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ABSTRACTAfter the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP), the Nuclear Regulation Authority (NRA) has adopted measures based on radiation monitoring in residential areas, instead of making decisions based on predictive radioactive material diffusion to protect residents in emergency preparedness and response. Protective actions such as sheltering and evacuation based on the measured radiation levels have been defined in each municipality where a nuclear facility is locating, but the measures to be taken when radioactive materials arrive from a neighboring country have not been discussed. We employed historical data of environmental γ-ray dose rate in 20 cities for more than 60 years, stored in the Environmental Radiation Database. In order to identify the abnormal signal from radiation emergency at neighboring country, we propose a new peak detection, detrended fluctuation analysis with the centered moving average, with real-time systematic analysis by checking the changes in radiation levels at multiple or all residential areas. Finally, we discuss a radiation monitoring strategy for taking protective actions for the local population.