Innuclear physics, theisland of stability is a predicted set ofisotopes ofsuperheavy elements that may have considerably longerhalf-lives than known isotopes of these elements. It is predicted to appear as an "island" in thechart of nuclides, separated from knownstable and long-livedprimordial radionuclides. Its theoretical existence is attributed to stabilizing effects of predicted "magic numbers" ofprotons andneutrons in the superheavy mass region.[3][4]
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Several predictions have been made regarding the exact location of the island of stability, though it is generally thought to center nearcopernicium andflerovium isotopes in the vicinity of the predicted closed neutronshell atN = 184.[2] These models strongly suggest that the closed shell will confer further stability towardsfission andalpha decay. While these effects are expected to be greatest nearatomic numberZ = 114 (flerovium) andN = 184, the region of increased stability is expected to encompass several neighboring elements, and there may also be additional islands of stability around heavier nuclei that aredoubly magic (having magic numbers of both protons and neutrons). Estimates of the stability of the nuclides within the island are usually around a half-life of minutes or days; some optimists propose half-lives on the order of millions of years.[5]
Although the nuclear shell model predicting magic numbers has existed since the 1940s, the existence of long-lived superheavy nuclides has not been definitively demonstrated. Like the rest of the superheavy elements, the nuclides within the island of stability have never been found in nature; thus, they must be created artificially in anuclear reaction to be studied. Scientists have not found a way to carry out such a reaction, for it is likely that new types of reactions will be needed to populate nuclei near the center of the island. Nevertheless, the successful synthesis of superheavy elements up toZ = 118 (oganesson) with up to 177 neutrons demonstrates a slight stabilizing effect around elements110 to114 that may continue in heavier isotopes, consistent with the existence of the island of stability.[2][6]
The composition of anuclide (atomic nucleus) is defined by thenumber of protonsZ and thenumber of neutronsN, which sum tomass numberA. Proton numberZ, also named the atomic number, determines the position of anelement in theperiodic table. The approximately 3300 known nuclides[7] are commonly represented in achart withZ andN for its axes and thehalf-life forradioactive decay indicated for each unstable nuclide (see figure).[8] As of 2019[update], 251 nuclides are observed to bestable (having never been observed to decay);[9] generally, as the number of protons increases, stable nuclei have a higherneutron–proton ratio (more neutrons per proton). The last element in the periodic table that has a stableisotope islead (Z = 82),[a][b] with stability (i.e., half-lives of the longest-lived isotopes) generally decreasing in heavier elements,[c][12] especially beyond curium (Z = 96).[13] The half-lives of nuclei also decrease when there is a lopsided neutron–proton ratio, such that the resulting nuclei have too few or too many neutrons to be stable.[14]
The stability of a nucleus is determined by itsbinding energy, higher binding energy conferring greater stability. The binding energy per nucleon increases with atomic number to a broad plateau aroundA = 60, then declines.[15] If a nucleus can be split into two parts that have a lower total energy (a consequence of themass defect resulting from greater binding energy), it is unstable. The nucleus can hold together for a finite time because there is apotential barrier opposing the split, but this barrier can be crossed byquantum tunneling. The lower the barrier and the masses of thefragments, the greater the probability per unit time of a split.[16]
Protons in a nucleus are bound together by thestrong force, which counterbalances theCoulomb repulsion between positivelycharged protons. In heavier nuclei, larger numbers of uncharged neutrons are needed to reduce repulsion and confer additional stability. Even so, as physicists started tosynthesize elements that are not found in nature, they found the stability decreased as the nuclei became heavier.[17] Thus, they speculated that the periodic table might come to an end. The discoverers ofplutonium (element 94) considered naming it "ultimium", thinking it was the last.[18] Following the discoveries of heavier elements, of which some decayed in microseconds, it then seemed that instability with respect tospontaneous fission would limit the existence of heavier elements. In 1939, an upper limit of potential element synthesis was estimated aroundelement 104,[19] and following the first discoveries oftransactinide elements in the early 1960s, this upper limit prediction was extended toelement 108.[17]
