Natural radioactivity and external dose assessment of surface soils in Vietnam

In this study, natural radioactivity in surface soils of Vietnam and external dose assessment to human population, deduced from activities of Ra-226, Th-232 and K-40 nuclides, were determined. From 528 soil samples collected in 63 provinces of Vietnam, including five centrally governed cities, the average activities were obtained and equal to 42.77 18.15 Bq kg(1) for Ra-226, 59.84 19.81 Bq kg(1) for Th-232 and 411.93 230.69 Bq kg(1) for K-40. The outdoor absorbed dose rates (OADRs) in air at 1 m above the ground level for 63 provinces were calculated, and their average value was 71.72 24.72 nGy h(1), with a range from 17.45 to 149.40 nGy h(1). The population-weighted OADR of Vietnam was 66.70 nGy h(1), which lies in the range of 1893 nGy h(1) found in the World. From the OADRs obtained, it was estimated that the outdoor annual effective dose and indoor annual effective dose to the population were 0.082 and 0.458 mSv, which are higher than the corresponding values 0.07 and 0.41 mSv, respectively, of the World. The radium equivalent activity Ra-eq and the external hazard index H-ex of surface soils of Vietnam are lower than the corresponding permissible limits of 370 Bq kg(1) and 1, respectively. Therefore, soil from Vietnam is safe for the human population when it is used as a building material.