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 (131)I, (132)I, (132)Te, (134)Cs, and (137)Cs. The largest deposition was observed during 7:00-16:00 on 21 March. The estimated total deposition densities at NIRS were 1.40 × 10(5) Bq m(-2) for (131)I, 4.12 × 10(4) Bq m(-2) for (132)Te, 1.45 × 10(4) Bq m(-2) for (134)Cs, and 1.48 × 10(4) Bq m(-2) for (137)Cs (corrected to 11 March 2011). The obtained densities of (134)Cs and (137)Cs were also supported by the accumulated amount of (134)Cs and (137)Cs 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.