Efficiency Calibration of Ge Detector for 131I and 134Cs in Soil Samples and a Simplified Calculation of Cascade Summing Corrections for Volume Source

The accident of the Fukushima Daiichi Nuclear Power Plant dispersed a huge amount of radionuclides in the natural environment, and part of them were deposited on the ground surface in the extensive region of east Japan. Assessments of the deposition densities of radionuclides and their geographical distributions provide important quantitative bases for estimations of radiation exposures of the residents, transfer of radionuclides into agricultural products, costs of decontaminations, etc. The Japanese government in collaboration with a few hundreds of researchers in universities and associations carried out detailed measurements of radionuclides deposited on soil over the whole area of Fukushima prefecture. Independent of this work, we carried out measurements of radionuclides in soil samples collected in the north and east area of Ibaraki prefecture and in the east area of Tochigi prefecture. The soil samples were collected on May 21–22 and June 22 in 2011, and γ rays originating from the β decay of I, Cs, and Cs were observed with Ge detectors, whose details are described in a separate paper. To measure radioactivities in soil with a Ge detector, we have to know γ-ray detection efficiencies of the Ge detector for soil samples. Typically soil samples have a large volume and density. Thus, to determine the efficiencies of Ge detectors, it is necessary to prepare the standard soil sample which contains a known amount of radionuclides and whose shape, density, and elemental contents are approximately the same as those of soil samples. At first, we used a standard soil sample distributed by IAEA to measure the efficiencies. This soil sample contains 4.0–60 Bq kg of Am, Cd, Cs, Cs, Mn, and Co. However, these radioactivity concentrations were too weak to determine the detector efficiencies precisely. Moreover, this standard source does not contain radionuclides which emit γ rays with energies at the range of 90–600 keV. This makes it difficult to determine the efficiency for the 364 keV γ ray of I. A mixed γ-ray standard source which resembles soil samples in shape, density, and elemental contents is commercially available. However, typically it does not contain I and Cs. The efficiency at 364 keV for I is extracted from the efficiency calibration curve determined by using γ rays of, e.g., Hg (279 keV), Cr (320 keV), Sn (392 keV), Sr (514 keV), and Cs (662 keV). However, these radionuclides except Cs are not always available because of their short half-lives. For Cs, even if the efficiency calibration curve is determined precisely, one has to take account of the influence of the cascade summing effect which reduces or increases observed γ-ray peak counts, especially when the sample is placed at a close source-to-detector distance. For point sources, correction factors for the cascade summing effect are easily calculated using total efficiencies and full-energy peak efficiencies of Ge detectors determined experimentally. For volume sources, however, the efficiencies are different at different parts of the source, which leads to different summing corrections at each part. This forces us to confront a difficulty in obtaining the total and full-energy peak efficiencies at any parts of the volume source. The correction factor for a certain γ ray depends only on the decay scheme of the nuclide and on the geometry of the source and the detector. Thus, if we can prepare the standard source which contains the same radionuclide and has the same geometry (shape, density, and elemental contents) as those of soil samples, we can determine the contents of radionuclides in soil samples only through a comparison of observed γ-ray peak counts between the standard source and a soil sample, because the cascade summing corrections for both the standard source and a soil sample are identical, and thus they are cancelled out. The aim of the present study is to determine γ-ray detection efficiencies of Ge detectors for I, Cs, and Cs in soil samples, and to evaluate the cascade summing corrections for I and Cs in soil samples which has a large volume and density. For this purpose, we make standard soil samples containing Cs, Cs, Hf, and Zr.