AMS analysis of 129I in Japanese soil samples collected from background areas far from nuclear facilities

Analytical procedures in the determination of iodine-129 (half-life: 1.6 x 10(7) y) have been studied using accelerator mass spectrometry (AMS), with special references to the separation procedures of iodine from soil samples for the AMS measurement. Iodine was successfully volatilized from soil samples by pyrohydrolysis at 1000 degrees C and collected in a trap solution. Iodine was purified from the matrix by solvent extraction. Finally, it was precipitated as silver iodide to make a target for AMS. In order to obtain information on the I-129/I-127 ratio in a chemical blank (or iodine carrier), we have determined the ratios in several iodine reagents and found that the ratios fell in a narrow range around 1.7 x 10(-13). The detection limit for soil sample (1 g material) by the present method was about 0.01 mBq/kg or 4 x 10(-11) as the ratio of stable iodine (I-129/I-127 ratio), i.e. these values were much better than that by neutron activation analysis (NAA) used in our previous studies. We have applied this method in the analysis of soil samples collected from different places in Japan. We could successfully determine 1291 in soil samples with low 1291 concentrations, which could not be detected by NAA. Sample size necessary for the soil analysis by AMS was only about 0.5g or less, whereas about 100g of the sample were required for NAA [Muramatsu, Y., Ohmomo, Y., 1986. Iodine-129 and iodine-127 in environmental samples collected from Tokaimura/ Ibaraki, Japan. Sci. Total Environ. 48, 33-43]. Using this method, new data were obtained for the I-129 levels in 20 soil samples collected from background areas far from nuclear facilities, and the ranges were 1.4 x 10(-5)-4.5 x 10(-3) Bq/kg as I-129 concentrations and 3.9 x 10(-11)-2.2 x 10(-8) as I-129/I-127 ratios. These values are useful in understanding the I-129 levels in Japanese environments. Higher I-129 concentrations were observed in forest soils than those in field and rice paddy soils should be related to the interception effect of atmospheric I-129 due to tree canopies. Relatively high I-129/I-127 ratios found in rice paddy soils could be explained by their low stable iodine concentrations which were caused by the desorption of stable iodine from the rice paddies during the cultivation. (c) 2007 Elsevier Ltd. All rights reserved.