Distribution of Ra-226, Th-232, and K-40 in soils of Rio Grande do Norte (Brazil)

A survey programme aimed at studying the environmental radioactivity in the Brazilian state of Rio Grande do Norte was undertaken. Fifty-two soil samples, together with two rock and two uraniferrous ore samples were collected from the eastern and central regions of this state. Concentrations of radioelements in samples were determined by gamma-ray spectrometry. The average concentrations of Ra-226, Th-232, and K-40 in the surveyed soils were 29.2 +/- 19.5 (SD), 47.8 +/- 37.3, and 704 +/- 437 Bq kg(-1), respectively. Higher values were found in the rock samples. The distributions of Ra-226 and Th-232 were fitted by log-normal curves. Radiological measurements carried out with a portable scintillometer at the sampled sites revealed an average absorbed dose rate of 55 +/- 27 (SD) nGy h(-1), and their distribution fitted a log-normal curve. An annual average effective dose equivalent of 552 mu S nu (range: 117-1361 mu S nu) was estimated for 51 sites in Rio Grande do Norte.