The temporal changes of air dose rate along the mountain trail at Mt. Ryozen in Fukushima prefecture and the relationship between the air dose rate and the ¹³⁷Cs activity concentration in moss

We measured the air dose rate at the fixed 17 points along the Mt. Ryozen mountain trail during the 6 years after 2011, when ¹³⁷Cs fallout was released by the Fukushima Daiichi Nuclear Power Plant accident, and we collected the 7 and 16 samples of moss at the 6 point in 2015 and at the 13 point in 2016 to validate the relationship between the air dose rate and the ¹³⁷Cs activity concentration in moss. The air dose rate under cedar and red pine was kept higher than that of other survey sites throughout the survey period. The ratio of the mean of air dose rate to that measured on 10 September 2011 was approximately 10% lower than that of the theoretical value which calculated due to the physical decay since 2015, when the effect of the weathering accelerated, whereas 5% lower ratio of measured value than that of the theoretical value was kept until 2014 from 2011. Temporal changes of the air dose rate at the open space surrounded the deciduous tree or on the rock were explained by a one or two-component negative exponential model. However those under cedar and red pine were not explained by the component negative exponential model. Consequently, the effective half-life of the mean of air dose rate at 17 points was 1016 day. A significant positive correlation was found between the ¹³⁷Cs activity concentration in the moss and the air dose rate in both 2015 and 2016, indicating that moss is effective as an indicator plant for comparing ¹³⁷Cs contamination status among different points. (author)