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.
The element with Z=113 has been named as Nihonium and the names of the new elements up to Z=118 have been determined. Synthesis of these superheavy elements is a fusion–fission process under a very strong Coulomb repulsion. It revealed many new aspects of nuclear reactions such as quasi-fission which were not carefully studied so far. In this manuscript, we describe the processes of the synthesis of supreheavy elements from the view point of fluctuation-dissipation theory. We apply Langevin equation to calculate the competition between the formation of superheavy compound nuclei and the decay through the quasi-fission reaction. We also discuss the importance of the shell-correction energy in the survival process.
Nuclear structure and decay modes in the heavy and superheavy nuclear mass region is reviewed from a theoretical view, and a prediction of ‘island of stability of superheavy nuclei is shown with some results by the author. In the region there exists various decay modes as alpha decay, beta decay, spontaneous fission, etc. The stability of nucleus is governed by nuclear decay properties. We found a periodicity of nuclear shell closure with numbers of neutron as 126, 184 and 228. We also found an alpha-decay dominant nucleus with approximately three-hundred years of half-life, 294Ds (Z=110), as the nucleus with the longest half-life in the island of stability of superheavy nuclei.
Forefront of synthesis of the superheavy elements is reviewed. Nuclear fusion reactions and experimental techniques to synthesize superheavy elements are presented, especially for new elements with atomic number Z=113–118. Perspectives to search for new elements with Z≥119 are also described.
Existence of superheavy elements essentially depends on shell structure of superheavy nuclei. It is theoretically predicted that stable doubly-closed-shell spherical superheavy nuclei should exist around the region of the proton number 114 to 126 and the neutron number 172 to 184, although their exact locations and degree of stability have not been established because of large variety of theoretical predictions. On the other hand, we have already succeeded in producing superheavy nuclei with the proton number up to 118 and the neutron number up to 177, indicating that we have already reached the doubly-closed-shell superheavy region. In addition, some pioneering experiments have been performed to directly establish the level structure of superheavy nuclei through spectroscopic methods. This paper introduces the current status of experimental studies on nuclear shell structure of superheavy nuclei.
A project for precision mass measurements of super-heavy elements including newly named nihonium is in progress. A gas cell for efficient collection of fusion reaction product into an ion-trap and a fast, precise and accurate mass spectrograph, MRTOF-MS, play important role for the SHE-Mass project. In the first stage of the project, the masses of more than 80 isotopes including seven isotopes of Z≧100 elements were measured.