Deposition of fission and activation products after the Fukushima Dai-ichi nuclear power plant accident

► We collected environmental samples near the Fukushima nuclear power plant. ► We observed 239Np and 59Fe along with many fission products. ► 239Np is evidently an activation product of 238U contained in nuclear fuel. ► 239Np is also parent nuclide of 239Pu. ► Our results show that activation products diffused within a month of the accident.