Natural radioactivity and radon emanation coefficient in the soil of Ninh Son region, Vietnam

Abstract

The natural radioactivity (238U, 226Ra, 232Th and 40K) and radon emanation coefficient for 57 soil samples belonging to alluvial, red, forest surface, slip-debris, metamorphic and sandy soil of the Ninh Son region in Ninh Thuan province have been determined. The soil gas radon was measured by in-situ with RAD7 radon monitor coupled with a soil gas probe while activity concentrations of 238U, 226Ra, 232Th, and 40K were measured by an HPGe gamma-ray spectrometry system. The 226Ra/238U disequilibrium occurred in the soil samples and a great majority of the 226Ra/238U values lie above 1. Average activity concentrations of 226Ra, 232Th, and 40K are significantly higher than the worldwide average concentrations in soils published by UNSCEAR, 2008. The gamma dose rate ranged from 55 ± 2 to 248 ± 7 nGy h−1 with an average of 130 ± 4 nGy h−1 which is greater than the world value. Strong positive correlations were recorded between 238U and 226Ra, 232Th and 226Ra, 232Th and 238U, and 226Ra and 222Rn. The results of weathering and alteration processes were proposed to be dominated reasons for the 226Ra/238U disequilibrium occurred in the soil samples. Most of the radon in soil gas samples are considered “normal risk” or low radon index. The mean values of the emanation coefficient for alluvial, red, forest surface, slip-debris, metamorphic and sandy soil were found to be 0.51 ± 0.03, 0.40 ± 0.02, 0.36 ± 0.02, 0.30 ± 0.02, 0.26 ± 0.02 and 0.15 ± 0.01, respectively. Radon emanation was found to be an inverse function of grain size for grain sizes larger than 0.1 mm in diameter and independent on the radium content of the soil sample.