The property of soil material was measured in order to clarify the relation between the radon and thoron exhalation rate and the physical and chemical characteristics of soil material. The examined factor is the physical property (particle size and porosity) of soil, the mineral component (quartz, microcline, biotite and kaolinite) of soil particle, and the chemical component of soil. The weathered granite soil was used in this experiment. The soil particle size was classified by using the standard mesh into six classes, i. e. >1000μm, 500-1000μm, 355-500μm, 250-355μm, 106-250μm and <106μm. The weight ratios of the classified soil were 48%, 12%, 10%, 9%, 13%, and 8% in each class, respectively. The exhalation rates showed a tendency to decrease with an increase in soil particle size. The measurement of the porosity suited the range of 0.46-0.62. A positive correlation between the exhalation rate and the porosity was found. Also, a positive correlation between the exhalation rate and kaolinite was obtained in each class. Moreover, a negative correlation between the exhalation rate and quartz and microcline was shown in each class. The relationship between the exhalation rate and the chemical components of SiO2 and Al2O3 that are the major elements in soil were negative and positive correlations, respectively.
