Abstract
The aim of this study was to evaluate the level of natural radioactivity in river sediments and riverbank surface soils collected along the Chao Phraya River and its tributaries in Thailand. The activity concentrations of naturally occurring radioactive materials in the 238U and 232Th decay chains and from 40K were determined by means of a gamma-ray spectrometry system using a hyper-pure germanium (HPGe) detector in a low background configuration. The ranges of activity concentrations of 238U, 232Th and 40K were found to be 13.9±0.4 ↔ 76.8±1.7, 12.9±0.3 ↔ 142.9±2.8 and 178.4±6.1 ↔ 810.7±26.7 Bq.kg-1, respectively. The results of this current study have been compared with the world mean values of 35, 30 and 400 Bq. kg-1, respectively, specified by the UNSCEAR (2000). The artificial radionuclide, 137Cs, was not observed in statistically significant amounts above the background level in the current study. Additionally, the distribution of the activity concentrations to various particle sizes of soil was investigated in this current study. It can be assumed from the results that the grain size or the fraction of sand and clay content is one of the factors that affects the activity concentrations of 238U and 232Th in the soil samples, while only a weak correlation between the activity concentration of 40K and the particle size in soil samples was observed in this current study. Concerning radiological risk to human health, the absorbed gamma dose rate (D) in air at 1 metre above the ground surface was estimated to lie in the range 21.7±0.4 to 155.7±2.2 nGy.h-1; the outdoor annual effective dose equivalent (AEDE) was evaluated to vary from 26.6±0.4 to 190.9±2.7 μSv.y-1, with the arithmetic mean value of 79.06±33.23 μSv.y-1, which is comparable to the worldwide effective dose of 70μSv.y-1. Also, the values of the Raeq and the Hex for all soil samples in the present work are lower than the accepted safety limit value of 370 Bq.kg-1 and below the limit of unity, respectively. The results indicate that the radiation hazard from primordial radionuclides in all soil samples from the area studied in this current work is not significant.