Natural Variation of Ambient Dose Rate in Air of Izu-Oshima Island After the Fukushima Daiichi Nuclear Power Plant Accident

The ambient dose rate in air and radioactivity concentration in soil samples collected on Izu-Oshima Island were observed in 2012, 2013 and 2014, i.e., one, two and three years after the severe accident at the Fukushima Daiichi Nuclear Power Plant. A car-borne survey for the ambient dose rate in air was carried out for the entire island. Soil samples were collected for the radioactivity concentration measurements from 22 points. The ambient dose rates in air were 36 nGy h⁻¹ in 2012, 34 nGy h⁻¹ in 2013 and 29 nGy h⁻¹ in 2014, respectively. The corresponding radioactivity concentrations in those years for ¹³⁴Cs were 53 Bq kg⁻¹, 39 Bq kg⁻¹ and 29 Bq kg⁻¹ and for ¹³⁷Cs, 87 Bq kg⁻¹, 73 Bq kg⁻¹ and 75 Bq kg⁻¹. All the values have decreased every year