As early as 1914, the possible existence ofsuperheavy elements with atomic numbers well beyond that of uranium—then the heaviest known element—was suggested, when German physicist Richard Swinne proposed that superheavy elements aroundZ = 108 were a source of radiation incosmic rays. Although he did not make any definitive observations, he hypothesized in 1931 thattransuranium elements aroundZ = 100 orZ = 108 may be relatively long-lived and possibly exist in nature.[22] In 1955, American physicistJohn Archibald Wheeler also proposed the existence of these elements;[23] he is credited with the first usage of the term "superheavy element" in a 1958 paper published with Frederick Werner.[24] This idea did not attract wide interest until a decade later, after improvements in thenuclear shell model. In this model, the atomic nucleus is built up in "shells", analogous toelectron shells in atoms. Independently of each other, neutrons and protons haveenergy levels that are normally close together, but after a given shell is filled, it takes substantially more energy to start filling the next. Thus, the binding energy per nucleon reaches a local maximum and nuclei with filled shells are more stable than those without.[25] This theory of a nuclear shell model originates in the 1930s, but it was not until 1949 that German physicistsMaria Goeppert Mayer andJohannes Hans Daniel Jensen et al. independently devised the correct formulation.[26]
The numbers of nucleons for which shells are filled are calledmagic numbers. Magic numbers of 2, 8, 20, 28, 50, 82 and 126 have been observed for neutrons, and the next number is predicted to be 184.[6][27] Protons share the first six of these magic numbers,[28] and 126 has been predicted as a magic proton number since the 1940s.[29] Nuclides with a magic number of each—such as16O (Z = 8,N = 8),132Sn (Z = 50,N = 82), and208Pb (Z = 82,N = 126)—are referred to as "doubly magic" and are more stable than nearby nuclides as a result of greater binding energies.[30][31]
In the late 1960s, more sophisticated shell models were formulated by American physicist William Myers and Polish physicistWładysław Świątecki, and independently by German physicist Heiner Meldner (1939–2019[32][33]). With these models, taking into account Coulomb repulsion, Meldner predicted that the next proton magic number may be 114 instead of 126.[34] Myers and Świątecki appear to have coined the term "island of stability", and American chemistGlenn Seaborg, later a discoverer of many of the superheavy elements, quickly adopted the term and promoted it.[29][35] Myers and Świątecki also proposed that some superheavy nuclei would be longer-lived as a consequence of higherfission barriers. Further improvements in the nuclear shell model by Soviet physicistVilen Strutinsky led to the emergence of the macroscopic–microscopic method, a nuclear mass model that takes into consideration both smooth trends characteristic of theliquid-drop model and local fluctuations such as shell effects. This approach enabled Swedish physicistSven Nilsson et al., as well as other groups, to make the first detailed calculations of the stability of nuclei within the island.[34] With the emergence of this model, Strutinsky, Nilsson, and other groups argued for the existence of the doubly magic nuclide298
114Fl (Z = 114,N = 184), rather than310Ubh (Z = 126,N = 184) which was predicted to be doubly magic as early as 1957.[34] Subsequently, estimates of the proton magic number have ranged from 114 to 126, and there is still no consensus.[6][21][36][37]
| Element | Atomic number | Most stable isotope | Half-life[d] | |
|---|---|---|---|---|
| Publications [38][39] | NUBASE 2020 [40] | |||
| Rutherfordium | 104 | 267Rf | 48 min[41] | 2.5 h |
| Dubnium | 105 | 268Db | 16 h[42] | 1.2 d |
| Seaborgium | 106 | 269Sg | 14 min[43] | 5 min |
| Bohrium | 107 | 270Bh[e] | 2.4 min[45] | 3.8 min |
| Hassium | 108 | 269Hs | 9.7 s[46] | 16 s |
| Meitnerium | 109 | 278Mt[f][g] | 4.5 s | 6 s |
