Relationships Between the Concentrations of Natural Radionuclides and the Mineral Composition of the Surface Soil

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Abstract
Hundreds of samples of surface soil in the central region of Japan were collected in areas classified into granite, gabbro, basalt, intrusive rocks, andesite, ultrabasic rocks, paleozoic and alluvium by a sub-surface geological map. That part of the soil sample larger than 10 mesh was excluded by sieving. The concentrations of the U series nuclides (238U, 226Ra), Th series nuclide (228Ra) and 40K were then determined by gamma ray spectrometer measurement with coaxial and planar types of Ge detector and the mineral compositions established by powder X ray diffraction analysis. For the soil samples containing 75-1400 Bq.kg-1 of 40K, 5-185 Bq.kg-1 of 228Ra and 5-130 Bq.kg-1 of 226Ra, the ranges of the concentrations of these three nuclides correspond to the main mineral composition and all of the samples could be classified into 8 groups. The lowest ranges of the concentration, 75-150 Bq.kg-1 of 40K, 5-20 Bq.kg-1 of 228Ra, 5-60 Bq.kg-1 of 226Ra, respectively, were observed in the soil sample mainly composed of amphibole, quartz, olivine and chlorite. The soil samples containing 960-1400 Bq.kg-1 of 40K, 55-185 Bq.kg-1 of 228Ra and 37-130 Bq.kg-1 of 226Ra, which are the highest ranges, are chiefly composed of quartz, microcline, albite and biotite. Although there is some scatter in the concentrations of 228Ra and 238U relative to the 40K content, the concentrations of 228Ra and 238U increase with the 40K content. The scattering of the ratios of the concentrations of 228Ra and 238U to the 40K content may be explained by the surface area of soil particles.

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