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放射性核種データの改訂が作業者に対する内部被ばく線量係数に及ぼす影響

Impact of a revision of nuclear decay data on internal dose coefficients for workers

真辺 健太郎; 遠藤 章

Manabe, Kentaro; Endo, Akira

国際放射線防護委員会(ICRP)が、従来、線量計算に利用してきた放射性核種データICRP Publication 38 (ICRP38)と、2008年に新たに出版された放射性核種データICRP107を用いて、データの更新が作業者の経口摂取に対する線量係数に与える影響について調べた。また、報告者らが以前実施した吸入摂取に対する線量係数の分析結果とあわせて、放射性核種データの改訂が内部被ばく線量係数に及ぼす影響を総合的に検討した。774核種,876の化学形について経口摂取に対する線量係数を計算し比較した結果、線量係数が10%以上増加又は減少した化学形の数は、それぞれ68及び21であった。これらの比較的大きな違いを示した原因は、放出放射線のエネルギーの変化,放出される電子と光子のエネルギーの割合,壊変様式,半減期の変化等であった。これらの結果を吸入摂取に対する線量係数の分析結果と比較したところ、電子と光子のエネルギーの割合の変化は、経口摂取の方が、線量係数に及ぼす影響が大きいことがわかった。一方、$$beta$$線エネルギースペクトルの形状の変化は、吸入摂取の場合にのみ線量係数に影響を与えることが明らかになった。

The impact of revisions of nuclear decay data on internal dose coefficients for workers was studied using revised data of ICRP Publication 107 (ICRP107) and existing data of ICRP Publication 38 (ICRP38). At first, two sets of ingestion dose coefficients for workers based on ICRP107 and ICRP38 were calculated using the dose and risk calculation software DCAL for 876 chemical forms of 774 nuclides. Ingestion dose coefficients based on ICRP107 increased by over 10% compared with those based on ICRP38 in 61 chemical forms, and decreased by over 10% in 28 chemical forms. It was revealed that the differences in ingestion dose coefficients mainly originated from the revision of energy data as is the case in inhalation dose coefficients. And, the revisions of the proportion of energy of electrons to that of photons, decay modes and half-lives also resulted in changes in the dose coefficients. Then, the extent and causes of changes in ingestion and inhalation dose coefficients were also compared with each other in light of our previous work concerning inhalation dose coefficients. As a result of comparing the dose coefficients for ingestion and inhalation, it was found that the change in proportion of energy of electrons to that of photons impacted on dose coefficients in ingestion more significantly than in inhalation. On the other hand, the change in shape of $$beta$$ energy spectrum, which was one of the causes of change in inhalation dose coefficients, did not impact on ingestion dose coefficients.

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