| Darmstadtium | 110 | 281Ds[f] | 12.7 s | 14 s |
| Roentgenium | 111 | 282Rg[f][h] | 1.7 min | 2.2 min |
| Copernicium | 112 | 285Cn[f] | 28 s | 30 s |
| Nihonium | 113 | 286Nh[f] | 9.5 s | 12 s |
| Flerovium | 114 | 289Fl[f][i] | 1.9 s | 2.1 s |
| Moscovium | 115 | 290Mc[f] | 650 ms | 840 ms |
| Livermorium | 116 | 293Lv[f] | 57 ms | 70 ms |
| Tennessine | 117 | 294Ts[f] | 51 ms | 70 ms |
| Oganesson | 118 | 294Og[f] | 690 μs | 700 μs |
Interest in a possible island of stability grew throughout the 1960s, as some calculations suggested that it might contain nuclides with half-lives of billions of years.[48][5] They were also predicted to be especially stable against spontaneous fission in spite of their high atomic mass.[34][49] It was thought that if such elements exist and are sufficiently long-lived, there may be several novel applications as a consequence of their nuclear and chemical properties. These include use inparticle accelerators asneutron sources, innuclear weapons as a consequence of their predicted lowcritical masses and high number of neutrons emitted per fission,[50] and asnuclear fuel to power space missions.[36] These speculations led many researchers to conduct searches for superheavy elements in the 1960s and 1970s, both in nature and throughnucleosynthesis in particle accelerators.[23]
During the 1970s, many searches for long-lived superheavy nuclei were conducted. Experiments aimed at synthesizing elements ranging in atomic number from 110 to 127 were conducted at laboratories around the world.[51][52] These elements were sought in fusion-evaporation reactions, in which a heavy target made of one nuclide isirradiated by accelerated ions of another in acyclotron, and new nuclides are produced after these nucleifuse and the resulting excited system releases energy by evaporating several particles (usually protons, neutrons, or alpha particles). These reactions are divided into "cold" and "hot" fusion, which respectively create systems with lower and higherexcitation energies; this affects the yield of the reaction.[53] For example, the reaction between248Cm and40Ar was expected to yield isotopes of element 114, and that between232Th and84Kr was expected to yield isotopes of element 126.[54] None of these attempts were successful,[51][52] indicating that such experiments may have been insufficiently sensitive if reactioncross sections were low—resulting in lower yields—or that any nuclei reachable via such fusion-evaporation reactions might be too short-lived for detection.[j] Subsequent successful experiments reveal that half-lives and cross sections indeed decrease with increasing atomic number, resulting in the synthesis of only a few short-lived atoms of the heaviest elements in each experiment;[55] as of 2022[update], the highest reported cross section for a superheavy nuclide near the island of stability is for288Mc in the reaction between243Am and48Ca.[42]
Similar searches in nature were also unsuccessful, suggesting that if superheavy elements do exist in nature, their abundance is less than 10−14moles of superheavy elements per mole of ore.[56] Despite these unsuccessful attempts to observe long-lived superheavy nuclei,[34] new superheavy elements were synthesizedevery few years in laboratories throughlight-ion bombardment and cold fusion[k] reactions; rutherfordium, the firsttransactinide, was discovered in 1969, and copernicium, eight protons closer to the island of stability predicted atZ = 114, was reached by 1996. Even though the half-lives of these nuclei are very short (on the order ofseconds),[40] the very existence of elements heavier than rutherfordium is indicative of stabilizing effects thought to be caused by closed shells; amodel not considering such effects would forbid the existence of these elements due to rapid spontaneous fission.[19]
