Nihonium

Nihonium is a synthetic chemical element; it has symbol Nh and atomic number 113. It is extremely radioactive: its most stable known isotope, nihonium-286, has a half-life of about 10 seconds. In the periodic table, nihonium is a transactinide element in the p-block. It is a member of period 7 and group 13.
Nihonium was first reported to have been created in experiments carried out between 14 July and 10 August 2003, by a Russian–American collaboration at the Joint Institute for Nuclear Research in Dubna, Russia, working in collaboration with the Lawrence Livermore National Laboratory in Livermore, California, and on 23 July 2004, by a team of Japanese scientists at Riken in Wakō, Japan. The confirmation of their claims in the ensuing years involved independent teams of scientists working in the United States, Germany, Sweden, and China, as well as the original claimants in Russia and Japan. In 2015, the IUPAC/IUPAP Joint Working Party recognised the element and assigned the priority of the discovery and naming rights for the element to Riken. The Riken team suggested the name nihonium in 2016, which was approved in the same year. The name comes from the common Japanese name for Japan.
Very little is known about nihonium, as it has been made only in very small amounts that decay within seconds. The anomalously long lives of some superheavy nuclides, including some nihonium isotopes, are explained by the island of stability theory. Experiments to date have supported the theory, with the half-lives of the confirmed nihonium isotopes increasing from milliseconds to seconds as neutrons are added and the island is approached. Nihonium has been calculated to have similar properties to its homologues boron, aluminium, gallium, indium, and thallium. All but boron are post-transition metals, and nihonium is expected to be a post-transition metal as well. It should also show several major differences from them; for example, nihonium should be more stable in the +1 oxidation state than the +3 state, like thallium, but in the +1 state nihonium should behave more like silver and astatine than thallium. Preliminary experiments have shown that elemental nihonium is not very volatile, and that it is less reactive than its lighter homologue thallium.

Introduction

History

Early indications

The syntheses of elements 107 to 112 were conducted at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, from 1981 to 1996. These elements were made by cold fusion reactions, in which targets made of lead and bismuth, which are around the stable configuration of 82 protons, are bombarded with heavy ions of period 4 elements. This creates fused nuclei with low excitation energies due to the stability of the targets' nuclei, significantly increasing the yield of superheavy elements. Cold fusion was pioneered by Yuri Oganessian and his team in 1974 at the Joint Institute for Nuclear Research in Dubna, Soviet Union. Yields from cold fusion reactions were found to decrease significantly with increasing atomic number; the resulting nuclei were severely neutron-deficient and short-lived. The GSI team attempted to synthesise element 113 via cold fusion in 1998 and 2003, bombarding bismuth-209 with zinc-70; both attempts were unsuccessful.
Faced with this problem, Oganessian and his team at the JINR turned their renewed attention to the older hot fusion technique, in which heavy actinide targets were bombarded with lighter ions. Calcium-48 was suggested as an ideal projectile, because it is very neutron-rich for a light element and would minimise the neutron deficiencies of the nuclides produced. Being doubly magic, it would confer benefits in stability to the fused nuclei. In collaboration with the team at the Lawrence Livermore National Laboratory in Livermore, California, United States, they made an attempt on element 114.
In 1998, the JINR–LLNL collaboration started their attempt on element 114, bombarding a target of plutonium-244 with ions of calcium-48:
A single atom was observed which was thought to be the isotope ²⁸⁹114: the results were published in January 1999. Despite numerous attempts to repeat this reaction, an isotope with these decay properties has never again been found, and the exact identity of this activity is unknown. A 2016 paper by Sigurd Hofmann et al. considered that the most likely explanation of the 1998 result is that two neutrons were emitted by the produced compound nucleus, leading to ²⁹⁰114 and electron capture to ²⁹⁰113, while more neutrons were emitted in all other produced chains. This would have been the first report of a decay chain from an isotope of element 113, but it was not recognised at the time, and the assignment is still uncertain. A similar long-lived activity observed by the JINR team in March 1999 in the ²⁴²Pu + ⁴⁸Ca reaction may be due to the electron-capture daughter of ²⁸⁷114, ²⁸⁷113; this assignment is also tentative.

JINR–LLNL collaboration

The now-confirmed discovery of element 114 was made in June 1999 when the JINR team repeated the first ²⁴⁴Pu + ⁴⁸Ca reaction from 1998; following this, the JINR team used the same hot fusion technique to synthesize elements 116 and 118 in 2000 and 2002 respectively via the ²⁴⁸Cm + ⁴⁸Ca and ²⁴⁹Cf + ⁴⁸Ca reactions. They then turned their attention to the missing odd-numbered elements, as the odd protons and possibly neutrons would hinder decay by spontaneous fission and result in longer decay chains.
The first report of element 113 was in August 2003, when it was identified as an alpha decay product of element 115. Element 115 had been produced by bombarding a target of americium-243 with calcium-48 projectiles. The JINR–LLNL collaboration published its results in February 2004:
Four further alpha decays were observed, ending with the spontaneous fission of isotopes of element 105, dubnium.

