RADIOISOTOPES

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.