Online ISSN: 1884-4111 Print ISSN: 0033-8303

## 連載講座Serial Lecture

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

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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