Time-Dependent Change of Ambient Radiation Dose Rate and Gamma-Ray Spectrum in Shinjuku, Tokyo, after the Fukushima Daiichi Nuclear Power Plant Accident

Abstract

We investigated the influence on Shinjuku, Tokyo of the Fukushima Daiichi Nuclear Power Plant accident by pulse height distribution using measured data of monitoring post. As a result, the time variation of the artificial radioactive nuclides which reached Shinjuku-ku from the nuclear plant became clear.Acute rises of the air dose rate were seen 4 times between 15 and 16 March 2011, and they were brought by many artificial radioactive materials passing around the monitoring post, and some nuclides such as ¹³¹I which fell as slight dry deposition. Main cause of the air dose rate increase between 21 and 23 March was considered to be ¹³¹I,¹³⁴Cs and ¹³⁷Cs as wet deposition, brought by the rainfall in this period.