Abstract

Mrima Hill, located in the South coast of Kenya is known for high natural background radiation, due to the presence of radiogenic heavy minerals such as monazites and carbonatites. The activity concentration of ²³⁸U, ²³²Th and ⁴⁰K in soil samples from the hill have been determined by gamma ray spectrometry using NaI(Tl) detector and decomposition of measured gamma-spectra. As a measure of radiation hazard to the public, gamma radiation dose rates were also estimated. The average activity concentrations of ²³⁸U, ²³²Th and ⁴⁰K were 207.0±11.3, 500.7±20.0 and 805.4±20.0 Bqkg^-1, respectively. The mean absorbed dose rate in air is 440.7±16.8 nGyh^-1 while the estimated annual average effective dose rate is 1.11±0.01 mSvy^‑1. The absorbed dose rate due to gamma radiation from naturally occurring radioactive materials is above the global average value of 60 nGyh^-1 (UNSCEAR, 2000).

Key words: High natural background radiation, gamma ray spectrometry, NaI(Tl) detector, Mrima Hill.