Natural radioactivity mapping of Greek surface soils

Abstract

A total of 1440 samples of soil, collected across Greece from the 1 cm surface layer, were analysed for natural radioactivity ( 226Ra, 232Th, and 40K). 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 escaped from them. Secular equilibrium of 226Ra and 232Th 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 and is for 226Ra: 25 ± 19 Bqkg −1, for 228Ra: 21 ± 16 Bqkg −1, and for 40K: 355 ± 220 Bqkg −1. They entail a gamma-ray dose rate outdoors of 40 nGyh −1. A statistical analysis showed that only 90 samples, collected at 67 locations, have 226Ra content exceeding their mean value by more than 1.65σ. The 226 Ra 232 Th and 226 Ra 40 K concentration ratios were 1.10 ± 0.48 and 0.06 ± 0.03, respectively.