Computational procedures and estimation of effective dose received by villagers residing at natural high and normal background areas of coastal regions of Tamil Nadu

Radiation exposure and effective dose received by residents of 8 villages, 4 belonging to Natural High Background Radiation Areas (NHBRA) and 4 of Low Background Radiation Areas of coastal regions of Tamil Nadu, were studied. 5 houses in each village were selected. The houses were of similar construction pattern (brick wall, tiled roof, cement flooring). External doses were estimated by gamma measurement using scintilometer and TLDs. Internal dose due to radon/thoron progeny was estimated using twin chamber Cup, SSNTD, LR-115-II, as the detector. The Cups were exposed adjacent to the TLDs, and the exposure was for a period of three months each, developed by standard procedures (10%NaOH, etching for 90 min, at 60 degC) and the tracks were counted in a spark counter. Earlier the SSNTD were calibrated using U and Th sources and calibration factors were obtained. Equilibrium factor of radon was taken as 0.4 and that of thoron as 0.03, for indoor exposure. Inhalation dose due to 232 Th was estimated by collecting air samples from each house, for one to two hours, during the replacement time of the TLDs and SSNTD Cups. For inhalation dose estimation the occupancy factor was assumed to be 0.8. Analyses of the results showed that in LBRAs, the dose contribution from external and internal sources was almost equal, 1 mSv, each. But in NHBRAs, the external dose was higher than that of the internal dose by a factor of 1.8 to 2.5. It was estimated that for the villagers of LBRAs average annual effective dose was 2.5±0.8 mSv, where as that in the NHBRAs, it is 7.00 ±2.45 mSv, the maximum being at Chinnavillai (9.84 mSv). (author)