Artificial Radioactivities in Rainwater and Air Dust Gathered in Bihoku Area of Chugoku Region after Fukushima Daiichi Nuclear Power Plant Accident

摘要

At a location in the central mountain area of the Chugoku region (Bihoku area), concentrations of ~(131)I,~(137)Cs and ~(134)Cs in rain and air dust were measured before and after the Tohoku-Pacific Ocean Earthquake that occurred on 11 March 2011. Rainwater was also sampled at a location near the Inland Sea. The rainwater was dried on a Naflon sheet. Airborne particles were collected using an air dust sampler. A pure Ge detector shielded by lead blocks was used to measure the gamma rays from the samples. In both locations of the Chugoku region, initial detections of ~(131)I in rainwater occurred on 8 April 2011. On this date, the ~(131)I concentrations in rainwater were 0.23 ± 0.03 Bq/kg in the mountain area and 0.19 ± 0.01 Bq/kg near the Inland Sea. The concentrations of radioactive cesium in rainwater were high from 19 April to 23 April 2011, and were 0.12 ± 0.02 Bq/kg of ~(134)Cs and 0.12 ± 0.03 Bq/kg of ~(137)Cs at maximum, respectively. The atmospheric concentration of ~(131)I was 9 ± 2 mBq/m~3 at maximum (6 April 2011), which was estimated without evaluating the ~(131)I activities of the gaseous molecules that passed through an ordinary filter used in the air dust sampler. Maximum concentrations of ~(134)Cs and ~(137)Cs on 7 April 2011 were 10.2 ±1.3 mBq/m~3 and 9.7 ± 1.4 mBq/m~3, respectively. Few radioactive cesium and iodine were observed in March. Western and northwestern winds in March probably prevented these artificial radioactive materials from the Fukushima Daiichi Nuclear Power Plant from reaching the mountain area of the Chugoku region.