Study on decreasing trend of air dose rates observed in Fukushima Prefecture and neighboring prefectures

Ando, Masaki  ; Kanno, Takashi; Saito, Kimiaki  

The ratios of air dose rate averaged in prefecture-wise measured by car borne surveys in wide area has been performed by with the use of the KURAMA and KURAMA-II systems were investigated. The changes in air dose rate from the first (June 2011) car borne survey to the fourth (September 2012) and seventh (November 2013) car borne surveys in Fukushima Prefecture was similar to those in Tochigi Prefecture, and the ratio in Miyagi Prefecture showed quicker decay than those in the other prefectures in the groups of less than 0.5Sv/h. Distribution maps of the ratios of air dose rate obtained in 100m mesh wise in Fukushima, Tochigi and Miyagi prefectures showed that the ratios (i.e. decreasing rates) were depending on the area or road. Further, decreasing in the air dose rate was quicker than the physical decay in the big cities where population is big and the traffic is huge.