NATURAL RADIOACTIVITY IN SURFACE SOIL SAMPLES FROM DWELLING AREAS IN TEHRAN CITY, IRAN

The study was carried out to determine radioactivity concentrations in surface soil samples of the city of Tehran and associated potential radiological hazards. The natural radionuclide (Ra-226, Th-232 and K-40) contents were determined for 50 locations throughout the geological surface formations in a representative area in the city of Tehran, Iran, using high-resolution gamma-spectrometric analysis. The range of activity concentrations of Ra-226, Th-232 and K-40 in the soil from the studied areas varies from 30.5 +/- 0.6 to 45.4 +/- 0.9, 27.3 +/- 0.5 to 57.1 +/- 1.1 and 328.0 +/- 4.6 to 768.5 +/- 13.4 Bq kg(-1) with overall mean values of 38.8 +/- 0.7, 43.4 +/- 0.8 and 555.1 +/- 8.9 Bq kg(-1), respectively. The mean radium equivalent activity, external hazard index, internal hazard index to quantify the internal exposure to radon and its daughter products, as well as the gamma activity concentration index for each sample are 143.6 +/- 4.6 Bq kg(-1), 0.39, 0.49 and 0.53, respectively. The average estimated radium equivalent is comparable with reported values for many countries in the world. Therefore, these areas may not pose radiological risks to the inhabitants due to harmful effects of the ionising radiation from the natural radioactivity in soil. The calculated average external and internal hazard indexes were found to be less than unity, as a recommended safe level. Estimates of the measured radionuclide content have been made for calculating the absorbed dose rate in the outdoor air at 1 m above the ground level. The absorbed dose rates resulting from those concentrations ranged from 48.1 to 88.7 nGy h(-1). Assuming a 20 % occupancy factor proposed by UNSCEAR, 2000, the corresponding effective dose rates in outdoor air equivalent to the population were calculated to be between 0.06 and 0.11 mSv y(-1). The measurement results and calculated values obtained from this study indicate that the dwelling areas in Tehran city, Iran, have background radioactivity levels within natural limits.