Determination of Naturally Occurring Radioactive Elements and Radiation Exposure Levels in the Soapstone Quarries of Tabaka Region of Kisii District, Kenya

Vincent Otwori Atambo


A radiological study was carried out in the soapstone quarries of Tabaka region of Kisii district in the Southern Nyanza province, Kenya, where soapstone is mined and used as a carving medium. In this study, 14 soil and rock samples collected from five quarries were analyzed using high-resolution gamma-ray spectroscopy. The absorbed dose rates were measured 1metre above the ground at each quarry using a model 2000 Canberra radiagem. Various radionuclides were identified in the samples. The activity concentrations of radionuclides 232Th, 40K and 226Ra in the samples as well as other radiological parameters were determined. The activity concentrations for 232Th ranged from 38.60 to 271.70 Bqkg-1, 226Ra ranged from 43.10 to 360.00 Bqkg-1 and 40K ranged from 245.00 to 1780.00 Bqkg-1. The average absorbed dose rate for the five quarries measured 1metre above the ground was 541.40 nGyh-1. The calculated absorbed dose rates were found to be in the range of 87.49 to 356.77 nGyh-1, with an overall average value of 183.79 nGyh-1 which was 4 times higher than the world average (43 nGyh-1). The corresponding annual effective dose rates due to the radionuclides 232Th, 226Ra and 40K in the quarries ranged from 0.22 to 0.88 mSvy-1, with a mean of 0.44 mSvy-1, assuming a 40% occupancy factor. Both the internal and external hazard indices were found to be more than unity (1.02 and 1.28 respectively), hence exceeding the permissible limits set by International Commission on Radiological Protection, 2000. The annual effective dose in the quarries was less than 1 mSvy-1, the limit acceptable for the public.


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