Flerovium, with the expected magic 114 protons, was first synthesized in 1998 at theJoint Institute for Nuclear Research inDubna, Russia, by a group of physicists led byYuri Oganessian. A single atom of element 114 was detected, with a lifetime of 30.4 seconds, and itsdecay products had half-lives measurable in minutes.[57] Because the produced nuclei underwent alpha decay rather than fission, and the half-lives were severalorders of magnitude longer than those previously predicted[l] or observed for superheavy elements,[57] this event was seen as a "textbook example" of a decay chain characteristic of the island of stability, providing strong evidence for the existence of the island of stability in this region.[59] Even though the original 1998 chain was not observed again, and its assignment remains uncertain,[44] further successful experiments in the next two decades led to the discovery of all elements up tooganesson, whose half-lives were found to exceed initially predicted values; these decay properties further support the presence of the island of stability.[6][47][60] However, a 2021 study on the decay chains of flerovium isotopes suggests that there is no strong stabilizing effect fromZ = 114 in the region of known nuclei (N = 174),[61] and that extra stability would be predominantly a consequence of the neutron shell closure.[37] Although known nuclei still fall several neutrons short ofN = 184 where maximum stability is expected (the most neutron-rich confirmed nuclei,293Lv and294Ts, only reachN = 177), and the exact location of the center of the island remains unknown,[62][6] the trend of increasing stability closer toN = 184 has been demonstrated. For example, the isotope285Cn, with eight more neutrons than277Cn, has a half-life almost five orders of magnitude longer. This trend is expected to continue into unknown heavier isotopes in the vicinity of the shell closure.[63]
Though nuclei within the island of stability aroundN = 184 are predicted to bespherical, studies from the early 1990s—beginning with Polish physicists Zygmunt Patyk and Adam Sobiczewski in 1991[65]—suggest that some superheavy elements do not have perfectly spherical nuclei.[66][67] A change in the shape of the nucleus changes the position of neutrons and protons in the shell. Research indicates that large nuclei farther from spherical magic numbers aredeformed,[67] causing magic numbers to shift or new magic numbers to appear. Current theoretical investigation indicates that in the regionZ = 106–108 andN ≈ 160–164, nuclei may be more resistant to fission as a consequence of shell effects for deformed nuclei; thus, such superheavy nuclei would only undergo alpha decay.[68][69][70] Hassium-270 is now believed to be a doubly magic deformed nucleus, with deformed magic numbersZ = 108 andN = 162.[71] It has a half-life of 9 seconds.[40] This is consistent with models that take into account the deformed nature of nuclei intermediate between the actinides and island of stability nearN = 184, in which a stability "peninsula" emerges at deformed magic numbersZ = 108 andN = 162.[72][73] Determination of the decay properties of neighboring hassium and seaborgium isotopes nearN = 162 provides further strong evidence for this region of relative stability in deformed nuclei.[49] This also strongly suggests that the island of stability (for spherical nuclei) is not completely isolated from the region of stable nuclei, but rather that both regions are instead linked through an isthmus of relatively stable deformed nuclei.[72][74]
Thehalf-lives of nuclei in the island of stability itself are unknown since none of the nuclides that would be "on the island" have been observed. Many physicists believe that the half-lives of these nuclei are relatively short, on the order of minutes or days.[62] Some theoretical calculations indicate that their half-lives may be long, on the order of 100 years,[2][55] or possibly as long as 109 years.[5]
The shell closure atN = 184 is predicted to result in longerpartial half-lives for alpha decay and spontaneous fission.[2] It is believed that the shell closure will result in higher fission barriers for nuclei around298Fl, strongly hindering fission and perhaps resulting in fission half-lives 30 orders of magnitude greater than those of nuclei unaffected by the shell closure.[34][75] For example, the neutron-deficient isotope284Fl (withN = 170) undergoes fission with a half-life of 2.5 milliseconds, and is thought to be one of the most neutron-deficient nuclides with increased stability in the vicinity of theN = 184 shell closure.[43] Beyond this point, some undiscovered isotopes are predicted to undergo fission with still shorter half-lives, limiting the existence[m] and possible observation[j] of superheavy nuclei far from the island of stability (namely forN < 170 as well as forZ > 120 andN > 184).[14][19] These nuclei may undergo alpha decay or spontaneous fission in microseconds or less, with some fission half-lives estimated on the order of 10−20 seconds in the absence of fission barriers.[68][69][70][75] In contrast,298Fl (predicted to lie within the region of maximum shell effects) may have a much longer spontaneous fission half-life, possibly on the order of 1019 years.[34]
