Unbiunium

Unbiunium, also known as eka-actinium or element 121, is a hypothetical chemical element; it has symbol Ubu and atomic number 121. Unbiunium and Ubu are the temporary systematic IUPAC name and symbol respectively, which are used until the element is discovered, confirmed, and a permanent name is decided upon. In the periodic table of the elements, it is expected to be the first of the superactinides, and the third element in the eighth period. It has attracted attention because of some predictions that it may be in the island of stability. It is also likely to be the first of a new g-block of elements.
Unbiunium has not yet been synthesized. It is expected to be one of the last few reachable elements with current technology; the limit could be anywhere between element 120 and 124. It will also likely be far more difficult to synthesize than the elements known so far up to 118, and still more difficult than elements 119 and 120. The teams at RIKEN in Japan and at the JINR in Dubna, Russia have indicated plans to attempt the synthesis of element 121 in the future after they attempt elements 119 and 120.
The position of unbiunium in the periodic table suggests that it would have similar properties to lanthanum and actinium; however, relativistic effects may cause some of its properties to differ from those expected from a straight application of periodic trends. For example, unbiunium is expected to have a s²p valence electron configuration, instead of the s²d of lanthanum and actinium or the s²g expected from the Madelung rule, but this is not predicted to affect its chemistry much. It would on the other hand significantly lower its first ionization energy beyond what would be expected from periodic trends.

Introduction

History

reactions producing superheavy elements can be divided into "hot" and "cold" fusion, depending on the excitation energy of the compound nucleus produced. In hot fusion reactions, very light, high-energy projectiles are accelerated toward very heavy targets, giving rise to compound nuclei at high excitation energies that may fission or evaporate several neutrons. In cold fusion reactions, the fused nuclei produced have a relatively low excitation energy, which decreases the probability that these products will undergo fission reactions. As the fused nuclei cool to the ground state, they require emission of only one or two neutrons. However, hot fusion reactions tend to produce more neutron-rich products because the actinides have the highest neutron-to-proton ratios of any element that can presently be made in macroscopic quantities; it is currently the only method to produce the superheavy elements from flerovium onward.
Attempts to synthesize elements 119 and 120 push the limits of current technology, due to the decreasing cross sections of the production reactions and their probably short half-lives, expected to be on the order of microseconds. Heavier elements, beginning with element 121, would likely be too short-lived to be detected with current technology, decaying within a microsecond before reaching the detectors. Where this one-microsecond border of half-lives lies is not known, and this may allow the synthesis of some isotopes of elements 121 through 124, with the exact limit depending on the model chosen for predicting nuclide masses. It is also possible that element 120 is the last element reachable with current experimental techniques, and that elements from 121 onward will require new methods.
Because of the current impossibility of synthesizing elements beyond californium in sufficient quantities to create a target, with einsteinium targets being currently considered, the practical synthesis of elements beyond oganesson requires heavier projectiles, such as titanium-50, chromium-54, iron-58, or nickel-64. This, however, has the drawback of resulting in more symmetrical fusion reactions that are colder and less likely to succeed. For example, the reaction between ²⁴³Am and ⁵⁸Fe is expected to have a cross section on the order of 0.5 fb, several orders of magnitude lower than measured cross sections in successful reactions; such an obstacle would make this and similar reactions infeasible for producing unbiunium.

Past synthesis attempt

The synthesis of unbiunium was first attempted in 1977 by bombarding a target of uranium-238 with copper-65 ions at the Gesellschaft für Schwerionenforschung in Darmstadt, Germany:
No atoms were identified.

