Summary of survey

  1. This data was created based on results of soil deposition densities of strontium 89 and strontium 90 measured in "the Second Survey on the Distribution Condition of Radioactive Substances by MEXT due to the Accident at TEPCO’s Fukushima Dai-ichi NPP".
  2. This survey targeted the following two areas (63 samples from 60 locations).
    • 1) Areas more than 80 km from Fukushima Dai-ichi NPS (50 samples from 50 locations)
      • Excluding areas 80 km from Fukushima Dai-ichi NPS covered by the First Distribution Survey, we collected new soil samples in areas where airborne monitoring of all of eastern Japan had shown relatively high air dose rates (0.2 µSv/h or higher),*1 and measured the deposition densities of Sr-89 and Sr-90. Survey points were determined by apportioning areas showing air dose rates of 0.2 µSv/h or higher into 5 km×5 km grids and selecting points evenly from these grid areas (50 locations: 50 samples). Sampling period was December 17, 2011 - February 9, 2012.
    • 2) Soma City and surrounding areas (13 samples from 10 locations)
      • The First Distribution Survey ascertained that a sample collected at one point in Soma City showed very large ratios of deposition densities of Sr-89 and Sr-90 against those of Cs-137 (hereinafter referred to as “ratios of Sr-89 and Sr-90 against Cs-137”), compared with samples collected at other points. This time, we analyzed the other four samples collected at this point (hereinafter referred to as “Soma City Point 1”) during the First Distribution Survey, and also conducted an analysis of Sr-89 and Sr-90 with regard to one sample each selected from those collected from nine points surrounding Soma City Point 1 in the First Distribution Survey that contained the largest amount of Cs-137.
  3. We conducted radiochemical analysis of the following soil samples using low background beta-ray counters for around 60 minutes. The amounts sampled and the detection limits were as follows.
    • 1) Soil samples collected in areas more than 80 km from Fukushima Dai-ichi NPS
      • We used sample amounts approximately three times larger than those used in the First Distribution Survey (100 g compared with 30 g in the First Distribution Survey) and also lowered the detection limits for Sr-89 and Sr-90. The detection limits adopted were around 100 Bq/m2 for Sr-89 and around 15 Bq/m2 for Sr-90.
    • 2) Soil samples collected at Soma City Point 1 and the surrounding areas
      • We used 30 g samples collected at the relevant points as we did at the time of the First Distribution Survey, and measured the deposition densities of Sr-89 and Sr-90. The detection limits adopted were around 300 Bq/m2 for Sr-89 and around 40 Bq/m2 for Sr-90.
  4. "ND" in the column "Deposition", "-" in the column "Error", and detection limit value as reference in the column "detection limit": The peak of relevant nuclide was not clearly detected or calculated deposition was equal to or below 0.0.
  5. Reference value in the column "Deposition density" smaller than the detection limit in the column "detection limit": The peak of the relevant nuclide was detected even though the calculated deposition was below detection limit.
  6. Considering the physical half-life of Sr-89 and Sr-90 (Sr-89: 50.53 days; Sr-90: 28.79 years), the obtained values were corrected for their decay as of January 13, 2012, the midpoint of the survey period (December 17, 2011 - February 9, 2012).
  7. *1: Measurements were made in some areas of Tokyo and ten other Prefectures (Fukushima, Ibaraki, Iwate, Kanagawa, Gunma, Saitama, Chiba, Tochigi, Miyagi, and Yamanashi). Areas in which air dose rates are high due to the influence of natural radionuclides were excluded.
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