Determination of low concentrations of 90Sr, 137Cs and 210Pb in fallout on Bulgarian territory

Fallout was sampled on five sites once a month and the distribution of 90Sr, 137Cs and 210Pb was determined. A low-background scintillation beta detector (background 1.2+-0.2 imp/min) was used in activity determination. Relations were studied between fallout density and 90Sr, 137Cs and 210Pb concentrations in the period 1970 to 1974. The results show that the concentration and density of 90Sr and 137Cs fallout increases with altitude. For 210Pb this dependence was only observed for samples from significantly different altitudes. The concentration and density of 90Sr and 137Cs markedly decreased with time. For all sampling sites seasonal variations were observed. The highest 90Sr and 137Cs concentrations were found for May to June while for 210Pb for April to May. The results show a linear correlation between fallout density and radionuclide concentration. It is shown that the mechanism of the self-cleaning of the atmosphere from long-lived 90Sr and 137Cs fission products and from natural 210Pb and 210Bi radioisotopes is the same. In so far as seasonal variations are not considered, the mean concentration of long-lived artificial radioisotopes decreases with time. (J.P.)