Unbiquadium

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Unbiquadium, also known as element 124 or eka-uranium, is a hypothetical chemical element; it has placeholder symbol Ubq and atomic number 124. Unbiquadium and Ubq are the temporary IUPAC name and symbol, respectively, until the element is discovered, confirmed, and a permanent name is decided upon. In the periodic table, unbiquadium is expected to be a g-block superactinide and the sixth element in the 8th period. Unbiquadium has attracted attention, as it may lie within the island of stability, leading to longer half-lives, especially for 308Ubq which is predicted to have a magic number of neutrons (184).

Despite several searches, unbiquadium has not been synthesized, nor have any naturally occurring isotopes been found to exist. It is believed that the synthesis of unbiquadium will be far more challenging than that of lighter undiscovered elements, and nuclear instability may pose further difficulties in identifying unbiquadium, unless the island of stability has a stronger stabilizing effect than predicted in this region.

As a member of the superactinide series, unbiquadium is expected to bear some resemblance to its possible lighter congener uranium. The valence electrons of unbiquadium are expected to participate in chemical reactions fairly easily, though relativistic effects may significantly influence some of its properties; for example, the electron configuration has been calculated to differ considerably from the one predicted by the Aufbau principle.
Introduction

A graphic depiction of a nuclear fusion reaction

External videos
[2]

The heaviest[a] atomic nuclei are created in nuclear reactions that combine two other nuclei of unequal size[b] into one; roughly, the more unequal the two nuclei in terms of mass, the greater the possibility that the two react.[8] The material made of the heavier nuclei is made into a target, which is then bombarded by the beam of lighter nuclei. Two nuclei can fuse into one only if they approach each other closely enough; normally, nuclei (all positively charged) repel each other due to electrostatic repulsion. The strong interaction can overcome this repulsion but only within a very short distance from a nucleus; beam nuclei are thus greatly accelerated in order to make such repulsion insignificant compared to the velocity of the beam nucleus.[9] Coming close alone is not enough for two nuclei to fuse: when two nuclei approach each other, they usually remain together for approximately 10−20 seconds and then part ways (not necessarily in the same composition as before the reaction) rather than form a single nucleus.[9][10] If fusion does occur, the temporary merger—termed a compound nucleus—is an excited state. To lose its excitation energy and reach a more stable state, a compound nucleus either fissions or ejects one or several neutrons,[c] which carry away the energy. This occurs in approximately 10−16 seconds after the initial collision.[11][d]

The beam passes through the target and reaches the next chamber, the separator; if a new nucleus is produced, it is carried with this beam.[14] In the separator, the newly produced nucleus is separated from other nuclides (that of the original beam and any other reaction products)[e] and transferred to a surface-barrier detector, which stops the nucleus. The exact location of the upcoming impact on the detector is marked; also marked are its energy and the time of the arrival.[14] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[17] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[14]

Stability of a nucleus is provided by the strong interaction. However, its range is very short; as nuclei become larger, their influence on the outermost nucleons (protons and neutrons) weakens. At the same time, the nucleus is torn apart by electrostatic repulsion between protons, as it has unlimited range.[18] Nuclei of the heaviest elements are thus theoretically predicted[19] and have so far been observed[20] to primarily decay via decay modes that are caused by such repulsion: alpha decay and spontaneous fission;[f] these modes are predominant for nuclei of superheavy elements. Alpha decays are registered by the emitted alpha particles, and the decay products are easy to determine before the actual decay; if such a decay or a series of consecutive decays produces a known nucleus, the original product of a reaction can be determined arithmetically.[g] Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.[h]

The information available to physicists aiming to synthesize one of the heaviest elements is thus the information collected at the detectors: location, energy, and time of arrival of a particle to the detector, and those of its decay. The physicists analyze this data and seek to conclude that it was indeed caused by a new element and could not have been caused by a different nuclide than the one claimed. Often, provided data is insufficient for a conclusion that a new element was definitely created and there is no other explanation for the observed effects; errors in interpreting data have been made.[i]
History
Synthesis attempts

Because complete nuclear shells (or, equivalently, a magic number of protons or neutrons) may confer additional stability on the nuclei of superheavy elements, moving closer to the center of the island of stability, it was thought that the synthesis of element 124 or nearby elements would populate longer-lived nuclei within the island. Scientists at GANIL (Grand Accélérateur National d'Ions Lourds) attempted to measure the direct and delayed fission of compound nuclei of elements with Z = 114, 120, and 124 in order to probe shell effects in this region and to pinpoint the next spherical proton shell. In 2006, with full results published in 2008, the team provided results from a reaction involving the bombardment of a natural germanium target with uranium ions:[32]

