Natural radioactivity and external dose in the high background radiation area of Lambwe East, Southwestern Kenya

Absorbed dose rates around the North and South Ruri hills in Lambwe east, southwestern Kenya were measured using survey meters. The area lies roughly between latitudes 030S and 100S. It is bounded on the east by longitude 3430E and on the west by the shores of Lake Victoria and Winnam Gulf. The measured absorbed dose varies from 0.7 to 6.0 Gy h(1), and the mean is 2.3 Gy h(1). Assuming an outdoor occupancy factor of 0.4, the corresponding range and mean annual effective doses are 1.714.7 and 5.7 mSv, respectively. The average activity concentrations (ACs) of Th-232, Ra-226 and K-40 in samples of soils and rocks from the area are 1397, 179 and 509 Bq kg(1), respectively. Values of absorbed dose rates calculated from the ACs are general lower than those obtained from in situ measurements. Possible causes of these discrepancies were discussed in the paper.