Unbibium

Unbibium, also known as element 122 or eka-thorium, is a hypothetical chemical element; it has placeholder symbol Ubb and atomic number 122. Unbibium and Ubb 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 follow unbiunium as the second element of the superactinides and the fourth element of the 8th period. Similarly to unbiunium, it is expected to fall within the range of the island of stability, potentially conferring additional stability on some isotopes, especially ³⁰⁶Ubb which is expected to have a magic number of neutrons.
Despite several attempts, unbibium has not yet been synthesized, nor have any naturally occurring isotopes been found to exist. There are currently no plans to attempt to synthesize unbibium. In 2008, it was claimed to have been discovered in natural thorium samples, but that claim has now been dismissed by recent repetitions of the experiment using more accurate techniques.
Chemically, unbibium is expected to show some resemblance to cerium and thorium. However, relativistic effects may cause some of its properties to differ; for example, it is expected to have a ground state electron configuration of [Og] 7d¹ 8s² 8p¹ or 8s² 8p², despite its predicted position in the g-block superactinide series.

Introduction

History

Synthesis attempts

Fusion-evaporation

Two attempts were made to synthesize unbibium in the 1970s, both propelled by early predictions on the island of stability at N = 184 and Z > 120, and in particular whether superheavy elements could potentially be naturally occurring. The first attempts to synthesize unbibium were performed in 1972 by Flerov et al. at the Joint Institute for Nuclear Research, using the heavy-ion induced hot fusion reactions:
Another unsuccessful attempt to synthesize unbibium was carried out in 1978 at the GSI Helmholtz Center, where a natural erbium target was bombarded with xenon-136 ions:
No atoms were detected and a yield limit of 5 nb was measured. Current results have shown that the sensitivity of these experiments were too low by at least 3 orders of magnitude. In particular, the reaction between ¹⁷⁰Er and ¹³⁶Xe was expected to yield alpha emitters with half-lives of microseconds that would decay down to isotopes of flerovium with half-lives perhaps increasing up to several hours, as flerovium is predicted to lie near the center of the island of stability. After twelve hours of irradiation, nothing was found in this reaction. Following a similar unsuccessful attempt to synthesize unbiunium from ²³⁸U and ⁶⁵Cu, it was concluded that half-lives of superheavy nuclei must be less than one microsecond or the cross sections are very small. More recent research into synthesis of superheavy elements suggests that both conclusions are true.
In 2000, the Gesellschaft für Schwerionenforschung Helmholtz Center for Heavy Ion Research performed a very similar experiment with much higher sensitivity:
These results indicate that the synthesis of such heavier elements remains a significant challenge and further improvements of beam intensity and experimental efficiency is required. The sensitivity should be increased to 1 fb in the future for more quality results.

Compound nucleus fission

Several experiments studying the fission characteristics of various superheavy compound nuclei such as ³⁰⁶Ubb were performed between 2000 and 2004 at the Flerov Laboratory of Nuclear Reactions. Two nuclear reactions were used, namely ²⁴⁸Cm + ⁵⁸Fe and ²⁴²Pu + ⁶⁴Ni. The results reveal how superheavy nuclei fission predominantly by expelling closed shell nuclei such as ¹³²Sn. It was also found that the yield for the fusion-fission pathway was similar between ⁴⁸Ca and ⁵⁸Fe projectiles, suggesting a possible future use of ⁵⁸Fe projectiles in superheavy element formation.

Claimed discovery as a naturally occurring element

In 2008, a group led by Israeli physicist Amnon Marinov at the Hebrew University of Jerusalem claimed to have found single atoms of unbibium-292 in naturally occurring thorium deposits at an abundance of between 10⁻¹¹ and 10⁻¹² relative to thorium. This was the first time in 69 years that a new element had been claimed to be discovered in nature, after Marguerite Perey's 1939 discovery of francium. The claim of Marinov et al. was criticized by the scientific community, and Marinov says he has submitted the article to the journals Nature and Nature Physics but both turned it down without sending it for peer review. The unbibium-292 atoms were claimed to be superdeformed or hyperdeformed isomers, with a half-life of at least 100 million years.
A criticism of the technique, previously used in purportedly identifying lighter thorium isotopes by mass spectrometry, was published in Physical Review C in 2008. A rebuttal by the Marinov group was published in Physical Review C after the published comment.
A repeat of the thorium experiment using the superior method of accelerator mass spectrometry failed to confirm the results, despite a 100-fold better sensitivity. This result throws considerable doubt on the results of the Marinov collaboration with regards to their claims of long-lived isotopes of thorium, roentgenium, and unbibium. Current understanding of superheavy elements indicates that it is very unlikely for any traces of unbibium to persist in natural thorium samples.

