Assessment of Background Radioactivity of Surface Soils in Ondo City, Ondo State, Nigeria

The natural radioactivity of surface soils collected from various locations in Ondo City were measured by means of well calibrated High Purity Germanium (HPGe) detector, Canberra model coupled to a versatile Canberra Multichannel Analyzer, at the Centre for Energy Research and Development of Obafemi Awolowo University. Ile Ife. The soil samples were collected from the fourteen zones that constituted the town with four samples taken from each zone. The emission line spectra used in identifying the natural radioisotopes belong to 238U and 232Th decay series and that of naturally occurring 40K. The gamma activity obtained for 40K ranged from 212.1 Bq/kg to 895.7 Bq/kg with a mean value of 470.4 ± 26.8 Bq/kg. For 232Th, the gamma activity ranged from 30.2 Bq/kg to 72.4 Bq/kg with a mean value of 48.8 ± 8.4 Bq/kg, and from 3.6 Bq/kg to 14.2 Bq/kg with a mean value of 6.9 ± 1.2 Bq/kg for 238U. The mean absorbed dose rate in these locations amounts to 45.36 nGy/h which by approximate conversion gives an indoor annual effective dose of 0.223 mSv/year and outdoor annual effective dose of 0.056 mSv/year. These values constitute the baseline dose equivalent, which is below the 1.0 mSv/year recommended by the International Commission for Radiological Protection (ICRP, 1990) as the maximum permissible dose equivalent for members of public. The results obtained showed that the study area is free from radiological contamination. Keywords: Radioactivity, Absorbed dose, Radioisotopes, Environment