In the center of the island, there may be competition between alpha decay and spontaneous fission, though the exact ratio is model-dependent.[2] The alpha decay half-lives of 1700 nuclei with 100 ≤ Z ≤ 130 have been calculated in a quantum tunneling model with both experimental and theoretical alpha decayQ-values, and are in agreement with observed half-lives for some of the heaviest isotopes.[68][69][70][79][80][81]
The longest-lived nuclides are also predicted to lie on thebeta-stability line, forbeta decay is predicted to compete with the other decay modes near the predicted center of the island, especially for isotopes of elements 111–115. Unlike other decay modes predicted for these nuclides, beta decay does not change the mass number. Instead, a neutron is converted into a proton or vice versa, producing an adjacentisobar closer to the center of stability (the isobar with the lowestmass excess). For example, significant beta decay branches may exist in nuclides such as291Fl and291Nh; these nuclides have only a few more neutrons than known nuclides, and might decay via a "narrow pathway" towards the center of the island of stability.[1][2] The possible role of beta decay is highly uncertain, as some isotopes of these elements (such as290Fl and293Mc) are predicted to have shorter partial half-lives for alpha decay. Beta decay would reduce competition and would result in alpha decay remaining the dominant decay channel, unless additional stability towards alpha decay exists insuperdeformed isomers of these nuclides.[82]
.svg)
Considering all decay modes, various models indicate a shift of the center of the island (i.e., the longest-living nuclide) from298Fl to a lower atomic number, and competition between alpha decay and spontaneous fission in these nuclides;[83] these include 100-year half-lives for291Cn and293Cn,[55][78] a 1000-year half-life for296Cn,[55] a 300-year half-life for294Ds,[75] and a 3500-year half-life for293Ds,[69][70] with294Ds and296Cn exactly at theN = 184 shell closure. It has also been posited that this region of enhanced stability for elements with 112 ≤ Z ≤ 118 may instead be a consequence of nuclear deformation, and that the true center of the island of stability for spherical superheavy nuclei lies around306Ubb (Z = 122,N = 184).[20] This model defines the island of stability as the region with the greatest resistance to fission rather than the longest total half-lives;[20] the nuclide306Ubb is still predicted to have a short half-life with respect to alpha decay.[2][70] The island of stability for spherical nuclei may also be a "coral reef" (i.e., a broad region of increased stability without a clear "peak") aroundN = 184 and 114 ≤ Z ≤ 120, with half-lives rapidly decreasing at higher atomic number, due to combined effects from proton and neutron shell closures.[84]
Another potentially significant decay mode for the heaviest superheavy elements was proposed to becluster decay by Romanian physicistsDorin N. Poenaru and Radu A. Gherghescu and German physicistWalter Greiner. Itsbranching ratio relative to alpha decay is expected to increase with atomic number such that it may compete with alpha decay aroundZ = 120, and perhaps become the dominant decay mode for heavier nuclides aroundZ = 124. As such, it is expected to play a larger role beyond the center of the island of stability (though still influenced by shell effects), unless the center of the island lies at a higher atomic number than predicted.[85]
Even though half-lives of hundreds or thousands of years would be relatively long for superheavy elements, they are far too short for any such nuclides to existprimordially on Earth. Additionally, instability of nuclei intermediate between primordial actinides (232Th,235U, and238U) and the island of stability may inhibit production of nuclei within the island inr-process nucleosynthesis. Various models suggest that spontaneous fission will be the dominant decay mode of nuclei withA > 280, and that neutron-induced or beta-delayedfission—respectively neutron capture and beta decay immediately followed by fission—will become the primary reaction channels. As a result, beta decay towards the island of stability may only occur within a very narrow path or may be entirely blocked by fission, thus precluding the synthesis of nuclides within the island.[86] The non-observation of superheavy nuclides such as292Hs and298Fl in nature is thought to be a consequence of a low yield in ther-process resulting from this mechanism, as well as half-lives too short to allow measurable quantities to persist in nature.[87][n] Various studies utilizingaccelerator mass spectroscopy andcrystal scintillators have reported upper limits of the natural abundance of such long-lived superheavy nuclei on the order of10−14 relative to their stablehomologs.[90]