238
92U
+ nat
32Ge
→ 308,310,311,312,314
Ubq
* → fission

The team reported that they had been able to identify compound nuclei fissioning with half-lives > 10−18 s. This result suggests a strong stabilizing effect at Z = 124 and points to the next proton shell at Z > 120, not at Z = 114 as previously thought. A compound nucleus is a loose combination of nucleons that have not arranged themselves into nuclear shells yet. It has no internal structure and is held together only by the collision forces between the target and projectile nuclei. It is estimated that it requires around 10−14 s for the nucleons to arrange themselves into nuclear shells, at which point the compound nucleus becomes a nuclide, and this number is used by IUPAC as the minimum half-life a claimed isotope must have to potentially be recognised as being discovered. Thus, the GANIL experiments do not count as a discovery of element 124.[32]

The fission of the compound nucleus 312124 was also studied in 2006 at the tandem ALPI heavy-ion accelerator at the Laboratori Nazionali di Legnaro (Legnaro National Laboratories) in Italy:[33]

232
90Th
+ 80
34Se
→ 312
Ubq
* → fission

Similarly to previous experiments conducted at the JINR (Joint Institute for Nuclear Research), fission fragments clustered around doubly magic nuclei such as 132Sn (Z = 50, N = 82), revealing a tendency for superheavy nuclei to expel such doubly magic nuclei in fission.[34] The average number of neutrons per fission from the 312124 compound nucleus (relative to lighter systems) was also found to increase, confirming that the trend of heavier nuclei emitting more neutrons during fission continues into the superheavy mass region.[33]
Possible natural occurrence

A study in 1976 by a group of American researchers from several universities proposed that primordial superheavy elements, mainly livermorium, unbiquadium, unbihexium, and unbiseptium, could be a cause of unexplained radiation damage (particularly radiohalos) in minerals.[35] Unbiquadium was then suggested to exist in nature with its possible congener uranium in detectable quantities, at a relative abundance of 10−11.[36] Such unbiquadium nuclei were thought to undergo alpha decay with very long half-lives down to flerovium, which would then exist in natural lead at a similar concentration (10−11) and undergo spontaneous fission.[36][37] This prompted many researchers to search for them in nature from 1976 to 1983. A group led by Tom Cahill, a professor at the University of California at Davis, claimed in 1976 that they had detected alpha particles and X-rays with the right energies to cause the damage observed, supporting the presence of these elements. Others claimed that none had been detected, and questioned the proposed characteristics of primordial superheavy nuclei.[38] In particular, they cited that the magic number N = 228 necessary for enhanced stability would create a neutron-excessive nucleus in unbiquadium that would not be beta-stable. This activity was also proposed to be caused by nuclear transmutations in natural cerium, raising further ambiguity upon this claimed observation of superheavy elements.[39]

The possible extent of primordial superheavy elements on Earth today is uncertain. Even if they are confirmed to have caused the radiation damage long ago, they might now have decayed to mere traces, or even be completely gone.[40] It is also uncertain if such superheavy nuclei may be produced naturally at all, as spontaneous fission is expected to terminate the r-process responsible for heavy element formation between mass number 270 and 290, well before elements such as unbiquadium may be formed.[41]
Naming

Using the 1979 IUPAC recommendations, the element should be temporarily called unbiquadium (symbol Ubq) until it is discovered, the discovery is confirmed, and a permanent name chosen.[42] 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 124", with the symbol E124, (124), or 124.[43] Some researchers have also referred to unbiquadium as eka-uranium,[37] a name derived from the system Dmitri Mendeleev used to predict unknown elements, though such an extrapolation might not work for g-block elements with no known congeners and eka-uranium would instead refer to element 144[44] or 146[45] when the term is meant to denote the element directly below uranium.
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[46][47] and more recently tennessine in 2010.[48] 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.[49] Consequently, future experiments must be done at facilities such as the superheavy element factory (SHE-factory) at the Joint Institute for Nuclear Research (JINR) or RIKEN, which will allow experiments to run for longer stretches of time with increased detection capabilities and enable otherwise inaccessible reactions.[50] Even so, it is expected to be a great challenge to continue past elements 120 or 121 given short predicted half-lives and low predicted cross sections.[51]

