Radioactivity and elemental analysis of carbonatite rocks from parts of Gwasi area, South Western Kenya

Abstract

Rocks and soils from a number of areas under lied by carbonatite rocks in Kenya have been associated with high radioactivity. In this thesis, the analyzed results of the background radiation measurements in the field, activity concentrations of primordial radionuclides in the samples, approximated annual external effective dose rates and the elemental concentrations of samples from Ruri hills have been presented. Ruri hills are roughly between latitudes 0˚30’S and 1˚00’S, bounded on the east by longitude 34˚30’E and on the west by the shores of Lake Victoria and the Winnam Gulf. Altitudes range from about 1000m on the shores of Lake Victoria to above 1800m on top of the Ruri hills. Gamma spectrometric analyses of twenty one samples from South and North Ruri hills in Lambwe east location of Suba District, south western Kenya, have been carried out using high-purity germanium (HPGe) gamma-ray detector. The samples comprise rocks (mainly carbonatite) and soils. The activity concentrations ranged from 14.18 to 6559.99 Bqkg-1 with an average of 1396.85Bqkg-1 for thorium-232; from 2.73 to 499.24Bqkg-1 with an average of 178.69Bqkg-1 for uranium-238 equivalent; and from 56.67 to 1454.73Bqkg-1 with an average of 508.67Bqkg-1 for potassium-40. The corresponding ranges of elemental concentrations, and the mean values in ppm are 25.151346.67, 280.04; 6.89-137.33, 27.91; and 438.00-3060.00, 1345.90; for thorium, uranium and potassium respectively. Absorbed dose rates in air outdoors at approximately 1m above the ground were measured using survey meters. The values are in the range of 700 to 6000.00nGyh-1, with an overall mean value of 2325.84nGyh-1. These values correspond to individual annual effective dose rates ranging from 1.717 to 14.717mSvyr-1 with an overall mean of 5.705mSvyr-1, assuming a 40% (0.4) occupancy factor. Thus, annual external effective dose rates at Ruri hills are much higher than the global average of 0.46mSvyr-1 (i.e. approximately twelve times the global average).