Dose rate monitoring and mapping in the eastern part of the 23 wards of the Tokyo metropolitan area after the accident of Fukushima Daiichi Nuclear Power Plant

 Our concern for radioactivity or radiation has become higher after the accident of Fukushima Daiichi Nuclear Power Plant. It is important that we know the personal radiation dose. Environmental radiation dose rate monitoring and mapping in the eastern part of the 23 wards of the Tokyo metropolitan area was performed after the accident. Monitoring of the dose rate was measured in the outdoor and the room of the wooden mortar 2-story house at northern Sumida-ku, Tokyo. The dose rate was measured twice a day after the explosion accident on March 15 using Myrate pocket survey meter. The maximum dose rate was 1μSv/h which measured on March 15, 2011. However the radioactive nuclides did not fall out then. It rained on March 21 and 22, then the radionuclide fell out on the ground surface. There are not large fluctuations of the dose rate afterwards by November 25, 2011. Energy spectrum was measured by using the gamma ray spectrometer to estimate sources of radiation nuclides. Radioactive tellurium 132, iodine 132 of tellurium daughter, iodine 131, cesium 134 and cesium 137 were detected by response matrix method (Minato, 2011). The radioactivity of iodine 131 was detected 12 kBq/m² on March 23, however, the radioactivity decreased remarkably after one week. The radioactive nuclei fell out just after accident outbreak, and few radioactive nuclei fall out at present. The radioactive cesium was already absorbed and immobilized on the surface of concrete, asphalt or soil.<br>  Dose rate mapping performed in the eastern part of the 23 wards of the Tokyo metropolitan. The measurement range is approximately 10km east-west and 15km north-south. The measurement points are 566. Distribution of the dose rate tends to gradually become low in the northeast part towards the southwestern part. However there is locally higher value. Such that place is the slope of levees which faces to the northeast or the super levees which are planted trees in. Dose rate values accord roughly with the data of Ministry of Education, Culture, Sports, Science and Technology. Dose rate distribution of Katsushika-ku, Tokyo before the accident was shown in Hosoda et al. (2011). The dose rate before the accident was low in many places of the green tract of land, and concrete and asphalt were high in many places. The tendency reversed after the accident, namely the increase of the dose rate is remarkable in the green tract of land. The average dose rate before the accident was 39nGy/h in Katsushika-ku. However the average dose rate after the accident increased to 158nGy/h. There was the area where the ratio increased to approximately 4 times before the accident in Tokyo.<br>