The production of new superheavy elements will require projectiles heavier than 48Ca, which was successfully used in the discovery of elements 114–118, though this necessitates more symmetric reactions which are less favorable.[52] Hence, it is likely that the reactions between 58Fe and a 249Cf[51] or 251Cf target are most promising.[53] Studies on the fission of various superheavy compound nuclei have found that the dynamics of 48Ca- and 58Fe-induced reactions are similar, suggesting that 58Fe projectiles may be viable in producing superheavy nuclei up to Z = 124 or possibly 125.[49][54] It is also possible that a reaction with 251Cf will produce the compound nucleus 309Ubq* with 185 neutrons, immediately above the N = 184 shell closure. For this reason, the compound nucleus is predicted to have relatively high survival probability and low neutron separation energy, leading to the 1n–3n channels and isotopes 306–308Ubq with a relatively high cross section.[53] These dynamics are highly speculative, as the cross section may be far lower should trends in the production of elements 112–118 continue or the fission barriers be lower than expected, regardless of shell effects, leading to decreased stability against spontaneous fission (which is of growing importance).[51] Nonetheless, the prospect of reaching the N = 184 shell on the proton-rich side of the chart of nuclides by increasing proton number has long been considered; already in 1970, Soviet nuclear physicist Georgy Flyorov suggested bombarding a plutonium target with zinc projectiles to produce isotopes of element 124 at the N = 184 shell.[55]
Predicted properties
Nuclear stability and isotopes
This nuclear chart used by the Japan Atomic Energy Agency predicts the decay modes of nuclei up to Z = 149 and N = 256. For unbiquadium (Z = 124), there are predicted regions of increased stability around N = 184 and N = 228, though many intermediate isotopes are theoretically susceptible to spontaneous fission with half-lives shorter than 1 nanosecond.[56]

Unbiquadium is of interest to researchers because of its possible location near the center of an island of stability, a theoretical region comprising longer-lived superheavy nuclei. Such an island of stability was first proposed by University of California professor Glenn Seaborg,[57] specifically predicting a region of stability centered at element 126 (unbihexium) and encompassing nearby elements, including unbiquadium, with half-lives possibly as long as 109 years.[36] In known elements, the stability of nuclei decreases greatly with the increase in atomic number after uranium, the heaviest primordial element, so that all observed isotopes with an atomic number above 101 decay radioactively with a half-life under a day. Nevertheless, there is a slight increase in nuclear stability in nuclides around atomic numbers 110–114, which suggests the presence of an island of stability. This is attributed to the possible closure of nuclear shells in the superheavy mass region, with stabilizing effects that may lead to half-lives on the order of years or longer for some as-yet undiscovered isotopes of these elements.[36][52] While still unproven, the existence of superheavy elements as heavy as oganesson provides evidence of such stabilizing effects, as elements with an atomic number greater than approximately 104 are extremely unstable in models neglecting magic numbers.[58]

In this region of the periodic table, N = 184 and N = 228 have been proposed as closed neutron shells,[59] and various atomic numbers have been proposed as closed proton shells, including Z = 124.[j] The island of stability is characterized by longer half-lives of nuclei located near these magic numbers, though the extent of stabilizing effects is uncertain due to predictions of weakening of the proton shell closures and possible loss of double magicity.[59] More recent research predicts the island of stability to instead be centered at beta-stable copernicium isotopes 291Cn and 293Cn,[52][60] which would place unbiquadium well above the island and result in short half-lives regardless of shell effects. A 2016 study on the decay properties of unbiquadium isotopes 284–339Ubq predicts that 284–304Ubq lie outside the proton drip line and thus may be proton emitters, 305–323Ubq may undergo alpha decay, with some chains terminating as far as flerovium, and heavier isotopes will decay by spontaneous fission.[61] These results, as well as those from a quantum-tunneling model, predict no half-lives over a millisecond for isotopes lighter than 319Ubq,[62] as well as especially short half-lives for 309–314Ubq in the sub-microsecond range[61] due to destabilizing effects immediately above the shell at N = 184. This renders the identification of many unbiquadium isotopes nearly impossible with current technology, as detectors cannot distinguish rapid successive signals from alpha decays in a time period shorter than microseconds.[51][k]

Increasingly short spontaneous fission half-lives of superheavy nuclei and the possible domination of fission over alpha decay will also probably determine the stability of unbiquadium isotopes.[51][60] While some fission half-lives constituting a "sea of instability" may be on the order of 10−18 s as a consequence of very low fission barriers, especially in even–even nuclei due to pairing effects, stabilizing effects at N = 184 and N = 228 may allow the existence of relatively long-lived isotopes.[56] For N = 184, fission half-lives may increase, though alpha half-lives are still expected to be on the order of microseconds or less, despite the shell closure at 308Ubq. It is also possible that the island of stability may shift to the N = 198 region, where total half-lives may be on the order of seconds,[60] in contrast to neighboring isotopes that would undergo fission in less than a microsecond. In the neutron-rich region around N = 228, alpha half-lives are also predicted to increase with increasing neutron number, meaning that the stability of such nuclei would primarily depend on the location of the beta-stability line and resistance to fission. One early calculation by P. Moller, a physicist at Los Alamos National Laboratory, estimates the total half-life of 352Ubq (with N = 228) to be around 67 seconds, and possibly the longest in the N = 228 region.[36][63]
Chemical

