- This data was created based on measurements within a 20 km radius of TEPCO Fukushima Dai-ichi NPP, available in the Monitoring Information of Environmental Radioactivity Level, the portal site run by Nuclear Regulation Authority.
- The survey was conducted every week from March 2011 to March 2016. In FY 2016-FY 2017, the survey was conducted in September.
- Dosimeters which were used: NaI (NaI scintillator detector) and IC (ionization chamber type survey meter).