Measurement of activity concentrations of 40K, 226Ra and 232Th for assessment of radiation hazards from soils of the southwestern region of Nigeria

Activity concentrations of the selected radionuclides K-40, Ra-226 and Th-232 were measured in surface soil samples collected from 38 cities in the southwest region of Nigeria by means of gamma spectroscopy with a high-purity germanium detector. Measured activity concentration values of K-40 varied from 34.9 +/- A 4.4 to 1,358.6 +/- A 28.5 Bq kg(-1) (given on a dry mass (DM) basis) with a mean value of 286.5 +/- A 308.5 Bq kg(-1); that of Ra-226 varied from 9.3 +/- A 3.7 to 198.1 +/- A 13.8 Bq kg(-1) with a mean value of 54.5 Bq kg(-1) and a standard deviation of 38.7 Bq kg(-1), while that of Th-232 varied from 5.4 +/- A 1.1 to 502.0 +/- A 16.5 Bq kg(-1) with a mean value of 91.1 Bq kg(-1) and standard deviation of 100.9 Bq kg(-1). The mean activity concentration values obtained for Ra-226 and Th-232 are greater than the world average values reported by the United Nations Scientific Committee on Effects of Atomic Radiation for areas of normal background radiation. Radiological indices were estimated for the radiation/health hazards of the natural radioactivity of all soil samples. Estimated absorbed dose rates in air varied from 12.42 +/- A 2.25 to 451.33 +/- A 19.06 nGy h(-1), annual outdoor effective dose rates from 0.015 +/- A 0.003 to 0.554 +/- A 0.023 mSv year(-1), internal hazard index from 0.10 +/- A 0.03 to 3.02 +/- A 0.16, external hazard index from 0.07 +/- A 0.01 to 2.60 +/- A 0.11, representative level index from 0.19 +/- A 0.03 to 6.84 +/- A 0.29, activity index from 0.09 +/- A 0.02 to 3.42 +/- A 0.15, and radium equivalent activity from 26.95 +/- A 5.04 to 963.15 +/- A 41.87 Bq kg(-1). Only the mean value of the representative level index exceeds the limit for areas of normal background radiation. All other indices show mean values that are lower than the recommended limits.