A record of <sup>241</sup>Am, <sup>236</sup>U, <sup>238</sup>U, <sup>239</sup>Pu, <sup>240</sup>Pu, <sup>134</sup>Cs and <sup>137</sup>Cs in surface seawater and <sup>241</sup>Am in aerosols shortly after the FDNPP incident occurred

We obtained the rare data on low activity concentration of Am-241 in the surface seawater collected at about 210 km northeast of Fukushima Daiichi Nuclear Power Plant (FDNPP) on 29 April 2011. The concentration of U-236, U-238, Pu-239, Pu-240, Cs-134 and Cs-137 in that seawater was also determined and the atomic ratio U-236/U-238 and Pu-240/Pu-239 was calculated. The activity concentration of Cs-137 compared to that before the incident and the equal activity ratio of Cs-134/Cs-137 clearly indicate that the seawater was contaminated by the radioactive Cs discharged from the FDNPP, while the activity concentration of Am-241 as well as U-236, Pu-239 and Pu-240 was in the range of the values reported before the incident. Am-241 in the aerosols collected at about 120 km from the FDNPP, in which FDNPP derived U-236 and Pu isotopes were found, was analyzed. However, no significant larger amount compared to U-236 and Pu239+240 was found.