Natural Radionuclides and 137Cs in Mosses and Lichen in Eastern Serbia

Abstract

The paper presents the results of ra dionuclides determination in mosses [Homolothedum sp., Hypnum Cupressiforme sp.. and Brachythecium sp.) and lichen {Cladonia sp.), sampled in the region of Eastern Serbia, during 1996-2010. Activities of 7Be, 40K, 226Ra, 232Th, and 137Cs were determined on an HPGe/ORTEC/Ametek detector (rela tive efficiency 34 %, resolution 1.65 keV at 1.33 MeV) and HPGe/Canberra detector (relative efficiency 25 %, re solution of 1.95 keV at 1.33 MeV) by standard gamma spectrometry. The average counting time was 60 000 s. The standard error of the method was 10 %. Spectral analysis was performed with the Gamma Vision 32 software package. The majority of the 40K activities mea sured in moss was within the range (100-500) Bq/kg dw (dry weight), while most of the 226Ra and 232Th activities of were in the range (5-50) Bq/kg dw. The calculated "soil-tomoss" transfer factors for 40K, 226Ra, and 232Th were 0.45, 3, and 0.3, respectively. Over the study period, the majority of the 137Cs activities in moss were in the range (0-500) Bq/kg dw, with less than 10 % of the samples with the activity higher than 1000 Bq/kg dw. The spatial distribution of the 137Cs activity was highly non-uni form over the region. The variations in the content of natural radionuclides among the three most sampled moss species were not significant. Linear Pearson correlation coefficient was 0.68 for 226Ra versus 232Th, and 0.24 for 137Cs versus 40K. The content of natural radionuclides in lichen was to some extent less than in moss, as lichen was collected from hi gher points (trees), and the effect of resuspension from soil was thus less pronounced. Still, the frequency pat tern of activities of natural radionu clides in lichen resembled the pattern seen in moss. The 137Cs activities in li chen were also less than in moss, with most of the concentrations below 300 Bq/kg dw. The mean activities of 7Be in moss and lichen sampled in Eastern Serbia in 2006 and 2008 were in the range (41- 122) Bq/kg dw, with variations up to 71 %.The 7Be concentrations were si gnificantly (20-60 %) higher in 2008 than in 2006. The pronounced ’Be va riations (28-71 %) between the sites and within a year were mainly a result of its short half-life and the differences in microclimate and topology of the sites.