Island of stability

Its theoretical existence is attributed to stabilizing effects of predicted "magic numbers" of protons and neutrons in the superheavy mass region.

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.

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.

In heavier nuclei, larger numbers of uncharged neutrons are needed to reduce repulsion and confer additional stability.

[25] This theory of a nuclear shell model originates in the 1930s, but it was not until 1949 that German physicists Maria Goeppert Mayer and Johannes Hans Daniel Jensen et al. independently devised the correct formulation.

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.

[19] 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.

[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,[44] 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.

[72][73] 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.

[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 for N < 170 as well as for Z > 120 and N > 184).

[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.

[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 nuclide 306Ubb is still predicted to have a short half-life with respect to alpha decay.

[87] 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.

[89][n] Various studies utilizing accelerator mass spectroscopy and crystal scintillators have reported upper limits of the natural abundance of such long-lived superheavy nuclei on the order of 10−14 relative to their stable homologs.

[92] Despite these obstacles to their synthesis, a 2013 study published by a group of Russian physicists led by Valeriy Zagrebaev proposes that the longest-lived copernicium isotopes may occur at an abundance of 10−12 relative to lead, whereby they may be detectable in cosmic rays.

[63] Similarly, in a 2013 experiment, a group of Russian physicists led by Aleksandr Bagulya reported the possible observation of three cosmogenic superheavy nuclei in olivine crystals in meteorites.

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.

[93][94][95] 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.

[101] 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.

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.

[88] 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).

[97] This inverse quasifission (partial fusion followed by fission, with a shift away from mass equilibrium that results in more asymmetric products) mechanism[104] 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.

[106] This result is supported by a later calculation suggesting that the yield of superheavy nuclides (with Z ≤ 109) will likely be higher in transfer reactions using heavier targets.

[98] A 2018 study of the 238U + 232Th reaction at the 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.

[98][107] 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.

[110] Substantially greater electromagnetic 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.

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.

A diagram showing the measured and predicted half-lives of heavy and superheavy nuclides, as well as the beta stability line and predicted location of the island of stability.
A diagram by the Joint Institute for Nuclear Research showing the measured (boxed) and predicted half-lives of superheavy nuclides , ordered by number of protons and neutrons. The expected location of the island of stability around Z = 112 ( copernicium ) is circled. [ 1 ] [ 2 ]
Complete chart of nuclide half-lives plotted against atomic number Z and neutron number N axes.
Chart of half-lives of known nuclides
Diagram showing energy levels of known and predicted proton shells, with gaps at atomic number 82, 114, 120, and 126.
Diagram showing energy levels of known and predicted proton shells (left and right show two different models). [ 20 ] The gaps at Z = 82, 114, 120, and 126 correspond to shell closures, [ 20 ] which have particularly stable configurations and thus result in more stable nuclei. [ 21 ]
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. [ 44 ] According to another analysis, chain 3 (starting at element 120) is not a real decay chain, but is rather a random sequence of events. [ 64 ] 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.
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 291 Cn and 293 Cn. [ 2 ]
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 294 Ds; 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. [ 14 ] [ 75 ]
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
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 ( 294 Ds– 298 Fl) and N = 228 ( 354 126), separated by a gap of short-lived fissioning nuclei ( t 1/2 < 1 ns; not colored in the chart). [ 75 ]