In this study, natural radioactivity in surface soils of Vietnam and external dose assessment to human population, deduced from activities of (226)Ra, (232)Th and (40)K 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 (226)Ra, 59.84 ± 19.81 Bq kg(-1) for (232)Th and 411.93 ± 230.69 Bq kg(-1) for (40)K. 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 18-93 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.

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A database on U, Th, K, and Cs in surface soils was established to provide inputs for the assessment of the collective dose to the population of Vietnam and to support soil erosion studies using Cs as a tracer. A total of 292 soil samples was taken from undisturbed sites across the territory and the concentrations of radionuclides were determined by gamma spectrometry method. The multiple regression of Cs inventories against characteristics of sampling locations allowed us to establish the distribution of Cs deposition density and its relationship with latitude and annual rainfall. The Cs deposition density increases northward and varies from 178 Bq m to 1,920 Bq m. High rainfall areas in the northern and central parts of the country have received considerable Cs inputs exceeding 600 Bq m, which is the maximum value that can be expected for Vietnam from the UNSCEAR global pattern. The mean activity concentrations of naturally occurring radionuclides U, Th, and K are 45, 59, and 401 Bq kg, respectively, which entail an average absorbed dose rate in air of 62 nGy h, which is about 7% higher than the world average.

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