RADIOISOTOPES

The Japan Maritime Safety Board (now Japan Coast Guard) observation vessel Takuyo encountered nuclear fallout originating from a U.S. nuclear weapon test detonated at Bikini Atoll on July 12, 1958. The exposure occurred two days after the detonation when the vessel was sailing southbound, about 300 km west of the danger area set up around the test site. From a small amount of rain sampled at the beginning of a rainsquall, a gross beta radioactivity of 16 kBq L⁻¹ was observed, but no total precipitation measurement was made at that time. Therefore, the total amount of gross beta activity surface deposition density was alternatively derived based on an indication of a NaI(Tl) scintillation detector placed 0.3-m above the after deck of the Takuyo. By combining the maximum measured dose rate of 3.1 µSv h⁻¹ (0.31 mR h⁻¹) aboard with the complementary results of Monte Carlo simulations, the surface deposition density on the Takuyo was estimated to be 2 TBq km⁻², ～10 times higher than the past maximum observed in Japan in 1966. The resultant effective dose to crew members was also estimated to be below 100 µSv over the entire period of the voyage.