Island of stability

Source: Wikipedia, the free encyclopedia.
For the speech by Jimmy Carter, see Island of Stability (speech).

nuclides, ordered by number of protons and neutrons. The expected location of the island of stability around Z = 112 (copernicium) is circled.[1][2]

In

primordial radionuclides. Its theoretical existence is attributed to stabilizing effects of predicted "magic numbers" of protons and neutrons in the superheavy mass region.[3][4]

Nuclear physics
Models of the nucleus
Nuclides' classification
  • N − Z
Nuclear stability
Radioactive decay
Nuclear fission
Capturing processes
High-energy processes
Nucleosynthesis and
nuclear astrophysics
High-energy nuclear physics
Scientists

Several predictions have been made regarding the exact location of the island of stability, though it is generally thought to center near

doubly 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 think half-lives of millions of years.[citation needed
]

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 a nuclear 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 to Z = 118 (oganesson) with up to 177 neutrons demonstrates a slight stabilizing effect around elements 110 to 114 that may continue in heavier isotopes, consistent with the existence of the island of stability.[2][5]

Introduction

Nuclide stability

See also: Valley of stability
Complete chart of nuclide half-lives plotted against atomic number Z and neutron number N axes.
Chart of half-lives of known nuclides

The composition of a

number of neutrons N, which sum to mass number A. Proton number Z, also named the atomic number, determines the position of an element in the periodic table. The approximately 3300 known nuclides[6] are commonly represented in a chart with Z and N for its axes and the half-life for radioactive decay indicated for each unstable nuclide (see figure).[7] As of 2019, 251 nuclides are observed to be stable (having never been observed to decay);[8] generally, as the number of protons increases, stable nuclei have a higher neutron–proton ratio (more neutrons per proton). The last element in the periodic table that has a stable isotope is lead (Z = 82),[a][b] with stability (i.e., half-lives of the longest-lived isotopes) generally decreasing in heavier elements,[c][11] especially beyond curium (Z = 96).[12] 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.[13]

The stability of a nucleus is determined by its

fragments, the greater the probability per unit time of a split.[15]

Protons in a nucleus are bound together by the

transactinide elements in the early 1960s, this upper limit prediction was extended to element 108.[16]

Magic numbers

Johannes Hans Daniel Jensen et al. independently devised the correct formulation.[25]

The numbers of nucleons for which shells are filled are called magic 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.[5][26] Protons share the first six of these magic numbers,[27] and 126 has been predicted as a magic proton number since the 1940s.[28] Nuclides with a magic number of each—such as 16O (Z = 8, N = 8), 132Sn (Z = 50, N = 82), and 208Pb (Z = 82, N = 126)—are referred to as "doubly magic" and are more stable than nearby nuclides as a result of greater binding energies.[29][30]

In the late 1960s, more sophisticated shell models were formulated by American physicist William Myers and Polish physicist

liquid drop model and local fluctuations such as shell effects. This approach enabled Swedish physicist Sven Nilsson et al., as well as other groups, to make the first detailed calculations of the stability of nuclei within the island.[33] With the emergence of this model, Strutinsky, Nilsson, and other groups argued for the existence of the doubly magic nuclide 298Fl (Z = 114, N = 184), rather than 310Ubh (Z = 126, N = 184) which was predicted to be doubly magic as early as 1957.[33] Subsequently, estimates of the proton magic number have ranged from 114 to 126, and there is still no consensus.[5][20][35][36]

