Uranium-238 and its daughter products in Greek surface soils

High concentrations of natural radionuclides in the soil can result in high dose rates outdoors mainly due to external exposure. In dose calculations, the main radionuclides of interest are 226Ra, 232Th and 40K. Research till recent years has focused mainly into the evaluation of 226Ra concentration, due to radon exhalation from the ground, and due to its relatively simple determination through its gamma emitting decay products. Other radionuclides of the uranium series such as 238U and 210Pb, emitting low energy photons are not usually determined due to γ-spectroscopic difficulties, and for dosimetric calculations an assumption of radioactive equilibrium among the nuclides of the uranium (238U) series as well as the nuclides of the thorium (232Th) series is usually adopted. Among the nuclides of the uranium series, radioactive disequilibrium may exist, and this is the case of the relatively long-lived nuclides 238U (T½ = 4.47 109y), 226Ra (T½ = 1600y) and 210Pb (T½ = 22.2y) for several reasons, such as leaching, rock weathering, radon exhalation from the ground etc. Radioactive equilibrium may also be disturbed because of human activities, such as the existence of coal fired power plants and the resulting fly-ash fallout. An extensive research has been undertaken by the Nuclear Engineering Laboratory of the National Technical University of Athens (NES-NTUA) for the investigation of the natural and artificial radioactivity in greek surface soils. This research has resulted to the mapping of 226Ra, 232Th and 40K in greek surface soils, as well as the mapping of radionuclides of the Chernobyl fallout in Greece. The gamma spectroscopic determination of 238U and 210Pb, which emit low energy photons of 63.29keV and 46.5keV respectively, with very low yields, requires the use of detectors with high efficiency in the low region below 200keV, such as LEGe or XtRa detectors, and special techniques for self-absorption corrections inside the samples. Due to the time consuming and difficult determination of the low activities of 238U and 210Pb in environmental samples such as soil, only recently such results are found in the literature. In the framework of this research soil samples from allover Greece are analysed for 238U, 226Ra and 210Pb. From the samples analysed so far, the conclusion may be drawn that, when 226Ra concentration is much lower than about 25Bqkg-1, which is incidentally the mean value for 226Ra of greek surface soils, radioactive equilibrium is significantly disturbed, and the ratio 226Ra/238U may be as low as 0.3. In the case where 226Ra activity is much higher than 25Bqkg-1, radioactive equilibrium is again significantly disturbed with the ratio 226Ra/238U reaching values as much as 4. The above findings indicate that extreme values of 226Ra in surface soils may be the result of 226Ra leaching. This research is currently focused on the mapping of uranium at surface soils in specific locations in Greece, and on the investigation of its association with that of 226Ra. Concerning the concentration of 210Pb in surface soils, the analyses performed show that is was almost always higher than that of 226Ra, probably due to radon exhalation from the ground. Analyses of soil samples from deeper soil layers are also performed, in order to investigate the vertical profile of the radionuclides under study.