DEPOSITION IN CHIBA PREFECTURE, JAPAN, OF FUKUSHIMA DAIICHI NUCLEAR POWER PLANT FALLOUT

Radioactivity from the Fukushima Daiichi Nuclear Power Plant (F-1NPP) accident in deposition samples has been monitored at the National Institute of Radiological Sciences (NIRS) in Chiba Prefecture, Japan, located about 220 km south-southwest of the F-1NPP. Sampling was carried out from 15 March 2011, two to three times a day for 10 d and then once a day until 22 April 2011. Gamma-ray spectrometry of fallout deposition samples revealed the presence of I-131, I-132, Te-132, Cs-134, and Cs-137. The largest deposition was observed during 7:00-16:00 on 21 March. The estimated total deposition densities at NIRS were 1.40 x 10(5) Bq m(-2) for I-131, 4.12 x 10(4) Bq m(-2) for Te-132, 1.45 x 10(4) Bq m(-2) for Cs-134, and 1.48 x 10(4) Bq m(-2) for Cs-137 (corrected to 11 March 2011). The obtained densities of Cs-134 and Cs-137 were also supported by the accumulated amount of Cs-134 and Cs-137 in soil near the deposition sampling site. For the vertical profile of those radionuclides in soil at NIRS, about 94% of the total deposition was distributed in the top 10 mm depth on 26 April 2011. Health Phys. 104(2): 189-194; 2013