Abstract
Dolon village located 60 km from the border of the Semipalatinsk test site is known to be heavily contaminated by the radioactive plume originating from the first USSR atomic bomb test on August 29, 1949. External radiation of about 2 Sv was estimated in Dolon, while it was recently reevaluated to be about 0.5 Sv based on new TL measurements and other data. A possible explanation of this difference was that the previous estimates were based on the exposure rate measured just above the center-axis of the radioactive trace, but the location of Dolon village was several kilometers away from there. In October 2005, in order to confirm the position and width of the radioactive plume that contaminated Dolon village, an extensive soil sampling was carried out at 26 locations concentrating along the line perpendicular to the supposed center-axis of the radioactive plume. Radioactivities of 137Cs and 239,240Pu were measured and inventory values in soil (Bq/m2) were plotted as a function of the distance from the supposed center-axis. Peak-like shapes were observed in the spatial distribution both for 137Cs and 239,240Pu, making maxima near the supposed center-axis. By fitting Gaussian function to the obtained data, the position of plume centerline was estimated about 2 km north from Dolon, and the σ value of the plume width was 2 – 2.2 km. The width of the plume was very small considering the distance of 110 km between the ground-zero and Dolon. The peak 137Cs value of 15 kBq/m2 was evaluated at the centerline of the plume, while the average 137Cs deposition of 6 kBq/m2 was obtained in the area of Dolon village (1.8 – 2.6 km from the centerline). Meanwhile, 8.5 kBq/m2 of 137Cs deposition was previously obtained as the average of 49 soil samples taken around Dolon, which is not inconsistent with the current estimate based on the Gaussian spatial distribution.
