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Island of stability

A diagram showing the measured and predicted half-lives of nuclides, as well as the beta stability line and predicted location of the island of stability.
Measured (boxed) and predicted (shaded) half-lives of nuclides, sorted by number of protons and neutrons. The expected location of the island of stability around Z = 112 is circled.[1]

In nuclear physics, the island of stability is the prediction that a set of superheavy nuclides with magic numbers of protons and neutrons will temporarily reverse the trend of decreasing stability in elements heavier than uranium.[2]

Various predictions have been made regarding the exact location of the island of stability, though it is generally thought to center near copernicium and flerovium isotopes (such as 291Cn, 293Cn, and 298Fl) approaching the predicted closed shell at neutron number N = 184.[3] It is thought that the closed shell will confer additional stability towards fission, while also leading to longer half-lives towards alpha decay. While these effects are expected to be greatest near atomic number Z = 114 and N = 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 are doubly magic (having magic numbers of both protons and neutrons). Estimates of the stability of the elements on the island are usually around a half-life of minutes or days; however, some estimates predict half-lives of millions of years.[4]

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 on 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; 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 oganesson in recent years demonstrates a slight stabilizing effect around elements 110114 that may continue in unknown isotopes, supporting the existence of the island of stability.[3][5]

Introduction

Complete chart of nuclide half-lives plotted against atomic number Z and neutron number N axes.
Chart of isotope half-lives for nuclides

Nuclide stability

The composition of an atomic nucleus is determined by the number of protons Z and the number of neutrons N, which sum to mass number A. The atomic number Z determines the position of an element in the periodic table, but the more than 3000 nuclides 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).[6] 252 nuclides are thought to be stable (having never been observed to decay),[7] and these follow a general trend in which the number of neutrons rises more rapidly than the number of protons. The last element in the periodic table that has a stable isotope is lead (Z = 82), with stability generally decreasing in heavier elements.[a] 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.[10]

The stability of a nucleus is determined by its binding energy, with higher binding energy conferring greater stability. The binding energy per nucleon increases with atomic number to a broad plateau around A = 60, then declines.[11] If a nucleus can be split into two parts that have a lower total energy (a consequence of the mass defect resulting from greater binding energy), it is unstable. The nucleus can hold together for a finite time because there is a potential barrier opposing the split, but this barrier can be crossed by quantum tunnelling. The lower the barrier and the masses of the constituents, the greater the probability per unit time of a split.[12]

Protons in a nucleus are bound together by the strong force, which counterbalances the Coulomb repulsion between positively charged protons. In heavier nuclei, larger numbers of neutrons are needed to reduce repulsion and confer additional stability. Even so, as physicists started to synthesize elements that are not found in nature, they found the stability decreased as the nuclei became heavier.[13] Thus, they speculated that the periodic table might come to an end. The discoverers of plutonium (element 94) considered naming it "ultimium", thinking it was the last.[14] Following the discoveries of heavier elements, of which some decayed in microseconds, it then seemed that instability with respect to spontaneous fission would limit the existence of heavier elements. In 1939, an upper limit was estimated around element 104,[15] and later, it seemed that element 108 might be the limit.[13]

Magic numbers

The possible existence of superheavy elements with atomic numbers well beyond that of uranium had been suggested as early as 1919, when German physicist Richard Swinne proposed that superheavy elements around Z = 108 were a source of radiation in cosmic rays. Although he did not make any definitive observations, he hypothesized in 1931 that transuranium elements around Z = 100 or Z = 108 may be relatively long-lived and possibly exist in nature.[16] In 1955, John Archibald Wheeler also proposed the existence of these elements;[17] he is credited with the first usage of the term "superheavy element" in a 1958 paper published alongside Frederick Werner.[18] However, this idea did not attract wide interest until a decade later, after improvements in the nuclear shell model. In this model, the atomic nucleus is built up in "shells", analogous to electron shells in atoms. Independently of each other, neutrons and protons have energy 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.[19] This theory of a nuclear shell model originates in the 1930s, but it was not until 1949 that Maria Goeppert Mayer and Johannes Hans Daniel Jensen et al. independently devised the correct formulation.[20] 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.[21][22] Protons share the first six of these magic numbers,[23] and 126 has been predicted since the 1940s.[24] Nuclides with a magic number of each are referred to as "doubly magic" and are more stable than nearby nuclides as a result of greater binding energies.[25]