Despite these obstacles to their synthesis, a 2013 study published by a group of Russian physicists led byValeriy Zagrebaev proposes that the longest-lived copernicium isotopes may occur at an abundance of 10−12 relative to lead, whereby they may be detectable incosmic rays.[63] Similarly, in a 2013 experiment, a group of Russian physicists led by Aleksandr Bagulya reported the possible observation of threecosmogenic superheavy nuclei inolivine crystals in meteorites. The atomic number of these nuclei was estimated to be between 105 and 130, with one nucleus likely constrained between 113 and 129, and their lifetimes were estimated to be at least 3,000 years. Although this observation has yet to be confirmed in independent studies, it strongly suggests the existence of the island of stability, and is consistent with theoretical calculations of half-lives of these nuclides.[91][92][93]
The decay of heavy, long-lived elements in the island of stability is a proposed explanation for the unusual presence of the short-livedradioactive isotopes observed inPrzybylski's Star.[94]
The manufacture of nuclei on the island of stability proves to be very difficult because the nuclei available as starting materials do not deliver the necessary sum of neutrons. Radioactive ion beams (such as44S) in combination with actinide targets (such as248Cm) may allow the production of more neutron rich nuclei nearer to the center of the island of stability, though such beams are not currently available in the required intensities to conduct such experiments.[63][95][96] Several heavier isotopes such as250Cm and254Es may still be usable as targets, allowing the production of isotopes with one or two more neutrons than known isotopes,[63] though the production of several milligrams of these rare isotopes to create a target is difficult.[97] It may also be possible to probe alternative reaction channels in the same48Ca-induced fusion-evaporation reactions that populate the most neutron-rich known isotopes, namely those at a lowerexcitation energy (resulting in fewer neutrons being emitted during de-excitation), or those involving evaporation of charged particles (pxn, evaporating a proton and several neutrons, orαxn, evaporating analpha particle and several neutrons).[98] This may allow the synthesis of neutron-enriched isotopes of elements 111–117.[99] Although the predicted cross sections are on the order of 1–900 fb, smaller than when only neutrons are evaporated (xn channels), it may still be possible to generate otherwise unreachable isotopes of superheavy elements in these reactions.[98][99][100] Some of these heavier isotopes (such as291Mc,291Fl, and291Nh) may also undergoelectron capture (converting a proton into a neutron) in addition to alpha decay with relatively long half-lives, decaying to nuclei such as291Cn that are predicted to lie near the center of the island of stability. However, this remains largely hypothetical as no superheavy nuclei near the beta-stability line have yet been synthesized and predictions of their properties vary considerably across different models.[1][63] In 2024, a team of researchers at the JINR observed one decay chain of the known isotope289Mc as a product in thep2n channel of the reaction between242Pu and50Ti, an experiment targeting neutron-deficientlivermorium isotopes. This was the first successful report of a charged-particle exit channel in a hot fusion reaction between an actinide target and a projectile withZ ≥ 20.[101]
