Abstract
Damage to the Fukushima nuclear power plant caused by the 11 March 2011 earthquake and tsunami off the northeast coast of Japan resulted in the release into the environment of radioactive material. Airborne radioactive material was detected in metropolitan areas near Tokyo, and increases in radiation dose rate were observed at many locations. In this study, repeated measurements with the in situ Ge system were performed in Chiba City, which is about 220 km south of Fukushima. Increases in radiation dose rate were recorded on 15, 16, and 21 March, with a maximum of 0.5 μGy h(-1). This level is clearly higher than natural background in Japan. Airborne (99)Mo, (99m)Tc, (129m)Te, (129)Te, (132)Te, (131)I, (132)I, (133)I, (133)Xe,(133m)Xe, (135)Xe, (134)Cs, (136)Cs, (137)Cs, and (140)La were detected. Environmental radioactive contamination in the metropolitan area occurred mainly on 21 March by rainfall. The initial rates of decrease in radiation dose rate generally reflected radiological decay according to their physical (radiological) half-lives. However, the in situ half-lives of the long-lived radionuclides such as (134)Cs and (137)Cs reflected environmental dispersal rather than radiological decay.
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