Determination of 129I/127I ratios in environmental samplesusing neutron activation analysis

Studies on the behavior of 129I in the environment are greatly enhanced when the concentration of the radioiodine can be related to stable 127I. The background ratios of 129I/127I of 10-10 and lower, found in uncontaminated areas, are best measured using accelerator mass spectrometry. However, there are many examples of studies where ratios higher than 10-8 have been measured, even in places located remotely from nuclear reprocessing activities. In the vicinity of reprocessing plants it is possible to find ratios between 100 and 10-7, which can be detected easily using neutron activation analysis (NAA). Stable iodine is readily determined at concentrations below 1 mg/kg in environmental materials with instrumental NAA and radiochemical techniques can be used to measure 129I to below mBq concentrations. Therefore, where there are elevated concentrations of 129I it is possible to use a combination of neutron activation techniques to determine 129I/127I ratios. This paper describes how NAA is used to measure 129I/127I ratios in milk, vegetation, and atmospheric samples. Instrumental NAA is used to measure both 129I and 127I where the ratio is between 100 and 10-3. A radiochemical procedure is used to measure 129I at ratios between 10-3 and 10-7, with a thermal neutron flux of 1016 m-2·s-1.