The process of slowneutron capture used to produce nuclides as heavy as257Fm is blocked by short-livedisotopes of fermium that undergo spontaneous fission (for example,258Fm has a half-life of 370 μs); this is known as the "fermium gap" and prevents the synthesis of heavier elements in such a reaction. It might be possible to bypass this gap, as well as another predicted region of instability aroundA = 275 andZ = 104–108, in a series of controlled nuclear explosions with a higherneutron flux (about a thousand times greater than fluxes in existing reactors) that mimics the astrophysicalr-process.[63] First proposed in 1972 by Meldner, such a reaction might enable the production of macroscopic quantities of superheavy elements within the island of stability;[1] the role of fission in intermediate superheavy nuclides is highly uncertain, and may strongly influence the yield of such a reaction.[86]
It may also be possible to generate isotopes in the island of stability such as298Fl in multi-nucleontransfer reactions in low-energy collisions ofactinide nuclei (such as238U and248Cm).[95] This inverse quasifission (partial fusion followed by fission, with a shift away from mass equilibrium that results in more asymmetric products) mechanism[102] may provide a path to the island of stability if shell effects aroundZ = 114 are sufficiently strong, though lighter elements such asnobelium and seaborgium (Z = 102–106) are predicted to have higher yields.[63][103] Preliminary studies of the238U + 238U and238U + 248Cm transfer reactions have failed to produce elements heavier thanmendelevium (Z = 101), though the increased yield in the latter reaction suggests that the use of even heavier targets such as254Es (if available) may enable production of superheavy elements.[104] This result is supported by a later calculation suggesting that the yield of superheavy nuclides (withZ ≤ 109) will likely be higher in transfer reactions using heavier targets.[96] A 2018 study of the238U + 232Th reaction at theTexas A&M Cyclotron Institute by Sara Wuenschel et al. found several unknown alpha decays that may possibly be attributed to new, neutron-rich isotopes of superheavy elements with 104 < Z < 116, though further research is required to unambiguously determine the atomic number of the products.[96][105] This result strongly suggests that shell effects have a significant influence on cross sections, and that the island of stability could possibly be reached in future experiments with transfer reactions.[105]
Further shell closures beyond the main island of stability in the vicinity ofZ = 112–114 may give rise to additional islands of stability. Although predictions for the location of the next magic numbers vary considerably, two significant islands are thought to exist around heavier doubly magic nuclei; the first near354126 (with 228 neutrons) and the second near472164 or482164 (with 308 or 318 neutrons).[34][75][106] Nuclides within these two islands of stability might be especially resistant to spontaneous fission and have alpha decay half-lives measurable in years, thus having comparable stability to elements in the vicinity offlerovium.[34] Other regions of relative stability may also appear with weaker proton shell closures in beta-stable nuclides; such possibilities include regions near342126[107] and462154.[108] Substantially greaterelectromagnetic repulsion between protons in such heavy nuclei may greatly reduce their stability, and possibly restrict their existence to localized islands in the vicinity of shell effects.[109] This may have the consequence of isolating these islands from the mainchart of nuclides, as intermediate nuclides and perhaps elements in a "sea of instability" would rapidly undergo fission and essentially be nonexistent.[106] It is also possible that beyond a region of relative stability around element 126, heavier nuclei would lie beyond a fission threshold given by theliquid drop model and thus undergo fission with very short lifetimes, rendering them essentially nonexistent even in the vicinity of greater magic numbers.[107]
It has also been posited that in the region beyondA > 300, an entire "continent of stability" consisting of a hypothetical phase of stablequark matter, comprising freely flowingup anddown quarks rather thanquarks bound into protons and neutrons, may exist. Such a form of matter is theorized to be a ground state ofbaryonic matter with a greater binding energy perbaryon thannuclear matter, favoring the decay of nuclear matter beyond this mass threshold into quark matter. If this state of matter exists, it could possibly be synthesized in the same fusion reactions leading to normal superheavy nuclei, and would be stabilized against fission as a consequence of its stronger binding that is enough to overcome Coulomb repulsion.[110]
