Natural Radioactivity and Radiological Dose Assessment of Rocks from Mbeere South, Kenya

Natural radioactivity in rocks and building materials is a major contributor to outdoor gamma radiation exposure. This study determined the activity concentrations of ²³⁸U (estimated via ²²⁶Ra assuming secular equilibrium), ²³²Th and ⁴⁰K and assessed radiological hazard indices in 30 purposively sampled rocks from Mbeere South, Embu County, Kenya using high-resolution HPGe gamma-ray spectrometry. Mean activity concentrations were 19.02 ± 14.00 Bq∙kg⁻¹, 38.66 ± 34.43 Bq∙kg⁻¹ and 634.88 ± 302.94 Bq∙kg⁻¹ for ²³⁸U, ²³²Th and ⁴⁰K, respectively (⁴⁰K > ²³²Th > ²³⁸U). ²³⁸U was below the world average of 35 Bq∙kg⁻¹, while ²³²Th and ⁴⁰K were 28.9% and 58.7% above world averages of 30 and 400 Bq∙kg⁻¹. Mean radium equivalent activity (Ra_eq), absorbed dose rate (D) and outdoor annual effective dose (AED_out) were 123.19 ± 73.10 Bq∙kg⁻¹, 58.61 ± 33.21 nGy∙h⁻¹ and 0.0719 ± 0.0407 mSv∙y⁻¹, respectively. All Ra_eq values were below the 370 Bq∙kg⁻¹ screening limit, and mean D and AED_out were comparable to world averages of 59 nGy∙h⁻¹ and 0.07 mSv∙y⁻¹. These values were 53% - 91% lower than high-background areas in Kenya, Nigeria, Saudi Arabia and Brazil, and comparable to Jaintiapur, Bangladesh. The results indicate low external gamma hazard; suitability with respect to indoor radon was not assessed.