NATURAL RADIOACTIVITY MAPPING OF GREEK SURFACE SOILS

A total of 1440 samples of soil, collected across Greece from the 1 cm surface layer, were analysed for natural radioactivity (Ra-226, Th-232 & K-40). The samples were air-dried and then hermetically sealed in 0.282 L plastic boxes covered with a film of epoxy resin to ensure that no gases escape from them. Secular equilibrium of Ra-226 and Th-232 with their decay products was obtained before the samples were analysed using high resolution Ge gamma-ray detectors. A Unix-based in-house built complex Data Base/Geographical Information System (DBGIS) was used to analyze the data and present them in map form. The analysis of a second 1 cm thick sample, collected at several sites at a depth of 10 cm, led to the statistically justified conclusion that the natural radioactivity content of the surface and of 10 cm deep soil layers do not significantly differ. According to these results the natural radioactivity content of Greek surface soils is rather low (mean ± standard deviation in Bq/kg):

Ra-226(U-238): 25 ± 19 Ra-228(Th-232): 21 ± 16 K-40: 355 ± 220

and entails a gamma-ray dose rate outdoors equal to 40 nGy/h. A statistical analysis showed that only 90 samples, collected at 67 locations, have Ra-226 content exceeding their mean value by more than 1.65 std. dev. The Ra-226/Th-232 and Ra-226/K-40 concentration ratios were equal to (mean value ± % std. dev.) 1.10 ± 44% and 0.06 ± 56%, respectively.