Altitude-dependent distribution of ambient gamma dose rates in a mountainous area of Japan caused by the fukushima nuclear accident.

Large amounts of airborne radionuclides were deposited over a wide area in eastern Japan, including mountainous regions, during the devastating Fukushima Dai-ichi nuclear power plant accident. Altitudinal distributions of ambient gamma dose rate in air were measured in a mountainous area at the northern rim of the Kanto Plain, Japan, using a portable instrument carried along the mountain trails. In the Nikko Mountain area, located 120 km north of Tokyo, the altitudinal distribution exhibited a maxima at ∼900-2000 m above sea level (ASL). This area was not affected by precipitation until 2300 Japan Standard Time (JST) on March 15, 2011. By that time, a substantial amount of radionuclides had been transported from the damaged reactor, according to the numerical simulations using transport models. Meteorological sounding data indicated that the corresponding altitudes were within the cloud layer. A visual-range monitor deployed in an unmanned weather station at 1292 m ASL also recorded low visibility on the afternoon of March 15. From these findings, it was deduced that the altitude-dependent radioactive contamination was caused by the cloud/fog deposition process of the radionuclides contained in aerosols acting as cloud condensation nuclei.