Riken

While the JINR–LLNL collaboration had been studying fusion reactions with ⁴⁸Ca, a team of Japanese scientists at the Riken Nishina Center for Accelerator-Based Science in Wakō, Japan, led by Kōsuke Morita had been studying cold fusion reactions. Morita had previously studied the synthesis of superheavy elements at the JINR before starting his own team at Riken. In 2001, his team confirmed the GSI's discoveries of elements 108, 110, 111, and 112. They then made a new attempt on element 113, using the same ²⁰⁹Bi + ⁷⁰Zn reaction that the GSI had attempted unsuccessfully in 1998. Despite the much lower yield expected than for the JINR's hot fusion technique with calcium-48, the Riken team chose to use cold fusion as the synthesised isotopes would alpha decay to known daughter nuclides and make the discovery much more certain, and would not require the use of radioactive targets. In particular, the isotope ²⁷⁸113 expected to be produced in this reaction would decay to the known ²⁶⁶Bh, which had been synthesised in 2000 by a team at the Lawrence Berkeley National Laboratory in Berkeley.
The bombardment of ²⁰⁹Bi with ⁷⁰Zn at Riken began in September 2003. The team detected a single atom of ²⁷⁸113 in July 2004 and published their results that September:
The Riken team observed four alpha decays from ²⁷⁸113, creating a decay chain passing through ²⁷⁴Rg, ²⁷⁰Mt, and ²⁶⁶Bh before terminating with the spontaneous fission of ²⁶²Db. The decay data they observed for the alpha decay of ²⁶⁶Bh matched the 2000 data, lending support for their claim. Spontaneous fission of its daughter ²⁶²Db had not been previously known; the American team had observed only alpha decay from this nuclide.

Road to confirmation

When the discovery of a new element is claimed, the Joint Working Party of the International Union of Pure and Applied Chemistry and the International Union of Pure and Applied Physics assembles to examine the claims according to their criteria for the discovery of a new element, and decides scientific priority and naming rights for the elements. According to the JWP criteria, a discovery must demonstrate that the element has an atomic number different from all previously observed values. It should also preferably be repeated by other laboratories, although this requirement has been waived where the data is of very high quality. Such a demonstration must establish properties, either physical or chemical, of the new element and establish that they are those of a previously unknown element. The main techniques used to demonstrate atomic number are cross-reactions and anchoring decay chains to known daughter nuclides. For the JWP, priority in confirmation takes precedence over the date of the original claim. Both teams set out to confirm their results by these methods.

2004–2008

In June 2004 and again in December 2005, the JINR–LLNL collaboration strengthened their claim for the discovery of element 113 by conducting chemical experiments on ²⁶⁸Db, the final decay product of ²⁸⁸115. This was valuable as none of the nuclides in this decay chain were previously known, so that their claim was not supported by any previous experimental data, and chemical experimentation would strengthen the case for their claim, since the chemistry of dubnium is known. ²⁶⁸Db was successfully identified by extracting the final decay products, measuring spontaneous fission activities and using chemical identification techniques to confirm that they behave like a group 5 element. Both the half-life and decay mode were confirmed for the proposed ²⁶⁸Db which lends support to the assignment of the parent and daughter nuclei to elements 115 and 113 respectively. Further experiments at the JINR in 2005 confirmed the observed decay data.
In November and December 2004, the Riken team studied the ²⁰⁵Tl + ⁷⁰Zn reaction, aiming the zinc beam onto a thallium rather than a bismuth target, in an effort to directly produce ²⁷⁴Rg in a cross-bombardment as it is the immediate daughter of ²⁷⁸113. The reaction was unsuccessful, as the thallium target was physically weak compared to the more commonly used lead and bismuth targets, and it deteriorated significantly and became non-uniform in thickness. The reasons for this weakness are unknown, given that thallium has a higher melting point than bismuth. The Riken team then repeated the original ²⁰⁹Bi + ⁷⁰Zn reaction and produced a second atom of ²⁷⁸113 in April 2005, with a decay chain that again terminated with the spontaneous fission of ²⁶²Db. The decay data were slightly different from those of the first chain: this could have been because an alpha particle escaped from the detector without depositing its full energy, or because some of the intermediate decay products were formed in metastable isomeric states.
In 2006, a team at the Heavy Ion Research Facility in Lanzhou, China, investigated the ²⁴³Am + ²⁶Mg reaction, producing four atoms of ²⁶⁶Bh. All four chains started with an alpha decay to ²⁶²Db; three chains ended there with spontaneous fission, as in the ²⁷⁸113 chains observed at Riken, while the remaining one continued via another alpha decay to ²⁵⁸Lr, as in the ²⁶⁶Bh chains observed at LBNL.
In June 2006, the JINR–LLNL collaboration claimed to have synthesised a new isotope of element 113 directly by bombarding a neptunium-237 target with accelerated calcium-48 nuclei:
Two atoms of ²⁸²113 were detected. The aim of this experiment had been to synthesise the isotopes ²⁸¹113 and ²⁸²113 that would fill in the gap between isotopes produced via hot fusion and cold fusion. After five alpha decays, these nuclides would reach known isotopes of lawrencium, assuming that the decay chains were not terminated prematurely by spontaneous fission. The first decay chain ended in fission after four alpha decays, presumably originating from ²⁶⁶Db or its electron-capture daughter ²⁶⁶Rf. Spontaneous fission was not observed in the second chain even after four alpha decays. A fifth alpha decay in each chain could have been missed, since ²⁶⁶Db can theoretically undergo alpha decay, in which case the first decay chain would have ended at the known ²⁶²Lr or ²⁶²No and the second might have continued to the known long-lived ²⁵⁸Md, which has a half-life of 51.5 days, longer than the duration of the experiment: this would explain the lack of a spontaneous fission event in this chain. In the absence of direct detection of the long-lived alpha decays, these interpretations remain unconfirmed, and there is still no known link between any superheavy nuclides produced by hot fusion and the well-known main body of the chart of nuclides.