Measurement of radioactivity in airborne dust and estimation of public dose in Tokyo after the Fukushima Daiichi Nuclear Power Plant accident

We report on the results of emergency monitoring of environmental radiation in Tokyo after the severe accident at the Fukushima Daiichi Nuclear Power Plant, triggered by the Great East Japan Earthquake and the resulting tsunami on the 11th March 2011. We collected airborne dust samples at the Setagaya and Koto districts of Tokyo. The concentrations of radionuclides were measured by a Ge semiconductor detector. The highest concentrations of major fission products were detected at 10:00-11:00 a.m. on the 15th March 2011. The detected concentrations were: 2.4×10² Bq/m³ of ¹³¹I, 6.4×10¹ Bq/m³ of ¹³⁴Cs, and 6.0×10¹ Bq/m³ of ¹³⁷Cs. The concentration of ¹³¹I fell to undetectable levels by June 2011. In contrast, ¹³⁴Cs and ¹³⁷Cs were detected intermittently for one year. Based on the results on the concentrations of detected nuclides, the public internal doses due to inhalation were estimated on the assumption that those in the affected area stayed outdoors continuously for the period from the 13th March 2011 to the 12th March 2012 in the Tokyo metropolitan area. The committed effective dose due to inhalation for adults was estimated to be 25 μSv. (author)