Online ISSN: 1884-4111 Print ISSN: 0033-8303
Radioisotopes 67(6): 243-253 (2018)

連載講座Serial Lecture

新元素ニホニウムの発見Discovery of New Element, Nihonium

新潟大学自然科学系Institute of Science and Technology, Niigata University ◇ 950–2181 新潟市西区五十嵐2の町8050 ◇ 8050 Ikarashi 2-no cho, Nishi-ku, Niigata Pref. 950–2181, Japan

発行日:2018年6月15日Published: June 15, 2018

理化学研究所で合成された113番元素は正式にその発見が認められ,研究グループからの提案通り元素名nihonium(ニホニウム),元素記号NhとIUPACによって正式命名された。113番元素合成実験は,冷たい融合反応により理化学研究所線形加速器施設で気体充填型反跳分離装置を用いて行われた。110番元素271Ds,111番元素272Rg,そして112番元素277Cnを追試実験により確認したのち,209Biに70Znを照射して新元素113番元素の探索が行われた。実質照射時間576日の実験で,278113に起因する崩壊連鎖が3事象観測された。1番目と2番目の崩壊連鎖は278113から266Bhの4つのα崩壊からなり,262Dbの自発核分裂で終了した。3番目はα崩壊が6回連続して起こり,既知核種262Db, 258Lrを経由して254Mdに至っている。観測された3つの崩壊連鎖はお互い矛盾のないものであった。また,266Bhの崩壊特性を248Cm+23Na反応によって調査し,278113の崩壊連鎖の一員である266Bhと一致することが確認された。これらの結果より,278113の合成が明確に立証された。

The priority of the discovery of the element 113 was officially assigned to the work that was carried out at RIKEN. The name and symbol of nihonium and Nh which was proposed to the element 113 by the discovery group was formally approved by the IUPAC Bureau. Experiments for producing heaviest elements by a cold-fusion type reaction were performed using a gas-filled recoil ion separator at the RIKEN linear accelerator facility. After confirmation experiments of the reported nuclides, 271Ds, 272Rg, and 277Cn, a new superheavy nuclide 278113 has been searched by the reaction of 70Zn on 209Bi. Totally 3 decay chains due to 278113 were observed during the net irradiation time of 576 days. The 1st and 2nd decay chains consist of four alpha decays from 278113 to 266Bh and terminate by spontaneous fission of 262Db. The 3rd chain consists of 6 consecutive alpha decays down to 254Md via known nuclides 262Db and 258Lr. Observed decay properties from 3 decay chains were consistent with each other. We also examined the decay properties of 266Bh produced by the reaction of 23Na on 248Cm to establish the cross-reaction of 278113 decay chain. It was found that 266Bh can be regarded as an anchor nuclide of the 278113 decay chain, and thus the production of 278113 was clearly confirmed.

Key words: new element; nihonium; cold-fusion reaction; gas-filled recoil ion separator; 278113; bohrium-266

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