Atmospheric Activity Concentration of 90Sr and 137Cs after the Fukushima Daiichi Nuclear Accident

On March 11, 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident occurred and large amounts of radionuclides were discharged into the atmosphere. We have operated continuous aerosol samplings at four locations in Japan from the accident until the end of 2011. The activities of 90 Sr and 137 Cs in the aerosol samples were measured using low background liquid scintillation counters and high-purity germanium detectors, respectively. The atmospheric 90 Sr and 137 Cs concentrations decreased exponentially during 2011. The time variation of the 90 Sr/ 137 Cs ratio was obtained, and we found that the ratio rose from 1.2 × 10 –3 in March to 1.3 × 10 –1 in August 2011. One reason for the increase in the 90 Sr/ 137 Cs ratio could be the change in the primary emission source of activity at the FDNPP, which occurred near June 2011.