3-dimensional rendering of the theoretical Island of Stability.

The island of stability is a term from nuclear physics that describes the possibility of elements with particularly stable "magic numbers" of protons and neutrons. This would allow certain isotopes of some transuranic elements to be far more stable than others; that is, decay much more slowly (with half lives of at least minutes or days, compared to fractions of a second; some have even suggested the possibility of half lives on the order of millions of years^[1]).

[edit] History

The idea of the island of stability was first proposed by Glenn T. Seaborg. The hypothesis is that the atomic nucleus is built up in "shells" in a manner similar to the electron shells in atoms. In both cases shells are just groups of quantum energy levels that are relatively close to each other. Energy levels from quantum states in two different shells will be separated by a relatively large energy gap. So when the number of neutrons and protons completely fill the energy levels of a given shell in the nucleus, the binding energy per nucleon will reach a local maximum and thus that particular configuration will have a longer lifetime than nearby isotopes that do not have filled shells.^[2]

A filled shell would have "magic numbers" of neutrons and protons. One possible magic number of neutrons for spherical nuclei is 184, and some possible matching proton numbers are 114, 120 and 126 – which would mean that the most stable spherical isotopes would be ununquadium-298, unbinilium-304 and unbihexium-310. Of particular note is Ubh-310, which would be "doubly magic" (both its proton number of 126 and neutron number of 184 are thought to be magic) and thus the most likely to have a very long half-life. (The next lighter doubly-magic spherical nucleus is lead-208, the heaviest stable nucleus and most stable heavy metal.) Isotopes of elements in the range between 110 through 114 have been found to decay more slowly than isotopes of nuclei nearby in the periodic table.

However, recent research indicates that large nuclei are deformed, causing magic numbers to shift. Hassium-270 is now believed to be doubly-magic nucleus, with deformed magic numbers 108 and 162. Its half life may be as high as 23 seconds.^[3][4]

[edit] Half-lives of large isotopes

Periodic table with elements colored according to the half-life of their most stable isotope.      Stable elements;      Radioactive elements with very long-lived isotopes. Their half-lives of over four million years confers them very small, if not negligible radioactivities;      Radioactive elements that may present low health hazards. Their most stable isotopes have half-lives between 800 and 34.000 years. Because of this, they usually have some commercial applications;      Radioactive elements that are known to pose high safety risks. Their most stable isotopes have half-lifes between one day and 103 years. Their radioactivities confers them little potential for commercial uses;      Highly radioactive elements. Their most stable isotopes have half-lifes between one day and several minutes. They pose severe health risks. Few of them receive uses outside basic research;      Extremely radioactive elements. Very little is known about these elements due to their extreme instability and radioactivity.

Fermium is the heaviest element that can be produced in a nuclear reactor. The stability (half-life of the longest-lived isotope) of elements generally decreases from element 101 to element 109 and then approaches an island of stability with longer-lived isotopes in the range of elements 111 and 114.^[5] The longest-lived observed isotopes are shown in the following table.

The half lives of elements in the island are uncertain due to the small number of atoms manufactured and studied to date. Many physicists think they are relatively short, on the order of minutes, hours, or perhaps days. However, some theoretical calculations indicate that their half lives may be long (some calculations put it on the order of 10⁹ years).^[6] It is possible that these elements could have unusual chemical properties, and, if long lived enough, various applications (such as targets in nuclear physics and neutron sources). However, the isotopes of several of these elements still have too few neutrons to be stable. The island of stability still has not been reached, since the island's "shores" are more neutron rich than nuclides that have been experimentally produced.

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.^{[7][8][9][10][11][12]} The theoretical calculations are in good agreement with the available experimental data.

