Abstract
Measurements of ambient gamma radiation dose and radioactivity of soil and rocks samples have been carried out in 103 sites located in the region of Marrah mountain (western Sudan) using survey meters and gamma spectrometry techniques, respectively. The ambient dose varied from 0.05 to 0.25 mu Sv/h with mean value of 0.18 +/- 0.05 (SD) mu Sv/h. At high altitudes (> 1500 m), relatively high values were observed (0.18-0.22 mu Sv/h). The levels of U-238, Th-232 and K-40 in soil-rocks were in the ranges 12-80, 12-66 and 120-937 Bq/kg with mean +/- SD of (41 +/- 6.9), (24 +/- 12) and (540 +/- 99) Bq/kg, respectively, equivalent to total gamma absorbed dose of 23-142 (66 +/- 20) nGy/h. The ambient dose data were presented in cartography format, and correlation to geological maps was attempted. Surface map of the absorbed dose created using geostatistical kriging tools showed the spatial distribution of dose. Analyses showed that trends of high values were located in and around areas characterized by local flow and basaltic flow lithology, while the low values were observed at locations of basement rocks (granitic) in general. That was attributed to the differences in environmental conditions in various altitude and weathering processes. The highest readings (0.25 mu Sv/h and 142 nGy/h) were found in Deriba Complex Pumice air-fall deposits (younger explosive phase). Health risk was assessed by calculating annual effective dose for the inhabitants resulted in values range between 0.14 and 1.4 mSv/y (mean +/- SD of 0.40 +/- 0.12 mSv/y). The observed values of dose rates are comparable with some countries but higher than the worldwide average for outdoor annual affective dose of 0.07 mSv/y.