1999 Volume 48 Issue 9 Pages 567-576
The natural radiation levels in Niigata area were studied from aspect of the gamma-ray dose rate in Kaetsu district in the northern part of the prefecture around Niigata City, and in Sado Island.
The quantity of radionuclides and their contribution to the gamma-ray dose rate were obtained by an in situ measurement method and the total dose rate with a scintillation survey meter. The identified radioactivities from collecting soil samples were converted to the dose rate at a point 1 m above ground using the γ-spectrometry. In Kaetsu district, the dose rates of the northern area of Niigata City were higher than those of the south parts, probably reflecting the geological situation of the northern area, which consists of granitic base rock belonging to the highly radioactive rock. The most of the measuring points showed radionuclide contribution in the order of 40K>Th-series>U-series and the dose rate was significantly dependent on the different composition of radionuclides in the soils weathered from the original base rocks.
Some radioactivity concentration were also analyzed in coastal sands. The sand collected from the northern district of Niigata City showed high concentration of both 232Th and 40K. The black and high radioactive sands from the coast in the central area of Niigata City were considered to be attributable to the active erosion effects of wave action after deposition from Shinano River.
Annual dose equivalent due to natural radiation from ground was calculated based on average dose rate.