Prospects for future synthesis

Currently, the beam intensities at superheavy element facilities result in about 10¹² projectiles hitting the target per second; this cannot be increased without burning the target and the detector, and producing larger amounts of the increasingly unstable actinides needed for the target is impractical. The team at the Joint Institute for Nuclear Research in Dubna has built a new superheavy element factory with improved detectors and the ability to work on a smaller scale, but even so, continuing beyond element 120 and perhaps 121 would be a great challenge. It is possible that the age of fusion–evaporation reactions to produce new superheavy elements is coming to an end due to the increasingly short half-lives to spontaneous fission and the looming proton drip line, so that new techniques such as nuclear transfer reactions would be required to reach the superactinides.
Because the cross sections of these fusion-evaporation reactions increase with the asymmetry of the reaction, titanium would be a better projectile than chromium for the synthesis of element 121, though this necessitates an einsteinium target. This poses severe challenges due to the significant heating and damage of the target due to the high radioactivity of einsteinium-254, but it would nonetheless probably be the most promising approach. It would require working on a smaller scale due to the lower amount of ²⁵⁴Es that can be produced. This small-scale work could in the near future only be carried out in Dubna's SHE-factory.
The isotopes ²⁹⁹Ubu, ³⁰⁰Ubu, and ³⁰¹Ubu, that could be produced in the reaction between ²⁵⁴Es and ⁵⁰Ti via the 3n and 4n channels, are expected to be the only reachable unbiunium isotopes with half-lives long enough for detection. The cross sections would nevertheless push the limits of what can currently be detected. For example, in a 2016 publication, the cross section of the aforementioned reaction between ²⁵⁴Es and ⁵⁰Ti was predicted to be around 7 fb in the 4n channel, four times lower than the lowest measured cross section for a successful reaction. A 2021 calculation gives similarly low theoretical cross sections of 10 fb for the 3n channel and 0.6 fb for the 4n channel of this reaction, along with cross sections on the order of 1–10 fb for the reactions ²⁴⁹Bk+⁵⁴Cr, ²⁵²Es+⁵⁰Ti, and ²⁵⁸Md+⁴⁸Ca. However, ²⁵²Es and ²⁵⁸Md cannot currently be synthesized in sufficient quantities to form target material.
Should the synthesis of unbiunium isotopes in such a reaction be successful, the resulting nuclei would decay through isotopes of ununennium that could be produced by cross-bombardments in the ²⁴⁸Cm+⁵¹V or ²⁴⁹Bk+⁵⁰Ti reactions, down through known isotopes of tennessine and moscovium synthesized in the ²⁴⁹Bk+⁴⁸Ca and ²⁴³Am+⁴⁸Ca reactions. The multiplicity of excited states populated by the alpha decay of odd nuclei may however preclude clear cross-bombardment cases, as was seen in the controversial link between ²⁹³Ts and ²⁸⁹Mc. Heavier isotopes are expected to be more stable; ³²⁰Ubu is predicted to be the most stable unbiunium isotope, but there is no way to synthesize it with current technology as no combination of usable target and projectile could provide enough neutrons.
The teams at RIKEN and at JINR have listed the synthesis of element 121 among their future plans. These two laboratories are best suited to these experiments as they are the only ones in the world where long beam times are accessible for reactions with such low predicted cross-sections.

Naming

Using Mendeleev's nomenclature for unnamed and undiscovered elements, unbiunium should be known as eka-actinium. Using the 1979 IUPAC recommendations, the element should be temporarily called unbiunium until it is discovered, the discovery is confirmed, and a permanent name chosen. Although widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, the recommendations are mostly ignored among scientists who work theoretically or experimentally on superheavy elements, who call it "element 121", with the symbol E121, , or 121.

Nuclear stability and isotopes

The stability of nuclei decreases greatly with the increase in atomic number after curium, element 96, whose half-life is four orders of magnitude longer than that of any currently known higher-numbered element. All isotopes with an atomic number above 101 undergo radioactive decay with half-lives of less than 30 hours. No elements with atomic numbers above 82 have stable isotopes. Nevertheless, for reasons not yet well understood, there is a slight increase of nuclear stability around atomic numbers 110–114, which leads to the appearance of what is known in nuclear physics as the "island of stability". This concept, proposed by University of California professor Glenn Seaborg and stemming from the stabilizing effects of the closed nuclear shells around Z = 114 and N = 184, explains why superheavy elements last longer than predicted. In fact, the very existence of elements heavier than rutherfordium can be attested to shell effects and the island of stability, as spontaneous fission would rapidly cause such nuclei to disintegrate in a model neglecting such factors.
A 2016 calculation of the half-lives of the isotopes of unbiunium from ²⁹⁰Ubu to ³³⁹Ubu suggested that those from ²⁹⁰Ubu to ³⁰³Ubu would not be bound and would decay through proton emission, those from ³⁰⁴Ubu through ³¹⁴Ubu would undergo alpha decay, and those from ³¹⁵Ubu to ³³⁹Ubu would undergo spontaneous fission. Only the isotopes from ³⁰⁹Ubu to ³¹⁴Ubu would have long enough alpha-decay lifetimes to be detected in laboratories, starting decay chains terminating in spontaneous fission at moscovium, tennessine, or ununennium. This would present a grave problem for experiments aiming at synthesizing isotopes of unbiunium if true, because the isotopes whose alpha decay could be observed could not be reached by any presently usable combination of target and projectile. Calculations in 2016 and 2017 by the same authors on elements 123 and 125 suggest a less bleak outcome, with alpha decay chains from the more reachable nuclides ^300–307Ubt passing through unbiunium and leading down to bohrium or nihonium. It has also been suggested that cluster decay might be a significant decay mode in competition with alpha decay and spontaneous fission in the region past Z = 120, which would pose yet another hurdle for experimental identification of these nuclides.