The Distribution Map of Radiation Dose, etc., has been created on the basis of the results of a variety of monitoring activities conducted by the MEXT to give public access to the details of the influence of radioactive substances discharged from TEPCO Fukushima Daiichi Nuclear Power Plant.

Airborne monitoring map:
Using high-sensitivity radiation detectors installed on helicopters, second-by-second measurements are taken of the gamma rays emitted from radioactive substances deposited in circles of a diameter approximately twice the flight altitude (target altitude: 150 to 300 m) centering around ground locations right below the flight trajectories. Then, a dedicated software program is used to determine the hourly air dose rate (μSv/hour) 1 m above the ground surface at each location based on the value of gamma rays measured up in the air and the reading of the survey meter on the ground.
Meanwhile, the deposition density of radioactive cesium per unit area of ground surface (Bq/m²) is determined, using another dedicated software program, from the relationship between the results of ground surface measurement by a Ge semiconductor detector and the air dose rate 1 m above the ground surface. Note that values for points not flown (i.e., points not present on the flight trajectories) are determined by interpolation from measurement values obtained from points flown.

Airborne monitoring map:
As for measurements using an unmanned helicopter, the values converted on the basis of the results of measurements carried out once a second are displayed on the map.
[Measurement method]
Measurements using an unmanned helicopter installed with radiation measuring instruments (NaI scintillation type survey meter etc.).
*The detection section of the survey meter was installed on the bottom surface of the unmanned helicopter.

Distribution Map of Radiation Dose, etc.:
The hourly air dose rate (μSv/hour) 1 m above the ground surface at each reading point is determined as follows: individual reading points are identified using commercially available GPS devices; then, at each location with an air dose rate of 30 μSv per hour or less, measurements are taken using a calibrated NaI(Tl) scintillation survey meter; and measurements are taken using a calibrated ion chamber survey meter at each location with an air dose rate exceeding 30 μSv per hour.
Soil samples taken from up to five spots in each reading point are agitated in suspension for nuclide analysis using a calibrated Ge semiconductor detector. The averaged results of the nuclide analysis provide the basis for the surface deposition densities of iodine-131 and cesium-134 and 137 per unit area (Bq/m2) at each reading point.

Vehicle-Borne Survey:
The KURAMA System (Kyoto Univ. RAdiation MApping System) connected to on-vehicle NaI(Tl) scintillation survey meters is used to determine the hourly air dose rate (μSv/hour) 1 m above the road surface (as measured aboard). The values shown on the map as air dose rates measured by the on-vehicle survey meters are determined by factoring in the shielding effect of the construction of the monitoring cars, the installation height of the survey meters, and by multiplication with the correction coefficient for the air dose rates measured inside and outside the cars.