Efficiency calibration of Ge detector for ¹³¹I and ¹³⁴Cs in soil samples and a simplified calculation of cascade summing corrections for volume source

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. (author)