[edit] Island of relative stability

²³²Th (thorium), ²³⁵U and ²³⁸U (uranium) are the only naturally occurring isotopes beyond bismuth that are relatively stable over the current lifespan of the universe. Bismuth was found to be unstable in 2003, with an α-emission half-life of 1.9×10¹⁹ years for Bi-209. All other isotopes beyond bismuth are relatively or very unstable. So the main periodic table ends at bismuth, with an island at thorium and uranium. Between bismuth and thorium there is a "sea of instability", which renders such elements as astatine, radon, and francium extremely short-lived relative to all but the heaviest elements found so far.

Current theoretical investigation indicates that in the region Z=106–108 and N≈160–164, a small ‘island/peninsula’ might be stable with respect to fission and beta decay, such superheavy nuclei undergoing only alpha decay.^[8][9][10] Also, ²⁹⁸114 is not the center of the magic island as predicted earlier.^[13] On the contrary, the nucleus with Z=110, N=183 appears to be near the center of a possible 'magic island' (Z=104–116, N≈176–186). In the N≈162 region the beta-stable, fission survived ²⁶⁸106 is predicted to have alpha-decay half life ~3.2hrs that is greater than that (~28s) of the deformed doubly-magic ²⁷⁰108.^[14] The superheavy nucleus ²⁶⁸106 has not been produced in the laboratory as yet (2009). For superheavy nuclei with Z>116 and N≈184 the alpha-decay half-lives are predicted to be less than one second. The nuclei with Z=120, 124, 126 and N=184 are predicted to form spherical doubly-magic nuclei and be stable with respect to fission.^[15] Calculations in a quantum tunneling model show that such superheavy nuclei would undergo alpha decay within microseconds or less.^[8][9][10]

[edit] Synthesis problems

Manufacturing nuclei in the island of stability may be very difficult, because the nuclei available as starting materials do not deliver the necessary sum of neutrons. So for the synthesis of isotope 298 of element 114 by using plutonium and calcium, one would require an isotope of plutonium and one of calcium, which have together a sum of at least 298 nucleons (more is better, because at the nuclei reaction some neutrons are emitted). This would require, for example, the use of calcium-50 and plutonium-248 for the synthesis of element 114 . However these isotopes (and heavier calcium and plutonium isotopes) are not available in weighable quantities. This is also the case for other target-projectile combinations.

However it may be possible to generate the isotope 298 of element 114, if the multi-nucleon transfer reactions would work in low-energy collisions of actinide nuclei ^[16]. One of these reactions may be:

²⁴⁸Cm + ²³⁸U → ²⁹⁸Uuq + ¹⁸⁶W + 2 ¹n

[edit] Quest for the island of stability

"We search for the island of stability because, like Mount Everest, it is there. But, as with Everest, there is profound emotion, too, infusing the scientific search to test a hypothesis. The quest for the magic island shows us that science is far from being coldness and calculation, as many people imagine, but is shot through with passion, longing and romance." — Oliver Sacks^[17]

[edit] See also

• Island of stability: Ununquadium — Unbinilium — Unbihexium
• Table of nuclides — a visualization of the island of stability
• Periodic table and Periodic table (extended)

[edit] References

[edit] External links

• Hunting the biggest atoms in the universe (July 23, 2008)
• The hunt for superheavy elements (April 7, 2008)
• The synthesis of spherical superheavy nuclei in 48Ca induced reactions (needs login so can not access !)
• Uut and Uup Add Their Atomic Mass to Periodic Table (Feb 2004)
• 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)
• Second postcard from the island of stability (Oct 2001)
• Superheavy Elements "Island of Stability" (single text slide - undated)
• Superheavy elements (Jul 2004 Yuri Oganessian of JINR )
• Can superheavy elements (such as Z=116 or 118) be formed in a supernova? Can we observe them?
• NOVA - Island of Stability
• New York Times Editorial by Oliver Sacks regarding the Island of Stability theory (Feb 2004 re 113 and 115)
• Upper Limit of the Periodic Table and Synthesis of Superheavy Elements
• Upper Limit in Mendeleev's Periodic Table - Element No.155 by Albert Khazan