In the late 1960s, more sophisticated shell models by William Myers and Władysław Świątecki, and by Heiner Meldner, taking into account Coulomb repulsion, changed the prediction for the next proton magic number from 126 to 114.[26] Myers and Świątecki appear to have coined the term "island of stability", and Glenn Seaborg, later a discoverer of many of the superheavy elements, quickly adopted the term and promoted it.[24] Myers and Świątecki also proposed that some superheavy nuclei would be longer-lived as a consequence of a higher fission barrier. Further improvements in the nuclear shell model by Vilen Strutinsky led to the emergence of the macroscopic-microscopic method which takes into consideration both smooth trends characteristic of the liquid drop model and local fluctuations such as shell effects. This approach enabled Sven Nilsson et al., as well as other groups, to make the first detailed calculations of the stability of nuclei within the island.[26] 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.[26] Subsequently, estimates of the proton magic number have ranged from 114 to 126, and there is still no consensus.[21][27][28]

Discoveries

Most stable isotopes of superheavy elements (Z ≥ 104)
Element Atomic
number
Most stable
isotope
Half-life[b]
Publications[29][30] NUBASE 2016[9]
Rutherfordium 104 267Rf 1.3 h 2.5 h
Dubnium 105 268Db 1.2 d 1.1 d
Seaborgium 106 269Sg 14 min[31] 5 min
Bohrium 107 270Bh[c] 1 min 3.8 min
Hassium 108 269Hs 9.7 s[32] 16 s
Meitnerium 109 278Mt[d][e] 4.5 s 7 s
Darmstadtium 110 281Ds[d] 12.7 s 14 s
Roentgenium 111 282Rg[d][f] 1.7 min 1.6 min
Copernicium 112 285Cn[d] 28 s 32 s
Nihonium 113 286Nh[d] 9.5 s 7 s
Flerovium 114 289Fl[d][g] 1.9 s 2.4 s
Moscovium 115 290Mc[d] 650 ms 410 ms
Livermorium 116 293Lv[d] 57 ms 80 ms
Tennessine 117 294Ts[d] 51 ms 70 ms
Oganesson 118 294Og[d][h] 690 µs 1.15 ms
A diagram of observed decay chains of even Z superheavy nuclides, consisting of several alpha decays and terminating in spontaneous fission.
A summary of observed decay chains in even-Z superheavy elements, including tentative assignments in chains 3, 5, and 8.[32] There is a general trend of increasing stability for isotopes with a greater neutron excess (N − Z, the difference in the number of protons and neutrons), especially in elements 110, 112, and 114, which strongly suggests that the center of the island of stability lies among even heavier isotopes.

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.[34][35] They were also predicted to be especially stable against spontaneous fission in spite of their high atomic mass.[26][36] 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 probably low critical masses,[37] and as nuclear fuel to power space missions.[27] These speculations led many researchers to conduct searches for superheavy elements in the 1960s and 1970s, both in nature and through nucleosynthesis at particle accelerators.[17]

During the 1970s, many searches for long-lived superheavy nuclei were conducted. Experiments aimed at synthesizing various elements ranging in atomic number from 110 to 127 were conducted at various laboratories around the world, though none were successful,[34][38] indicating that such experiments may have been insufficiently sensitive if cross sections were low, or that any nuclei reachable via such fusion-evaporation reactions would be too short-lived for detection. More recent experiments reveal that this indeed may be the case, for only a few short-lived atoms are typically produced in one experiment.[39] Similar searches in nature were also unsuccessful, suggesting that if superheavy elements do exist in nature, their abundance is less than 10−14 moles of superheavy elements per mole of ore.[40] Despite these failures,[26] new superheavy elements were being discovered every few years in various laboratories through light-ion bombardment and cold fusion reactions; rutherfordium, the first 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),[9] 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.[41]

Flerovium, with the expected magic 114 protons, was first synthesized in 1998 by Yuri Oganessian et al. at the Joint Institute for Nuclear Research in Dubna, Russia. 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.[42] Because the produced nuclei underwent alpha decay rather than fission, and the half-lives were several orders of magnitude longer than predicted, 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.[43] Even though the original 1998 chain was not observed again, and its assignment remains uncertain,[32] 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][33][44] 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,[4][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 is expected to continue into unknown heavier isotopes.[45]

