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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.

Additional recommended knowledge

Contents


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 numbers of neutrons and protons completely fill the energy levels of a given shell in the nucleus, then the binding energy per nucleon will reach a local minimum and thus that particular configuration will have a longer lifetime than nearby isotopes that do not have filled shells[1].

A filled shell would have "magic numbers" of neutrons and protons. One possible magic number of neutrons is 184, and some possible matching proton numbers are 114, 120 and 126 — which would mean that the most stable possible 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 nucleus is Lead-208, the heaviest stable nucleus and most stable heavy metal.) None of these transuranic isotopes has yet been produced, but isotopes of elements in the range between 110 through 114 are slower to decay than isotopes of nearby nuclei on the periodic table.

Half-lives of large isotopes

Fermium is the largest 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[2]. This is illustrated in the following two tables.

Isotopes of elements 100 through 109[2]
Number Name Longest-lived
isotope
Half-life of
longest-lived isotope
Link
100 fermium 257Fm 101 days Isotopes of fermium
101 mendelevium 258Md 52 days Isotopes of mendelevium
102 nobelium 259No 58 minutes Isotopes of nobelium
103 lawrencium 262Lr 215 minutes Isotopes of lawrencium
104 rutherfordium 261Rf 13 hours Isotopes of rutherfordium
105 dubnium 262Db 32 hours Isotopes of dubnium
106 seaborgium 266Sg 2.4 minutes Isotopes of seaborgium
107 bohrium 267Bh 22 seconds Isotopes of bohrium
108 hassium 270Hs 22 seconds Isotopes of hassium
109 meitnerium 268Mt 720 milliseconds Isotopes of meitnerium

The following table shows information about the half-lives of isotopes of elements 110 through 120.

Isotopes of elements 110 through 120
Number Name # isotopes (known) # isotopes observed Longest half-life observed (ms) Link
110 darmstadtium 15 10 11,100 Isotopes of darmstadtium
111 roentgenium 12 4 3,600 Isotopes of roentgenium
112 ununbium 9 1 300,000 Isotopes of ununbium
113 ununtrium 6 2 480 Isotopes of ununtrium
114 ununquadium 5 4 2,800 Isotopes of ununquadium
115 ununpentium 5 2 87 Isotopes of ununpentium
116 ununhexium 5 4 61 Isotopes of ununhexium
117 ununseptium 2 0 N/A Isotopes of ununseptium
118 ununoctium 1 1 0.89 Isotopes of ununoctium
119 ununennium 0 0 N/A Isotopes of ununennium
120 unbinilium 0 0 N/A Isotopes of unbinilium

The half lives of elements in the island are uncertain. 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)[3]. 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 hasn't been reached, since the island's "shores" have neutron richer nuclides than those produced.

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 hypothetically 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 trough of severe instability, which renders such elements as astatine, radon, and francium extremely short-lived relative to all but the heaviest elements found so far.

Synthesis problems

Manufacturing nuclei in the island of stability may be very difficult, because the nuclei available would 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 in the case of synthesis of element 114 the usage of calcium-50 and plutonium-248. However these isotopes (and heavier calcium and plutonium isotopes) are not available in weighable quantities. This is also the fact for other target/projectile-combinations.

However it may be possible to generate the isotope 298 of element 114, if nuclear transfer reactions would work. One of these reactions may be:

204Hg + 136Xe → 298Uuq + 40Ca + 2n

References

  1. ^ Shell Model of Nucleus. HyperPhysics. Department of Physics and Astronomy, Georgia State University. Retrieved on 2007-01-22.
  2. ^ a b Emsley, John (2001). Nature's Building Blocks, (Hardcover, First Edition), Oxford University Press, (pages 143,144,458). ISBN 0198503407. 
  3. ^ Moller Theoretical Nuclear Chart 1997

See also

 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Island_of_stability". A list of authors is available in Wikipedia.
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