Results of Deposition of Radioactive Cesium of the Airborne Monitoring Survey in the Areas to which Evacuation Orders Have Been Issued ( Decay correction: March 11, 2013 )
- This data was created based on the measurement results from the airborne monitoring survey conducted by MEXT in the restricted areas and areas to which evacuation orders have been issued.
- This published data was prepared based on the results obtained from March 4 to March 11, 2013 by one helicopter, in a total 13 flights.
- Their flight altitudes were from about 300 m above ground, and the air dose rate at the ground surface is the averaged value of air dose rates in a roughly about 600 m diameter circle (varies by flight altitude) below the aircraft.
- The width of the track is around 1.85 km.
- The deposition densities of radioactive cesium were prepared by first assessing the characteristics of the energy spectra of gamma rays measured in the air by type of helicopter and measuring equipment used, and then sorting out areas where the energy spectra of radioactive cesium (Cs-134 and Cs-137) were detected in significant amounts and those where they were not.
- ① Areas where the energy spectra of radioactive cesium were detected in significant amounts; In order to calculate the amount of radioactive cesium in detail, we used the method for conducting detailed assessment of the influence of natural radionuclides based on the energy spectra of gamma rays measured in the sky that were applied in western Japan and Hokkaido. Refer to Attachment 9 of “Results of Airborne Monitoring Survey by MEXT in the Kyushu Region and Okinawa Prefecture” [Announced on May 11, 2012]. Based on this method, deposition densities of radioactive cesium were calculated by deducting the contribution by natural radionuclides from the measurement results of air dose rates at the respective measuring points, and also based on the correlation between air dose rates and the results of the in-situ measurement using germanium semiconductor detectors*1, which was conducted by the Japan Chemical Analysis Center in the course of the project by the 2011 Strategic Funds for the Promotion of Science and Technology, entitled “Establishment of the Base for Taking Measures for Environmental Impact of Radioactive Substances - Study on Distribution of Radioactive Substances” (June-August, 2011).
- ② Areas where the energy spectra of radioactive cesium were not detected in significant amounts; As with before, these were indicated as areas showing the minimum range of radioactive cesium "<1.0E+04" (Bq/m2) for the sake of simplicity.
- The values have been decay-compensated to the value for March 11, 2013, which is the final day in which the airborne monitoring survey was conducted in the restricted areas and areas to which evacuation orders have been issued. Regarding deposition densities of Cs-134 and Cs-137, physical attenuation from the measurement time up to a certain point in time (March 11, 2013) was taken into account.
- There was snow coverage in some part of the targeted areas of this monitoring. It has been expected that the air dose rate measurements are apt to be lower due to the effect of snow coverage. Therefore, in order to distinguish areas covered by snow, such areas are indicated as data. When identifying areas covered by snow, we used data obtained by NASA's earth observation satellites, Terra and Aqua*3, which are available on JASMES*2 released by the Japan Aerospace Exploration Agency.
- The data consists of radioactive cesium deposition densities at the median points of the Quarter Grid Squares (approximately 250 m x 250 m) defined by JIS X 0410. See the Ministry of Internal Affairs and Communications Statistics Bureau website for "Grid Square Statistics".
*1: In-situ measurement using germanium semiconductor detectors: A means to analyze the concentration of radionuclides accumulated in soil by setting up transportable germanium semiconductor detectors in the environment and detecting gamma rays that are emitted from radiation sources distributed in soil.
*2: JAXA Satellite Monitoring for Environmental Studies
*3: We used the data of snow coverage around Japan, which were observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) installed on NASA’s earth observation satellites, Terra and Aqua, and were analyzed by the Japan Aerospace Exploration Agency (JAXA) using its original processing algorithm. JAXA has been planning the Global Change Observation Mission (GCOM) to observe the Earth’s environment as a whole, including the atmosphere, ocean, land, snow and ice, from space on a long-term basis (10 to 15 years), thereby monitoring hydrologic circulation and climate change and figuring out the mechanisms thereof. Said data have been collected and processed for the purpose of assessing the analytical algorithm for optical sensors used in the Global Change Observation Satellite (GCOM-C). As they are 500 m-grid data, snow coverage over 5 cm with an even surface can be indicated correctly, but it is sometimes difficult to accurately identify snow coverage of a shallower depth or with an uneven surface.