Evaluation of the natural radioactivity level of Nigerian tar sand deposits, eastern Dahomey basin, southwestern Nigeria

Abstract

The specific activities of natural radionuclides in twenty seven samples collected from the east- ern Dahomey basin in southwestern Nigeria were evaluated. Experimental results were ob- tained by using a 3''×3'' sodium iodide NaI(Tl) detector. A major trace element assessment of the samples was made by Inductively Coupled Plasma Mass Spectrometry techniques. Gamma ray radioactive standard sources,137Cs and60Co, were used to calibrate the measurement sys- tem and the International Atomic Energy Agency SOIL-375 radioactive standard source was also used to analyze and compute the specific activities of desired natural radionuclides. Three radioelements, viz.238U,232Th and40K, were identified in the samples with the following specific activities of 238U ranging from 9.88 ± 4.70 Bq/kg to 69.15 ± 12.37 Bq/kg with an average of 26.50 ± 7.18 Bq/kg;232Th from 12.78 ± 5.16 Bq/kg to 36.86 ± 13.35 Bq/kg with an average of 22.77 ± 4.28 Bq/kg;40K ranging from 189.82 ± 79.51 Bq/kg to 518.77 ± ±.119.54 Bq/kg with an average of 297.69 ± 16.21 Bq/kg. The result was compared with the world mean values of 35, 30 and 400 Bq/kg, respectively, specified by the United Nations Sci- entific Committee on the Effects of Atomic Radiation. The mean result obtained for the radium equivalent, total absorbed dose rate, external hazard index, internal hazard index, and the an- nual effective dose equivalent, were 79.90 Bq/kg, 38.50 μSv/h, 0.22 Bq/kg, 0.29 Bq/kg, and 47.22 μSv, respectively. With respect to radiological risk to human health, the absorbed gamma dose rate in air was estimated to be in the range of 21.7 ± 0.4 to 155.7 ± 2.2 μSv/h; the outdoor annual effective dose equivalent was evaluated to vary from 26.6 ± 0.4 to 190.9 ± ±.2.7 μSv with the arithmetic mean value of 79.06 ± 33.23 μSv and compared to the world- wide effective dose of 70 μSv. Also, the values of the radium equivalent and the external hazard index for all samples in the study area were found to be lower than the accepted safety limit value of 370 Bq/kg and equally below the limit of unity, respectively. The results indicate that, at present, the radiation hazard from radionuclides in all samples analyzed is within permissible limits.