In the 2013 OLIMPIYA experiment, a group of Russian physicists led by A. V. Bagulya reported the possible observation of three superheavy nuclei in olivine crystals in meteorites, originating from cosmic rays. 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 models.[46][47][48]

Deformed nuclei

Studies from the early 1990s have shown that superheavy elements do not have perfectly spherical nuclei.[49] A change in the shape of the nucleus changes the position of neutrons and protons in the shell; recent research indicates that large nuclei are deformed, 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.[50][51][52] Hassium-270 is now believed to be a doubly magic deformed nucleus, with deformed magic numbers Z = 108 and N = 162. It has a half-life of 10 seconds.[53][54] 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.[15] Determination of the decay properties of neighboring hassium and seaborgium isotopes near N = 162 provides additional strong evidence for this region of relative stability in deformed nuclei.[36] This also strongly suggests that the island of stability 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.[15][55]

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 gray square near the bottom left represents uranium-238, the heaviest primordial nuclide. 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), isotopes near those currently known will undergo alpha decay, and isotopes closest to the center of the island may also have significant beta decay or electron capture (EC) branches. The nuclei 291Cn and 293Cn (denoted in white squares) are predicted to be beta-stable and have the longest total half-lives of approximately 100 years.[3]
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.
3-dimensional rendering of the theoretical island of stability around N = 178 and Z = 112

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.[4] Some theoretical calculations indicate that their half-lives may be long, on the order of 100 years,[3][39] or possibly as long as 109 years.[35]

The shell closure at N = 184 is predicted to result in longer partial half-lives for alpha decay and spontaneous fission.[3] 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.[26][56] For example, the doubly magic nucleus 298Fl may have a spontaneous fission half-life on the order of 1019 years;[26] this is much longer than the 2.5 ms half-life of the known neutron-deficient isotope 284Fl (with N = 170) believed to demarcate the limit of stabilizing effects.[31] Some undiscovered isotopes are predicted to undergo fission with still shorter half-lives, limiting the existence and possible observation of superheavy nuclei beyond the island of stability (namely for Z > 120 and N > 184;[41] these nuclei may undergo alpha decay or spontaneous fission in microseconds or less).[50][51][52] In the center of the island, there may be competition between alpha decay and spontaneous fission, though the exact ratio is strongly model dependent.[3] 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.[i] The longest lived isotopes are predicted to lie on the beta-stability line as well, for beta decay is predicted to compete with the other decay modes near the predicted center of the island, especially for isotopes of elements 111–115.[3] The possible role of beta decay is highly uncertain, as isotopes of these elements (such as 290Fl and 293Mc) are predicted to have shorter partial half-lives for alpha decay; this would reduce competition and result in alpha decay remaining the dominant decay channel, unless additional stability towards alpha decay exists in superdeformed isomers of these nuclides.[60] 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;[61] these include 100-year half-lives for 291Cn and 293Cn,[39] a 1000-year half-life for 296Cn,[39] and a 300-year half-life for 294Ds;[56] the latter two 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).[62] However, this model defines the island of stability as the region with the greatest resistance to fission;[62] the nuclide 306Ubb is still predicted to have a short half-life with respect to alpha decay.[3]

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 primordially on Earth. It is thought that they may occur in cosmic rays at an abundance of 10−12 relative to lead,[45] although instability of nuclei intermediate between primordial actinides (232Th, 235U, and 238U) and the island of stability may inhibit their production in r-process nucleosynthesis. Various models suggest that spontaneous fission will be the dominant decay mode of nuclei with A > 280, and that neutron-induced or beta-delayed 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.[63]

A possible stronger decay mode for the heaviest superheavy elements was shown to be cluster decay by Dorin N. Poenaru, R.A. Gherghescu, and Walter Greiner. This is predicted to have a greater impact above Z = 122, however, reducing its effect on the decay of isotopes near the center of the island, unless the center of the island is at a higher atomic number than predicted.[64]