Unbiquadium is the fourth member of the superactinide series and should be similar to uranium: both elements have six valence electrons over a noble gas core. In the superactinide series, the Aufbau principle is expected to break down due to relativistic effects, and an overlap of the 5g, 6f, 7d, and 8p orbitals is expected. The ground state electron configuration of unbiquadium is thus predicted to be [Og] 6f3 8s2 8p1[64] or 6f2 8s2 8p2,[65] in contrast to [Og] 5g4 8s2 derived from Aufbau. This predicted overlap of orbitals and uncertainty in order of filling, especially for f and g orbitals, renders predictions of chemical and atomic properties of these elements very difficult.[66]

One predicted oxidation state of unbiquadium is +6, which would exist in the halides UbqX6 (X = a halogen), analogous to the known +6 oxidation state in uranium.[1] Like the other early superactinides, the binding energies of unbiquadium's valence electrons are predicted to be small enough that all six should easily participate in chemical reactions.[44] The predicted electron configuration of the Ubq5+ ion is [Og] 6f1.[1]
Notes

In nuclear physics, an element is called heavy if its atomic number is high; lead (element 82) is one example of such a heavy element. The term "superheavy elements" typically refers to elements with atomic number greater than 103 (although there are other definitions, such as atomic number greater than 100[3] or 112;[4] sometimes, the term is presented an equivalent to the term "transactinide", which puts an upper limit before the beginning of the hypothetical superactinide series).[5] Terms "heavy isotopes" (of a given element) and "heavy nuclei" mean what could be understood in the common language—isotopes of high mass (for the given element) and nuclei of high mass, respectively.
In 2009, a team at JINR led by Oganessian published results of their attempt to create hassium in a symmetric 136Xe + 136Xe reaction. They failed to observe a single atom in such a reaction, putting the upper limit on the cross section, the measure of probability of a nuclear reaction, as 2.5 pb.[6] In comparison, the reaction that resulted in hassium discovery, 208Pb + 58Fe, had a cross section of ~20 pb (more specifically, 19+19
−11 pb), as estimated by the discoverers.[7]
The greater the excitation energy, the more neutrons are ejected. If the excitation energy is lower than energy binding each neutron to the rest of the nucleus, neutrons are not emitted; instead, the compound nucleus de-excites by emitting a gamma ray.[11]
The definition by the IUPAC/IUPAP Joint Working Party states that a chemical element can only be recognized as discovered if a nucleus of it has not decayed within 10−14 seconds. This value was chosen as an estimate of how long it takes a nucleus to acquire its outer electrons and thus display its chemical properties.[12] This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.[13]
This separation is based on that the resulting nuclei move past the target more slowly then the unreacted beam nuclei. The separator contains electric and magnetic fields whose effects on a moving particle cancel out for a specific velocity of a particle.[15] Such separation can also be aided by a time-of-flight measurement and a recoil energy measurement; a combination of the two may allow to estimate the mass of a nucleus.[16]
Not all decay modes are caused by electrostatic repulsion. For example, beta decay is caused by the weak interaction.[21]
Since mass of a nucleus is not measured directly but is rather calculated from that of another nucleus, such measurement is called indirect. Direct measurements are also possible, but for the most part they have remained unavailable for heaviest nuclei.[22] The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.[23] Mass was determined from the location of a nucleus after the transfer (the location helps determine its trajectory, which is linked to the mass-to-charge ratio of the nucleus, since the transfer was done in presence of a magnet).[24]
Spontaneous fission was discovered by Soviet physicist Georgy Flerov,[25] a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.[26] In contrast, the LBL scientists believed fission information was not sufficient for a claim of synthesis of an element. They believed spontaneous fission had not been studied enough to use it for identification of a new element, since there was a difficulty of establishing that a compound nucleus had only ejected neutrons and not charged particles like protons or alpha particles.[13] They thus preferred to link new isotopes to the already known ones by successive alpha decays.[25]
For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics in Stockholm, Stockholm County, Sweden.[27] There were no earlier definitive claims of creation of this element, and the element was assigned a name by its Swedish, American, and British discoverers, nobelium. It was later shown that the identification was incorrect.[28] The following year, LBNL was unable to reproduce the Swedish results and announced instead their synthesis of the element; that claim was also disproved later.[28] JINR insisted that they were the first to create the element and suggested a name of their own for the new element, joliotium;[29] the Soviet name was also not accepted (JINR later referred to the naming of element 102 as "hasty").[30] The name "nobelium" remained unchanged on account of its widespread usage.[31]
Atomic numbers 114, 120, 122, and 126 have also been proposed as closed proton shells in different models.

While such nuclei may be synthesized and a series of decay signals may be registered, decays faster than one microsecond may pile up with subsequent signals and thus be indistinguishable, especially when multiple uncharacterized nuclei may be formed and emit a series of similar alpha particles. The main difficulty is thus attributing the decays to the correct parent nucleus, as a superheavy atom that decays before reaching the detector will not be registered at all.

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Periodic table

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