Evaluation of cosmic and terrestrial gamma dose around Trombay

Contribution from cosmic radiation to atmosphere and evaluation of terrestrial, (naturally occurring and fallout) radionuclides in soil constitute significant component of background radiation exposure to population. The activity concentration of the radionuclides in soils is directly relevant to outdoor gamma exposure. In order to evaluate the total outdoor gamma exposure to the residents around Trombay, gamma spectrometry measurement of surface soils and direct dose measurements using survey meter were carried out. The average activity concentrations of 238U, 232Th, 40K and 137Cs were found to be 14 ± 0.7, 21.3 ± 1.3, 159 ± 7.9 and 3.5 ± 0.2 Bq/kg respectively. The calculated absorbed dose rate due to naturally occurring radionuclides ranged between 12.2 ± 0.4 to 76.2 ± 2.3 nGy/h and due to 137Cs ranged between 0.2 ± 0.006 to 3.8 ± 0.15 nGy/h. Thus contribution to dose from 137Cs is almost 5 to 60 times lesser than from natural radionuclides. The measured outdoor gamma dose rate in air at 1 m above ground ranged between 30-96 nGy/h, the difference being contributed by cosmic ray which calculates between 6.6 ± 0.3 to 24.2 ± 1.2 nGy/h. Using the data obtained in this study, the average annual effective dose equivalent for a person living in Anushaktinagar is found to be about 54 ± 4.2 μSv where the mean terrestrial outdoor gamma radiation dose due to fallout radionuclide (137Cs) is only about 1.6 ± 0.1 μSv/y. (author)