Radiometric Assessment of Natural Radioactivity Levels and Radiation Hazard Indices for Soil Samples in Kericho County, Kenya
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Throughout their lives, human beings are continually exposed to radiation from terrestrial radionuclides particularly 40K, 238U, 232Th and their decay products. The radiation, usually in the form of gamma rays and alpha particles, is largely attributed to building materials for instance rock and concrete mainly used in developed countries and cities around the world, or soil which is principally used in most parts of rural Africa. Radiation exposure from terrestrial sources may be more of a concern in indoor settings under certain conditions. As an example, houses constructed using building materials high in 238U and 232Th content often contain elevated concentrations of radon which, according to the World Health Organization, is the leading cause of lung cancer among non-smokers. Owing to the potential risk, many countries especially in the developed world have quantified 40K, 238U and 232Th in their environments and mapped the radiation risk-prone areas. In Kenya however as in most parts of Africa, data on the terrestrial radionuclides is largely absent. The reported study involved measurement of specific activities of 40K, 238U and 232Th in surface soil of Bureti Sub-County, Kericho County, Kenya, for purposes of evaluating the risk indices that included radium equivalent activity (Raeq), external hazard index (Hex), internal hazard index (Hin), outdoor gamma dose rate and outdoor annual effective dose rate. Bureti Sub-County has a rural setting and as such, soil forms the main building material for house construction. The region is also a relatively rich agricultural zone with tea and maize being the main crops grown. Soil samples were randomly collected from twenty one sampling stations within tea and maize plantations and after requisite preparation analyzed for 40K, 238U and 232Th activity concentrations using calibrated NaI (Tl) gamma ray spectrometer and the associated electronics. The specific activities of 40K varied from 39 ± 2 to 2129 ± 106 BqKg-1 with an average of 1086 ± 49 BqKg-1 , 238U ranged from 34 ± 2 to 265 ± 13 BqKg-1 with a mean of 143 ± 7 BqKg-1 whereas 232Th ranged from 3 ± 1 to 150 ± 9 BqKg-1 with a mean of 95 ±4 BqKg-1 . The average values of 40K, 238U and 232Th exceeded the world averages by factors of 2.6, 4.3 and 2.1 respectively. Raeq varied from 130 ±7 to 590 ± 30 BqKg-1 with a mean of 362±18 BqKg-1 which is within the recommended range while Hex and Hin varied from 0.35± 0.02 to 1.60± 0.08 with a mean of 0.98±0.05 and 0.44±0.02 to 2.16±0.11 with a mean of 1.36±0.07 respectively. While Hex average value is within the reference level of 1, that of Hin exceeds the value by 36%. The estimated outdoor absorbed gamma dose rate ranged from 59 ±3 to 273±14 nGyh-1 with a mean of 169 ±8 nGyh-1, about 3 times the world average. The outdoor annual effective gamma dose rate averaged 0.41 ± 0.02 mSvy-1 which though significantly higher than the world average of 0.07 mSvy-1 is less than half the maximum dose constraint of 1 mSvy-1 recommended by ICRP for the general public. Thus, radiation exposure in Bureti Sub-County is within the acceptable safety limit. Nonetheless, soil when used as a building material may to some degree contribute to indoor radon concentration hence the need to quantify radon concentration in earthen dwellings in the region.