Detection Systems for the Low Level Radiochemical Analysis of Iodine-131, Iodine-129 and Natural Iodine in Environmental Samples

A procedure based on chemical separation techniques and activation analysis has been developed for the sequential analysis of low levels of 131I, 129I and natural iodine in environmental samples. The iodine is first separated from the samples by oxidation. The separated iodine is then counted by low level, beta-gated gamm-jay spectrometry for the measurement of 131I. The chemical yield for the separation is measured by means of 125I tracer. Activation analysis is used for measurement of the separated natural iodine (127I) and 129I. The natural iodine is estimated from either the 126I or 128I activity produced in the sample. the induced 130I is used to estimate the 129I concentration. The measurement of 130I at low levels requires specialized counting methods. Interfering activities which may be present in the irradiated sample include 125I, 126I, shortlived fission-product iodine activities produced by activation of uranium impurities, and 82Br from bromine impurities. Since 130I decays with several coincident gamma-rays, multiple gamma-ray coincidence counting techniques can be used to reduce background and discriminate against interfering activities. Sevoral three-and four-segmented NaI(Tl) detectors have been used for these measurements and compared with other detector systems. The sensitivities for different coincidence counting modes were compared for several of the detector systems. Detection sensitivities-achieved for 131I and 129I are 0.02 dpm and 10-6 dpm, respectively. Measurements have been made of iodine radioactivity in the environment at concentrations below the limits established for radiation protection purposes.