Discoveries

Most stable isotopes of superheavy elements (Z ≥ 104)
Element Atomic
number 		Most
stable
isotope 		Half-life[d]
Publications
[37][38] 		NUBASE 2020
[39]
Rutherfordium 104 267Rf 48 min[40] 2.5 h
Dubnium 105 268Db 16 h[41] 1.2 d
Seaborgium 106 269Sg 14 min[42] 5 min
Bohrium 107 270Bh[e] 2.4 min[44] 3.8 min
Hassium 108 269Hs 9.7 s[45] 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.[47][48] They were also predicted to be especially stable against spontaneous fission in spite of their high atomic mass.[33][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 in particle accelerators as neutron sources, in nuclear weapons as a consequence of their predicted low critical masses and high number of neutrons emitted per fission,[50] and as nuclear fuel to power space missions.[35] These speculations led many researchers to conduct searches for superheavy elements in the 1960s and 1970s, both in nature and through nucleosynthesis in particle accelerators.[22]

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 is irradiated by accelerated ions of another in a cyclotron, and new nuclides are produced after these nuclei fuse 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 higher excitation energies; this affects the yield of the reaction.[53] For example, the reaction between 248Cm and 40Ar was expected to yield isotopes of element 114, and that between 232Th and 84Kr 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 reaction cross 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, the highest reported cross section for a superheavy nuclide near the island of stability is for 288Mc in the reaction between 243Am and 48Ca.[41]

Similar searches in nature were also unsuccessful, suggesting that if superheavy elements do exist in nature, their abundance is less than 10−14

transactinide, was discovered in 1969, and copernicium, eight protons closer to the island of stability predicted at Z = 114, was reached by 1996. Even though the half-lives of these nuclei are very short (on the order of seconds),[39] the very existence of elements heavier than rutherfordium is indicative of stabilizing effects thought to be caused by closed shells; a model not considering such effects would forbid the existence of these elements due to rapid spontaneous fission.[18]

Flerovium, with the expected magic 114 protons, was first synthesized in 1998 at the Joint Institute for Nuclear Research in Dubna, Russia, by a group of physicists led by Yuri Oganessian. A single atom of element 114 was detected, with a lifetime of 30.4 seconds, and its decay products had half-lives measurable in minutes.[57] Because the produced nuclei underwent alpha decay rather than fission, and the half-lives were several orders 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,[43] further successful experiments in the next two decades led to the discovery of all elements up to oganesson, whose half-lives were found to exceed initially predicted values; these decay properties further support the presence of the island of stability.[5][46][60] However, a 2021 study on the decay chains of flerovium isotopes suggests that there is no strong stabilizing effect from Z = 114 in the region of known nuclei (N = 174),[61] and that extra stability would be predominantly a consequence of the neutron shell closure.[36] Although known nuclei still fall several neutrons short of N = 184 where maximum stability is expected (the most neutron-rich confirmed nuclei, 293Lv and 294Ts, only reach N = 177), and the exact location of the center of the island remains unknown,[62][5] the trend of increasing stability closer to N = 184 has been demonstrated. For example, the isotope 285Cn, with eight more neutrons than 277Cn, 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]

Deformed nuclei

spherical, studies from the early 1990s—beginning with Polish physicists Zygmunt Patyk and Adam Sobiczewski in 1991[64]—suggest that some superheavy elements do not have perfectly spherical nuclei.[65][66] 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 are deformed,[66] causing magic numbers to shift or new magic numbers to appear. Current theoretical investigation indicates that in the region Z = 106–108 and N ≈ 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.[67][68][69] Hassium-270 is now believed to be a doubly magic deformed nucleus, with deformed magic numbers Z = 108 and N = 162.[70] It has a half-life of 9 seconds.[39] This is consistent with models that take into account the deformed nature of nuclei intermediate between the actinides and island of stability near N = 184, in which a stability "peninsula" emerges at deformed magic numbers Z = 108 and N = 162.[71][72] Determination of the decay properties of neighboring hassium and seaborgium isotopes near N = 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.[71][73]

Predicted decay properties

A diagram depicting the four major decay modes (alpha, electron capture, beta, and spontaneous fission) of known and predicted superheavy nuclei.
A diagram depicting predicted decay modes of superheavy nuclei, with observed nuclei given black outlines. The most neutron-deficient nuclei as well as those immediately beyond the shell closure at N = 184 are predicted to predominantly undergo spontaneous fission (SF), whereas alpha decay (α) may dominate in neutron-deficient nuclei closer to the island, and significant beta decay (β) or electron capture (EC) branches may appear closest to the center of the island around 291Cn and 293Cn.[2]

