Radiological investigations at the “Taiga” nuclear explosion site, part II: man-made γ-ray emitting radionuclides in the ground and the resultant kerma rate in air

Abstract

Samples of soil and epigeic lichens were collected from the “Taiga” peaceful nuclear explosion site (61.30°N 56.60°E, the Perm region, Russia) in 2009 and analyzed using high resolution γ-ray spectrometry. For soil samples obtained at six different plots, two products of fission (137Cs and 155Eu), five products of neutron activation (60Co, 94Nb, 152Eu, 154Eu, 207Bi) and 241Am have been identified and quantified. The maximal activity concentrations of 60Co, 137Cs, and 241Am for the soils samples were measured as 1650, 7100, and 6800 Bq kg−1 (d.w.), respectively. The deposit of 137Cs for the top 20 cm of soil on the tested plots at the “Taiga” site ranged from 30 to 1020 kBq m−2; the maximal value greatly (by almost 3 orders of magnitude) exceeded the regional background (from global fallout) level of 1.4 kBq m−2. 137Cs contributes approximately 57% of the total ground inventory of the man-made γ-ray emitters for the six plots tested at the “Taiga” site. The other major radionuclides –241Am and 60Co, constitute around 40%. Such radionuclides as 60Co, 137Cs, 241Am, and 207Bi have also been determined for the epigeic lichens (genera Cladonia) that colonized certain areas at the ground lip produced by the “Taiga” explosion. Maximal activity concentrations (up to 80 Bq kg−1 for 60Co, 580 Bq kg−1 for 137Cs, 200 Bq kg−1 for 241Am, and 5 Bq kg−1 for 207Bi; all are given in terms of d.w.) have been detected for the lower dead section of the organisms. The air kerma rates associated with the anthropogenic sources of gamma radiation have been calculated using the data obtained from the laboratory analysis. For the six plots tested, the kerma rates ranged from 50 to 1200 nGy h−1; on average, 51% of the dose can be attributed to 137Cs and 45% to 60Co. These estimates agree reasonably well with the results of the in situ measurements made during our field survey of the “Taiga” site in August 2009.