Evaluation and Measurement Methods of Radioactivity of the Soil - Measurements on the Campus of St. Marianna University School of Medicine

[Abstract] A quantitative method was applied that employs the correlation between the count rate of total β-rays and the count rate of γ-rays with fixed energy from a soil sample. Specimens were collected from four locations on the campus of St. Marianna University School of Medicine, Kawasaki, Japan. First, the radioactivity of 40K that emits both γ-rays and β-rays during its decay was determined in the specimen by γ-ray spectrometry. Second, total β-ray radioactivity of each specimen was obtained by a gas-flow counter. Finally, by analyzing the correlation between total β-ray counts and results of γ-ray spectrometry regarding 40K, 214Bi, 212Bi, and 137Cs, the components of β-ray radioactivity of soils originating from 40K, 238U series, 232Th series, and 137Cs were estimated. We found that the major component of β-ray radioactivity in the soils was 40K, and that its concentration ranged from 0.04 to 0.31 (Bq/g). The contribution of β-rays from 238U series and 232Th series to total β-ray activity of each soil specimen was estimated to be less than 42%. The estimated radioactivity was within the range of natural radioactivity in Japanese soils reported in the literature. The possibility of radioactive contamination in soil is very low in the vicinity of the laboratory, which utilized only small amounts of radioisotopes. Thus the evaluation of soil radioactivity is actually much like the evaluation of natural radioactivity in soil. The method examined in this study is so practical that it can be used in handling many samples in accidents or in case of removal or closure of radioactive facilities.