Rapid in-situ radiometric assessment of the Mrima-Kiruku high background radiation anomaly complex of Kenya

This paper presents the radiometric survey results of the Mrima-Kiruku high background radiation (HBR) anomaly complex of south coastal Kenya. Utilizing a portable γ-ray spectrometer consisting of a 2.0 l NaI(Tl) backpack detector integrated with GPS to perform the relevant in-situ radiometric measurements, a novel geospatial gating method was devised to represent the measurements. The goal of this study was to assess radiation exposure and associated natural radioactivity levels in the complex and to compare the results obtained with those from previous preliminary related studies. Absorbed dose-rates in air were found to range <60-2368 nGy h−1. These rates were observed to correspond with the spatial variability of the underlying geology and terrain, increasing toward the summits of both Mrima and Kiruku Hills which implies that the complex is a geogenic HBR anomaly. The activity concentrations of 232Th in the study area are generally higher than those of 40K and 238U: The means of 40K, 238U and 232Th ranged 235±19-603±28 Bq kg−1, 68±6-326±24 Bq kg−1 and 386±12-1817±51 Bq kg−1 respectively. It was concluded that the high air absorbed dose-rate values that were measured (>600 nGy h−1) are due to elevated activity concentrations of 232Th. Therefore there is significant (>1 mSv/y) radiological hazard to the inhabitants of the area particularly those who reside at the foothills of both Mrima and Kiruku Hills.