Island of stability
|Nucleus · Nucleons (p, n) · Nuclear matter · Nuclear force · Nuclear structure · Nuclear reaction|
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. 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 N = 184. 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 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 doubly magic nuclei. 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.
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 110–114 that may continue in unknown isotopes, supporting the existence of the island of stability.
The composition of an atomic nucleus is determined by the number of protons Z and the number of neutrons N. 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). 253 of the nuclides are thought to be stable (having never been observed to decay), and these follow a general trend in which the number of neutrons rises more rapidly than the number of protons. As the number of neutrons moves away from the stable region, the half-lives decrease rapidly, and the last element in the periodic table that has a stable isotope is lead (Z = 82).
The stability of a nucleus is determined by its binding energy. The binding energy per nucleon increases with atomic number to a broad plateau around A = 60, then declines. If a nucleus can be split into two parts that have a lower total 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 consituents, the greater the probability per unit time of a split.
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. 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. and later, it seemed that element 108 might be the limit.
The possible existence of superheavy elements with atomic numbers well beyond that of uranium had been suggested as far back as 1955 by John Archibald Wheeler, but the 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. The numbers of nucleons for which shells are filled are called magic numbers, and 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. Protons share the first six of these magic numbers, and 126 had been predicted since the 1940s. 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.
In the late 1960s, more sophisticated shell models by William Myers and Władysław Świątecki, and by H. Meldner, taking into account Coulomb repulsion, changed the prediction for the next proton magic number from 126 to 114. Some Russian physicists 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. 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. Subsequently, estimates of the proton magic number have ranged from 114 to 126, and there is still no consensus.
Interest in island of stability grew over the next few years, as some calculations suggested that it might contain nuclides with half-lives of billions of years. They were also predicted to be especially stable against spontaneous fission in spite of their high atomic mass. It was thought that if such elements existed, they may be used in particle accelerators as neutron sources, and in nuclear weapons as a consequence of their probably low critical masses. 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.
|Element||Atomic number||Most stable isotope||Half-life|
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, 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. Similar searches in nature were also unsuccessful, setting upper limits to the abundance of these elements between 10−14 and 10−11 moles of superheavy elements per mole of ore. Despite these failures, 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 was reached by 1996. Even though the half-lives of these nuclei are very short (on the order of seconds), 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.
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. This was seen as a "textbook example" of a decay chain characteristic of the island of stability, and provided strong evidence for the existence of the island of stability in this region. Further successful experiments in the next two decades led to the discovery of all elements up to oganesson, whose decay properties further support the presence of the island of stability. Although the known nuclei still fall several neutrons short of N = 184 where maximum stability is expected (the most neutron rich nuclei, 293Lv and 294Ts, only reach N = 177), and the exact location of the center of the island remains unknown, 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.
Studies from the early 1990s have shown that superheavy elements do not have perfectly spherical nuclei. A shell is considered stable when it is in a spherical form. 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 relative to those for spherical nuclei. 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. 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. Determination of the decay properties of neighboring hassium and seabogrium isotopes near N = 162 provides additional strong evidence for this region of relative stability in deformed nuclei.
Predicted decay properties
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. Some theoretical calculations indicate that their half-lives may be long, on the order of 100 years, or possibly as long as 109 years.
The shell closure at N = 184 is predicted to result in longer partial half-lives for alpha decay and spontaneous fission. 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. For example, the doubly magic nucleus 298Fl may have a spontaneous fission half-life on the order of 1019 years; 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. 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; these nuclei may undergo alpha decay or spontaneous fission in microseconds or less). In the center of the island, there may be competition between alpha decay and spontaneous fission, though the exact ratio is strongly model dependent. 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.[g] 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. 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; these include 100-year half-lives for 291Cn and 293Cn, a 1000-year half-life for 296Cn, and a 300-year half-life for 294Ds; the latter two exactly at the N = 184 shell closure.
Even though these half-lives 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, although instability of nuclei intermediate between primordial actinides (232Th, 235U, 238U) and the island of stability against spontaneous and induced fission may inhibit their production in r-process nucleosynthesis.
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. Nevertheless, this is predicted to have a greater impact above Z = 122, 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.
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. 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, though the production of several milligrams of these rare isotopes to create a target is difficult. 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. Although the predicted cross sections are smaller than those in the xn channels (on the order of 1-900 fb), it may still be possible to generate otherwise unreachable isotopes of superheavy elements in these reactions. 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.
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). This inverse quasifission (partial fusion followed by fission, with a shift away from mass equilibrium in the products) mechanism 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 are predicted to have higher yields. Preliminary studies of the 238U + 238U and 238U + 248Cm have failed to produce elements heavier than mendelevium, 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. 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.
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 354126 (with 228 neutrons) and the second near 472164 or 482164 (with 308 or 318 neutrons). 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. Such a concept was proposed by Yuri Oganessian at the 235th national meeting of the American Chemical Society in 2008. 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. 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.
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- Can superheavy elements (such as Z = 116 or 118) be formed in a supernova? Can we observe them? 2004 – "maybe"
- Second postcard from the island of stability (Oct 2001) nuclides with 116 protons and mass 292
- The synthesis of spherical superheavy nuclei in 48Ca induced reactions Reports the 1999 synthesis of Z = 114, N + Z = 287.
- New elements discovered and the island of stability sighted (Aug 1999; includes report on article later retracted)
- First postcard from the island of nuclear stability (1999; first few Z = 114 atoms)
- NOVA: Island of Stability (2006. 13 m TV segment, with transcript)
- New York Times editorial by Oliver Sacks regarding the Island of Stability theory (Feb 2004 re 113 and 115)
- Tendency equation and curve of stable nuclides