The

half-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.[48]

The shell closure at N = 184 is predicted to result in longer

partial half-lives for alpha decay and spontaneous fission.[2] It is believed that the shell closure will result in higher fission barriers for nuclei around 298Fl, strongly hindering fission and perhaps resulting in fission half-lives 30 orders of magnitude greater than those of nuclei unaffected by the shell closure.[33][74] For example, the neutron-deficient isotope 284Fl (with N = 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 the N = 184 shell closure.[42] 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 for N < 170 as well as for Z > 120 and N > 184).[13][18] 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.[67][68][69][74] 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.[33]

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 decay Q-values, and are in agreement with observed half-lives for some of the heaviest isotopes.[67][68][69][78][79][80]

The longest-lived nuclides are also predicted to lie on the

superdeformed isomers of these nuclides.[81]

A diagram depicting the four major decay modes (alpha, electron capture, beta, and spontaneous fission) of known and predicted superheavy nuclei, according to the KTUY model.
This chart of predicted decay modes, derived from theoretical research of the Japan Atomic Energy Agency, predicts the center of the island of stability around 294Ds; it would be the longest-lived of several relatively long-lived nuclides primarily undergoing alpha decay (circled). This is the region where the beta-stability line crosses the region stabilized by the shell closure at N = 184. To the left and right, half-lives decrease as fission becomes the dominant decay mode, consistent with other models.[13][74]

Considering all decay modes, various models indicate a shift of the center of the island (i.e., the longest-living nuclide) from 298Fl to a lower atomic number, and competition between alpha decay and spontaneous fission in these nuclides;[82] these include 100-year half-lives for 291Cn and 293Cn,[55][77] a 1000-year half-life for 296Cn,[55] a 300-year half-life for 294Ds,[74] and a 3500-year half-life for 293Ds,[83][84] with 294Ds and 296Cn exactly at the N = 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 around 306Ubb (Z = 122, N = 184).[19] This model defines the island of stability as the region with the greatest resistance to fission rather than the longest total half-lives;[19] the nuclide 306Ubb is still predicted to have a short half-life with respect to alpha decay.[2][69] 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") around N = 184 and 114 ≤ Z ≤ 120, with half-lives rapidly decreasing at higher atomic number, due to combined effects from proton and neutron shell closures.[85]

Another potentially significant decay mode for the heaviest superheavy elements was proposed to be

branching ratio relative to alpha decay is expected to increase with atomic number such that it may compete with alpha decay around Z = 120, and perhaps become the dominant decay mode for heavier nuclides around Z = 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.[86]

Possible natural occurrence

See also: Extinct isotopes of superheavy elements

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 exist

homologs.[91]

Despite these obstacles to their synthesis, a 2013 study published by a group of Russian physicists led by

cosmogenic superheavy nuclei in olivine 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.[92][93][94]

The decay of heavy, long-lived elements in the island of stability is a proposed explanation for the unusual presence of the short-lived radioactive isotopes observed in Przybylski's Star.[95]

Synthesis and difficulties

A 3D graph of stability of elements vs. number of protons Z and neutrons N, showing a "mountain chain" running diagonally through the graph from the low to high numbers, as well as an "island of stability" at high N and Z.
Three-dimensional rendering of the island of stability around N = 178 and Z = 112

