MEASUREMENT OF NATURAL RADIOACTIVITY OF SOILS FROM SELECTED CITIES IN OYO STATE, SOUTHWESTERN NIGERIA

Abstract

The concentrations of some of the natural radionuclides in the uranium, actinium and thorium series and of 40K in soil samples collected from 9 major towns of Oyo State, Southwestern Nigeria have been determined by gamma ray spectrometry (using high purity germanium detector). Spectral analyses were performed with the Genie2k spectrometry software, version 2.1 (Canberra Industries Inc.). A library of radionuclides, which contained the energy of the characteristic gamma peaks for each nuclide analysed and their corresponding emission probabilities were built from the data supplied in the software. The natural radionuclides that were discovered in the soil samples include, 40K, 208TI, 2l0Pb, 212Bi, 212Pb, 214Bi, 214Pb, 224Ra, 226Ra, 228Ac, 228Th 234Mpa 234Th and 235U 137Cs was the only artificial radionuclide found in trace form in some of the samples analysed. The specific activities ranged from -0.102 ± 0.29 for 137CS to 1,850.60 ± 36.83 for 40K. The outdoor gamma radiation absorbed dose rate in air due to each of the primordial radionuclides was calculated from the mean values of the activity concentration in each sample. The estimated annual outdoor effective dose equivalent for the urban areas in the state is 50.25 μSvy-1 while that of rural area is 108.57 μSvy-1. Average value for the state is 79.41 μSvy-1 Using the estimated population of 6,617,720 for the year 2005, the estimated collective effective dose equivalent for the state is 5.26 x 102 man-Svy-1. The expected number of people in the state at risk of incurring radiation-induced cancer per year was found to be 10 (i.e. risk G = 2 from the urban areas, and 8 from rural areas).