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測量船「拓洋」が遭遇した核実験フォールアウト

Nuclear fallout encountered by oceanographic observation vessel Takuyo

辻村 憲雄

Tsujimura, Norio

1958年7月、海上保安庁の測量船「拓洋」は、赤道海域に向かう途中で、米国がビキニ環礁で実施した水爆実験によって発生した核実験フォールアウトに遭遇した。当時船上で採取した雨水から観測された全$$beta$$放射能及び甲板に設置したNaI(Tl)シンチレーション検出器の計数率並びに計算シミュレーションの結果を基に、その単位面積当たりの全$$beta$$放射能を2.0TBq/km$$^{2}$$、外部被ばくによる実効線量を100$$mu$$Svに満たないと推定した。

The 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 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 $$mu$$Sv/h aboard with the results of Monte Carlo simulations, the surface deposition density on the Takuyo was estimated to be 2 PBq/km$$^{2}$$, about 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 $$mu$$Sv over the entire period of the voyage.

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