Early In situ measurement of radioactive fallout in Fukushima City due to Fukushima Daiichi nuclear accident

Using a high-purity germanium detector, both indoor and outdoor radionuclides that had deposited 1.5 d after the radioactive fallout events in the city of Fukushima were experimentally measured. Eleven artificial (131I, 132I, 134Cs, 136Cs, 137Cs, 129Te, 129mTe, 131mTe, 132Te, 140La and 99mTc) and 5 natural radionuclides were identified. Total air kerma rates were mainly due to 132I, 134Cs and 136Cs from 4 to 6 mGy/h at a 7.5-cm height from the ground. Radioactive contamination on the ground was contributed by 132I and 132Te, from 330 to 420 Bq/cm2. In a worst-case scenario, the maximum skin dose rates were estimated to be from 520 to 670 mGy/h. Effective dose rates were evaluated to be 10 to 15 mSv/h and reached 17.9 mSv/h at 4 a.m. on 16 March. In the effective dose rates, 132I, 134Cs and 132Te were the main contributors. Our measurements are useful for estimating dose levels in the public in the city of Fukushima during the days after radioactive fallout contamination. (authors)