Some Problems of the Radiometric Mapping of Carbonate Terrains - An Example From Istria (Croatia)

Abstract

The gamma dose rates were calculated from the results of gamma-spectrometric measurements of radionuclide concentrations in soils in the region of the Istrian peninsula, Croatia. Calculated gamma dose rate conversion factors (GDRCF) for external exposure above ground, give external dose rates per unit of natural radionuclide concentration in the soil and bedrock. GDRCF in air at a height of 100 cm above ground are tabulated for 40K, 232Th decay series, 238U - 230Th subseries and 226Ra - 210Pb subseries for various source depths in soil or bedrock up to a maximum depth of 120 cm. The radiometric maps must take into consideration gamma doses from soils as well as from underlying rocks in the first metre of ground profile. This is of crucial importance especially for carbonate terrains, where the differences between natural radionuclide concentrations in carbonate derived soils and carbonate bedrock could be of two orders of magnitude. Furthermore, for soils themselves the role of the drainage network in migration processes is undoubtedly very important. All these dependencies make us believe that gamma-spectrometric measurements and related dose rate conversions in connection with in situ dose measurements could be the best method for radiometric mapping at least for carbonate terrains with numerous rock outcrops, poor drainage and lithological variability on a local scale. Recalculation of natural background dose rate data, collected by airborne or carborne surveys, into radionuclide concentration in the ground in carbonate terrains is possible in two cases only: when soil is completely absent or where soils are well developed (thicknesses of 25 cm or more).