 decay series, U-238-Th-230 subseries and Ra-226- Pb-210 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 o f natural background dose rate data collected by airborne or carborne surveys above carbonate terrains into radionuclides concentration in t he ground is possible in two cases. Recalculated values present radion uclide concentrations in bedrock in the case where soil has been compl etely missing. In the case of well-developed soil (thickness of 25 cm or more) it represents concentrations in soil relatively well. The lab oratory gamma-spectrometry of K-40, Ra-226, Ac-228 or Tl-208 and U-238 in samples collected during conventional geochemical sampling provide s a good database of characteristic radionuclides. Such a database is applicable for assessment of ground components of natural background d ose rate, as well as for geochemical map cosntructions.