238U 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 Ra-226 Th-232 and K-40. Research till recent years has focused mainly on the evaluation of Ra-226 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 U-231 and Pb-210, emitting low energy photons, are not usually determined due to gamma-spectroscopic difficulties, and for dosimetric calculations an assumption of radioactive equilibrium among the nuclides of the uranium (U-238) series as well as the nuclides of the thorium (Th-232) series is usually adopted. Radioactive disequilibrium may exist among the nuclides of the uranium series, and this is the case for the relatively long-lived nuclides U-238 (T-1/2 = 4.47 x 10(9) y), Ra-226 (T-1/2 = 1600 y) and Pb-210 (T-1/2 = 22.2 y) 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 operation of coal-fired power plants and the resulting fly-ash dispersion and fallout. An extensive research program has been undertaken by the Nuclear Engineering Section 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 in the mapping of Ra-226, Th-232 and K-40 in Greek surface soils, as well as the mapping of radionuclides of the Chernobyl fallout in Greece. The gamma spectroscopic determination of U-218 and Pb-210, which emit low energy photons at 63.29 and 46.5 keV, respectively, with very low yields, requires the use of detectors with high efficiency in the low region below 200 keV, such as LEGe or XtRa detectors, and special techniques for correction due to self-absorption inside the samples. Due to the time consuming and difficult determination of the low activities of U-238 and Pb-210 in environmental samples, such as soil, it is only recently that such results are reported in the literature. In the framework of this research soil samples from all over Greece are analyzed for U-238, Ra-226 and Pb-210. From the samples analyzed so far, the conclusion may be drawn that, when Ra-226 concentration is much lower than about 25 Bq kg(-1), which is incidentally the mean value for Ra-226 in Greek surface soils, radioactive equilibrium is significantly disturbed, and the Ra-226/U-238 ratio may be as low as 0.3. In the case where Ra-226 activity is much higher than 25 Bq kg(-1), radioactive equilibrium is again significantly disturbed with the Ra-226/U-238 ratio reaching values as high as 4. The above findings indicate that very high values of Ra-226 in surface soils may be the result of Ra-226 transport to that soil. Concerning the concentration of Pb-210 in surface soils, the analyses performed show that it is usually higher than that of Ra-226, 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. (c) 2005 Elsevier Ltd. All rights reserved.