Gamma ray spectrometric analysis of sand samples from selected beaches on coastline of Kenya

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Odongo, Willis Otieno
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Kenyatta University
Background radiation from natural sources is the major source of external dose in our environment. The major radionuclides responsible for this are 40K, 232Th, 238U and their products of decay. Beach sands are weathering resistant residuals of geological formations which gets to the beach by erosion from mainland or deposition through waves and current action from the ocean. Waves wash back lighter grains leaving behind heavy ones which may be rich in minerals such as zircon, ilmenite, garnet and monazite, which are associated with elevated levels of naturally occurring radionuclides. In addition to that, Indian Ocean waters are polluted with traces of oil spillages and radioactive wastes from damping all of which may be brought to the beach by wave action and have great association with these radionuclides. Low level whole body background radiation exposure to the public for a long time can lead to some of the radiation health effects like acute leucopoenia, leukemia, cancer and genetic effects. Thus, public awareness of natural radiation levels in these beaches is necessary. Studies of beach sand in other countries have shown higher levels in the radiological parameters than the recommended limit. Radioactivity levels from natural radionuclides on Kenyan beaches is not known despite the fact that many people spend a lot of time relaxing in these beaches. The objective of this study is to assess human exposure to natural sources of radiation by measurements of their activity concentration using NaI (Tl) gamma ray detector, then to determine the radiological parameters like radium equivalent, absorbed dose rates and hazard indices. Sand samples were obtained from (Shanzu, Nyali, Kenyatta on North coast and Tiwi, Shelly, Diani on South coast). Activity concentration of 40K ranged between 653 ± 32 Bq/kg and 2117 ± 105 Bq/kg with a mean of 1254 ± 62 Bq/kg, 238U ranged between 43 ± 2 and 118 ± 5 Bq/kg with a mean of 87 ± 4 Bq/kg and 232Th ranged between 42 ± 2 Bq/kg to 127 ± 6 Bq/kg with a mean of 98 ± 4 Bq/kg. All the mean concentration values were higher than the world averages of 400 Bq/kg, 33Bq/kg and 45 Bq/kg for 40K, 238U and 232Th respectively. Analysis of variance for the mean activity concentration across all the beaches showed no significant difference at 0.05 level of significance except for 238U in Nyali beach. Radium equivalent varied from 220 ± 11 Bq/kg to 455 ± 23Bq/kg with a mean of 327 ± 16 Bq/kg which below recommended limit of 370 Bq/kg. Internal hazard index varied from 0.67 ± 0.03 to 1.37 ± 0.06 with a mean of 0.98 ± 0.04. External hazard index varied from 0.48 ± 0.02 to 1.02 ± 0.05 with a mean of 0.72 ± 0.03. The outdoor annual effective dose rate varied from 0.16 ± 0.01 mSv/y to 0.32 ± 0.02 mSv/y with a mean of 0.23 ± 0.01 mSv/y. Indoor annual effective dose rate varied from 0.43 ± 0.02 mSv/y to 0.89 ± 0.04 mSv/y with a mean 0.63 ± 0.03 mSv/y. All these parameters are below maximum recommended limit of one. Generally these results indicate no significant radiological health hazards for the studied beaches. I recommend more studies on ocean waters of south coast beaches since they have relatively higher activity concentration, houses made of beach sand to have more ventilations. Reduced level of oil spillages and radioactive waste damping in ocean waters as this gets to beaches by wave action thus increases the levels of the radionuclides
A thesis submitted in partial fulfillment of the requirement for the award of the degree of masters of science (physics) in the school of pure and applied sciences of kenyatta university. December, 2017