Synthesis problems

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.[45][65] 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,[45] though the production of several milligrams of these rare isotopes to create a target is difficult.[66] 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 the pxn and αxn (emission of a proton or alpha particle, respectively, followed by several neutrons) channels; these may allow the synthesis of neutron-enriched isotopes of elements 111–117.[67] Although the predicted cross sections are on the order of 1–900 fb, smaller than those in the xn (emission of neutrons only) channels, it may still be possible to generate otherwise unreachable isotopes of superheavy elements in these reactions.[67][68] Some of these heavier isotopes may also undergo electron capture 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, though this remains largely hypothetical as properties of superheavy nuclei near the beta-stability line remain unexplored.[45]

It may also be possible to generate isotopes in the island of stability such as 298Fl in multi-nucleon transfer reactions in low-energy collisions of actinide nuclei (such as 238U and 248Cm).[65] This inverse quasifission (partial fusion followed by fission, with a shift away from mass equilibrium in the products) mechanism[69] may provide a path to the island of stability if shell effects around Z = 114 are sufficiently strong, though lighter elements such as nobelium and seaborgium (Z = 102–106) are predicted to have higher yields.[45][70] Preliminary studies of the 238U + 238U and 238U + 248Cm transfer reactions have failed to produce elements heavier than mendelevium (Z = 101), though the increased yield in the latter reaction suggests that the use of even heavier targets such as 254Es (if available) may enable production of superheavy elements.[71] A later study of the 238U + 232Th reaction 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. This result strongly suggests that shell effects have a significant influence on cross sections, and that the island of stability may be reached in future experiments with transfer reactions.[72]

Other islands of stability

Further shell closures beyond the main island of stability around Z = 114, N = 184 may give rise to additional islands of stability. It is thought that two significant islands may exist around heavier doubly magic nuclei; the first near 342126 or 354126 (with 216 or 228 neutrons) and the second near 472164 or 482164 (with 308 or 318 neutrons).[26][56][73][74] These isotopes 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 of flerovium.[26] Such a concept was proposed by Yuri Oganessian at the 235th national meeting of the American Chemical Society in 2008.[75] However, substantially greater electromagnetic repulsion between protons in such heavy nuclei may reduce their stability, and possibly cause them to only briefly exist as unbound resonances. This may have the additional consequence of isolating these islands from the main chart of nuclides, as intermediate isotopes and perhaps elements in a "sea of instability" would rapidly undergo fission and essentially be nonexistent.[73] It is also possible that beyond a region of relative stability around element 126, heavier nuclei would lie beyond a fission threshold given by the liquid drop model, thus rendering them nonexistent even in the vicinity of greater magic numbers.[74]

As these elements are much heavier than any known elements, it is thought that a new, stronger particle accelerator will be needed for their synthesis.[76]

It has also been posited that in the region beyond A > 300, an entire "continent of stability" consisting of a hypothetical phase of stable quark matter with only up and down quarks may exist. Such a form of matter is theorized to be a ground state of baryonic matter with a greater binding energy per baryon, 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.[77]

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.[8] It is theoretically possible for other observationally stable nuclides to decay, though their predicted half-lives are so long that this process has never been observed.[9]
  2. ^ Different sources give different values for half-lives; the most recently published values in the literature and NUBASE are both listed below for reference.
  3. ^ The unconfirmed 278Bh may have a longer half-life of 11.5 minutes.[32]
  4. ^ 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.[33]
  5. ^ The unconfirmed 282Mt may have a longer half-life of 1.1 minutes.[32]
  6. ^ The unconfirmed 286Rg may have a longer half-life of 10.7 minutes.[32]
  7. ^ The unconfirmed 290Fl may have a longer half-life of 19 seconds.[32]
  8. ^ The unconfirmed 295Og may have a longer half-life of 181 milliseconds.[32]
  9. ^ [50][51][52][57][58][59]

References

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  2. ^ Moskowitz, Clara (7 May 2014). "Superheavy Element 117 Points to Fabled "Island of Stability" on Periodic Table". Scientific American. Retrieved 20 April 2019.
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Bibliography

  • Emsley, John (2011). Nature's Building Blocks: An A-Z Guide to the Elements (New ed.). New York, NY: Oxford University Press. ISBN 978-0-19-960563-7.
  • Hoffman, Darleane C.; Ghiorso, Albert; Seaborg, Glenn T. (2000). The Transuranium People: The Inside Story. World Scientific. ISBN 978-1-78-326244-1.
  • Kragh, Helge (2018). From Transuranic to Superheavy Elements: A Story of Dispute and Creation. Springer. ISBN 9783319758138.

External links