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 as 44S) in combination with actinide targets (such as 248Cm) 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][96][97] Several heavier isotopes such as 250Cm and 254Es 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.[98] It may also be possible to probe alternative reaction channels in the same 48Ca-induced fusion-evaporation reactions that populate the most neutron-rich known isotopes, namely those at a lower excitation energy (resulting in fewer neutrons being emitted during de-excitation), or those involving evaporation of charged particles (pxn, evaporating a proton and several neutrons, or αxn, evaporating an alpha particle and several neutrons).[99] This may allow the synthesis of neutron-enriched isotopes of elements 111–117.[100] 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.[99][100][101] Some of these heavier isotopes (such as 291Mc, 291Fl, and 291Nh) may also undergo electron capture (converting a proton into a neutron) in addition to alpha decay with relatively long half-lives, decaying to nuclei such as 291Cn 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]

The process of slow neutron capture used to produce nuclides as heavy as 257Fm is blocked by short-lived isotopes 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 around A = 275 and Z = 104–108, in a series of controlled nuclear explosions with a higher neutron flux (about a thousand times greater than fluxes in existing reactors) that mimics the astrophysical r-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.[87]

JAEA chart of nuclides up to Z = 149 and N = 256 showing predicted decay modes and the beta-stability line
This chart of nuclides used by the Japan Atomic Energy Agency shows known (boxed) and predicted decay modes of nuclei up to Z = 149 and N = 256. Regions of increased stability are visible around the predicted shell closures at N = 184 (294Ds–298Fl) and N = 228 (354126), separated by a gap of short-lived fissioning nuclei (t1/2 < 1 ns; not colored in the chart).[74]

It may also be possible to generate isotopes in the island of stability such as 298Fl in multi-nucleon

Texas 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.[97][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]

Other islands of stability

See also: Extended periodic table

Further shell closures beyond the main island of stability in the vicinity of Z = 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 near 354126 (with 228 neutrons) and the second near 472164 or 482164 (with 308 or 318 neutrons).

liquid 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 beyond A > 300, an entire "

baryonic matter with a greater binding energy per baryon than nuclear 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]

See also

Notes

  1. ^ The heaviest stable element was believed to be bismuth (atomic number 83) until 2003, when its only stable isotope, 209Bi, was observed to undergo alpha decay.[9]
  2. observationally stable nuclides to decay, though their predicted half-lives are so long that this process has never been observed.[10]
  3. age of the Earth. Elements intermediate between bismuth and thorium have shorter half-lives, and heavier nuclei beyond uranium become more unstable with increasing atomic number.[11]
  4. ^ Different sources give different values for half-lives; the most recently published values in the literature and NUBASE are both listed for reference.
  5. ^ The unconfirmed 278Bh may have a longer half-life of 11.5 minutes.[43]
  6. ^ a b c d e f g h i j For elements 109–118, the longest-lived known isotope is always the heaviest discovered thus far. This makes it seem likely that there are longer-lived undiscovered isotopes among the even heavier ones.[46]
  7. ^ The unconfirmed 282Mt may have a longer half-life of 1.1 minutes.[43]
  8. ^ The unconfirmed 286Rg may have a longer half-life of 10.7 minutes.[43]
  9. ^ The unconfirmed 290Fl may have a longer half-life of 19 seconds.[43]
  10. ^
    parent nucleus, as a superheavy atom that decays before reaching the detector will not be registered at all.[77]
  11. ^ This is a distinct concept from hypothetical fusion near room temperature (cold fusion); it instead refers to fusion reactions with lower excitation energy.
  12. ^ Oganessian stated that element 114 would have a half-life on the order of 10−19 s in the absence of stabilizing effects in the vicinity of the theorized island.[58]
  13. ^ The International Union of Pure and Applied Chemistry (IUPAC) defines the limit of nuclear existence at a half-life of 10−14 seconds; this is approximately the time required for nucleons to arrange themselves into nuclear shells and thus form a nuclide.[75]
  14. ^ The observation of long-lived isotopes of roentgenium (with A = 261, 265) and unbibium (A = 292) in nature has been claimed by Israeli physicist Amnon Marinov et al.,[89][90] though evaluations of the technique used and subsequent unsuccessful searches cast considerable doubt on these results.[52][91]

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Bibliography

External links

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