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.

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