Naming

Using Mendeleev's nomenclature for unnamed and undiscovered elements, unbibium should instead be known as eka-thorium. After the recommendations of the IUPAC in 1979, the element has since been largely referred to as unbibium with the atomic symbol of, as its temporary name until the element is officially discovered and synthesized, and a permanent name is decided on. Scientists largely ignore this naming convention, and instead simply refer to unbibium as "element 122" with the symbol of, or sometimes even E122 or 122.

Prospects for future synthesis

Every element from mendelevium onward was produced in fusion-evaporation reactions, culminating in the discovery of the heaviest known element oganesson in 2002 and most recently tennessine in 2010. These reactions approached the limit of current technology; for example, the synthesis of tennessine required 22 milligrams of ²⁴⁹Bk and an intense ⁴⁸Ca beam for six months. The intensity of beams in superheavy element research cannot exceed 10¹² projectiles per second without damaging the target and detector, and producing larger quantities of increasingly rare and unstable actinide targets is impractical.
Consequently, future experiments must be done at facilities such as the superheavy element factory at the Joint Institute for Nuclear Research or RIKEN, which will allow experiments to run for longer stretches of time with increased detection capabilities and enable otherwise inaccessible reactions.
It is possible that fusion-evaporation reactions will not be suitable for the discovery of unbibium or heavier elements. Various models predict increasingly short alpha and spontaneous fission half-lives for isotopes with Z = 122 and N ~ 180 on the order of microseconds or less, rendering detection nearly impossible with current equipment. The increasing dominance of spontaneous fission also may sever possible ties to known nuclei of livermorium or oganesson and make identification and confirmation more difficult; a similar problem occurred in the road to confirmation of the decay chain of ²⁹⁴Og which has no anchor to known nuclei. For these reasons, other methods of production may need to be researched such as multi-nucleon transfer reactions capable of populating longer-lived nuclei. A similar switch in experimental technique occurred when hot fusion using ⁴⁸Ca projectiles was used instead of cold fusion to populate elements with Z > 113.
Nevertheless, several fusion-evaporation reactions leading to unbibium have been proposed in addition to those already tried unsuccessfully, though no institution has immediate plans to make synthesis attempts, instead focusing first on elements 119, 120, and possibly 121. Because cross sections increase with asymmetry of the reaction, a chromium beam would be most favorable in combination with a californium target, especially if the predicted closed neutron shell at N = 184 could be reached in more neutron-rich products and confer additional stability. In particular, the reaction between and would generate the compound nucleus and reach the shell at N = 184, though the analogous reaction with a target is believed to be more feasible because of the presence of unwanted fission products from and difficulty in accumulating the required amount of target material. One possible synthesis of unbibium could occur as follows:
Should this reaction be successful and alpha decay remain dominant over spontaneous fission, the resultant ³⁰⁰Ubb would decay through ²⁹⁶Ubn which may be populated in cross-bombardment between ²⁴⁹Cf and ⁵⁰Ti. Although this reaction is one of the most promising options for the synthesis of unbibium in the near future, the maximum cross section is predicted to be 3 fb, one order of magnitude lower than the lowest measured cross section in a successful reaction. The more symmetrical reactions ²⁴⁴Pu + ⁶⁴Ni and ²⁴⁸Cm + ⁵⁸Fe have also been proposed and may produce more neutron-rich isotopes. With increasing atomic number, one must also be aware of decreasing fission barrier heights, resulting in lower survival probability of compound nuclei, especially above the predicted magic numbers at Z = 126 and N = 184.