Radioactivity of soil in Croatia II: 137Cs, 40K, and absorbed dose rate

We took samples of uncultivated soil from the surface layer (0-10 cm) at 138 sites from all over Croatia and measured their radionuclide activity concentrations with high-resolution gamma-ray spectrometry. This second part of our report brings the results on K-40 and Cs-137 to complement those on the Th-232 and U-238 decay chains addressed in the first part. Together they give the most complete picture of radioactivity of Croatian soil so far. Activity concentrations of K-40 were the highest in the Pannonian region, and there was an opposite trend for Cs-137. We found that the concentrations of Cs-137 tended to increase with altitude, annual precipitation, and vegetation density. The concentration ratio of Cs-137 and K in soil, which indicates the potential for Cs-137 entering food chains via uptake by plants, was the lowest in agriculturally important areas in the east of the Pannonian region. In addition, we used the obtained results on activity concentrations to calculate the related absorbed dose rate as a measure of external exposure to ionising radiation from soil. The sum of the absorbed dose rates for naturally occurring radionuclides and Cs-137 showed that external exposure was generally the highest in the Dinaric region and Istrian Peninsula.