Soil was separated according to particle size, and the concentrations of the uranium series nuclides 238U, 230Th, 226Ra, and 210Pb in each size‐separated fraction were determined by γ ray spectrometry with Ge(Li) detectors. The activity ratios of 230Th, 226Ra, and 210Pb to 238U were larger than unity in all fractions. The activity ratio of 230Th to 238U was unexpectedly high, reaching 3–5. In the particle size range below 0.15 mm the concentrations of these uranium series nuclides were found to increase with decrease in particle size. Such a trend was also found for the elements Fe, Mn, Zn, and Cu. The elements K and Ca, however, were found to have a direct correlation with particle size.
Concentrations of uranium series nuclides (238U, 226Ra, 210Pb), thorium series nuclides (228Ra, 212Bi), and potassium in soil of a granitic belt were examined in relation to their surface area of soil, clay mineral composition, and the SiO2 and Fe contents. In soil which is less weathered and has a surface area of around 10 m2/g, the 238U, 226Ra, 210Pb, 228Ra, and 40K contents increase with an increase the SiO2 content related to the elemental compositions of their parents rocks. In homologous soils resulting from progressive weathering, the 238U, 226Ra, 210Pb, and 228Ra contents are found to increase with increasing surface area. For an homologous soil which is free from the clay consisting of three‐layer units, the 40K content decrease with increasing surface area. However, in an homologous soil which contains a clay consisting of three‐layer units, the 40K content is not surface area dependent.
Samples of soil originating from the weathering of granite rock were sieved into eight classes of particle size. Each sieved sample was measured for its content of 226Ra (parent nuclide of radon), 224Ra (parent nuclide of thoron), 228Ac (grandparent nuclide of 224Ra), K, Ca, and Fe as well as for the radon and thoron emanation from it. There was a trend for the contents of 226Ra and 224Ra to increase as particle size decreased. On the other hand, the K content showed a different variation with particle size from the variations found for 226Ra and 224Ra. On the basis of the results of the emanation measurements, it was concluded that smaller soil particles make relatively more contribution to radon and thoron exhalations from the ground surface than larger soil particles, but they make absolutely less because of their smaller abundance.
The rate of radon (222Rn) and thoron (220Rn) exhalation from the ground was measured in the yard of our laboratory to investigate effects of soil conditions on the exhalation; radon and thoron gases released from the ground were directly adsorbed on granular‐activated charcoal, and the charcoal was subjected to γ ray spectrometry to estimate the activity of adsorbed radon and thoron gases. Although the rate of radon exhalation was not appreciably changed by a light rainfall, it apparently decreased after a heavy rainfall and remained low for several days. No appreciable seasonal difference was found in the radon exhalation. On the other hand, the thoron exhalation decreased more or less after every rainfall and depended strongly on the moisture of a thin surface soil layer. The rate of thoron exhalation in summer was about twice its value in winter.
Abstracts. We found that increasing sunspot number caused a significant negative effect on monthly and yearly average air concentration and yearly deposition of 7Be.
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