2012 Volume 12 Issue 1 Pages 5-10
To measure radioactivities in soil contaminated by the accident of the Fukushima Daiichi Nuclear Power Plant, efficiency calibration of Ge detectors for 131I, 134Cs, and 137Cs in volume sources was investigated. Gamma-ray detection efficiencies for these nuclei were determined precisely using standard soil samples containing 134Cs, 137Cs, 175Hf, and 88Zr. These standard sources were prepared by admixing radioactive solutions with soil, and point sources were made from the same solutions to determine their radioactivity concentrations. The efficiency for the 364 keV γ ray of 131I was extracted from the experimental efficiencies of 137Cs and 175Hf. Cascade summing corrections for γ rays of 134Cs in soil samples were evaluated experimentally. To easily calculate the cascade summing corrections for volume sources, we examined a simplified method using averaged efficiencies, and evaluated its validity through a comparison of the calculated correction factors with the experimental ones.