Island of stability

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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.

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 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 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 superheavy isotopes has yet been produced, but isotopes of elements in the range between 110 through 114 have been found to decay slower than isotopes of nuclei nearby in the periodic table.

[edit] Half-lives of large isotopes

Periodic table with elements colored according to the half-life of their most stable isotope .
(1) stable elements.
(2) radioactive elements with half-lives of over a million years. Their long half-lives give them very small, if not negligible, radioactivities. They may be handled without any precautions.
(3) radioactive elements with half-lives of over 500 years. They may present low health hazards due to the fact that their radiation levels are near the background radiation level. Their half-lives allow them to have commercial applications.
(4) radioactive elements with half-lives of over a day. Their short half-lives pose high safety risks. They are dangerous to health. They have little potential for any commercial use.
(5) radioactive elements with half-lives of over a minute. These are highly radioactive elements. They pose severe health risks. They have no potential for commercial use.
(6) radioactive elements with half-lives of under a minute. These elements are extremely radioactive. Very little is known about these elements. Their short life times makes it impossible for them to exist outside research laboratories.

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[2]. The longest-lived observed isotopes are shown in the following table.

Isotopes of elements 100 through 118[2]
Number Name Longest-lived
isotope		 Half-life of
longest-lived isotope		 Article
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 3.6 hours Isotopes of lawrencium
104 rutherfordium 267Rf 1.3 hours Isotopes of rutherfordium
105 dubnium 268Db 29 hours Isotopes of dubnium
106 seaborgium 271Sg 1.9 minutes Isotopes of seaborgium
107 bohrium 270Bh 61 seconds Isotopes of bohrium
108 hassium 277Hs 16.5 minutes Isotopes of hassium
109 meitnerium 278Mt 0.72 seconds Isotopes of meitnerium
110 darmstadtium 281Ds 11 seconds Isotopes of darmstadtium
111 roentgenium 280Rg 3.6 seconds Isotopes of roentgenium
112 ununbium 285Uub 29 seconds Isotopes of ununbium
113 ununtrium 284Uut 0.49 seconds Isotopes of ununtrium
114 ununquadium 289Uuq 2.6 seconds Isotopes of ununquadium
115 ununpentium 288Uup 88 ms Isotopes of ununpentium
116 ununhexium 293Uuh 61 ms Isotopes of ununhexium
117 ununseptium Yet unknown N/A Isotopes of ununseptium
118 ununoctium 294Uuo 0.89 ms Isotopes of ununoctium

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)[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 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. [4][5][6][7][8][9] 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 an "sea 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.

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.[5][6][7]. Also, 298114 is not the center of the magic island as predicted earlier. [10] 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.[11] The superheavy nucleus 268106 has not been produced in the laboratory as yet (2008). 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.[12] Calculations in a quantum tunneling model show that such superheavy nuclei would undergo alpha decay within microseconds or, less. [5][6][7].

[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 nuclear transfer reactions would work. One of these reactions may be:

204Hg + 136Xe → 298Uuq + 40Ca + 2n

[edit] See also

[edit] References

  1. ^ "Shell Model of Nucleus". HyperPhysics. Department of Physics and Astronomy, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/shell.html. Retrieved on 2007-01-22. 
  2. ^ a b Emsley, John (2001). Nature's Building Blocks ((Hardcover, First Edition) ed.). Oxford University Press. pp. (pages 143,144,458). ISBN 0198503407. 
  3. ^ Moller Theoretical Nuclear Chart 1997
  4. ^ P. Roy Chowdhury, C. Samanta, and D. N. Basu (26 January 2006). "α decay half-lives of new superheavy elements". Phys. Rev. C 73: 014612. doi:10.1103/PhysRevC.73.014612. http://link.aps.org/doi/10.1103/PhysRevC.73.014612. 
  5. ^ a b c C. Samanta, P. Roy Chowdhury and D.N. Basu (2007). "Predictions of alpha decay half lives of heavy and superheavy elements". Nucl. Phys. A 789: 142–154. doi:10.1016/j.nuclphysa.2007.04.001. 
  6. ^ a b c P. Roy Chowdhury, C. Samanta, and D. N. Basu (2008). "Search for long lived heaviest nuclei beyond the valley of stability". Phys. Rev. C 77: 044603. doi:10.1103/PhysRevC.77.044603. http://link.aps.org/doi/10.1103/PhysRevC.77.044603. 
  7. ^ a b c P. Roy Chowdhury, C. Samanta, and D. N. Basu (2008). "Nuclear half-lives for α -radioactivity of elements with 100 < Z < 130". At. Data & Nucl. Data Tables 94: 781. doi:10.1016/j.adt.2008.01.003. 
  8. ^ P. Roy Chowdhury, D. N. Basu and C. Samanta (26 January 2007). "α decay chains from element 113". Phys. Rev. C 75: 047306. doi:10.1103/PhysRevC.75.047306. http://link.aps.org/doi/10.1103/PhysRevC.75.047306. 
  9. ^ Chhanda Samanta, Devasish Narayan Basu, and Partha Roy Chowdhury (2007). "Quantum tunneling in 277112 and its alpha-decay chain". Journal of the Physical Society of Japan 76: 124201–124204. doi:10.1143/JPSJ.76.124201. 
  10. ^ Sven Gösta Nilsson, Chin Fu Tsang, Adam Sobiczewski, Zdzislaw Szymaski, Slawomir Wycech, Christer Gustafson, Inger-Lena Lamm, Peter Möller and Björn Nilsson (1969-02-14). "On the nuclear structure and stability of heavy and superheavy elements". Nuclear Physics A 131 (1): 1–66. doi:10.1016/0375-9474(69)90809-4. 
  11. ^ J. Dvorak, W. Brüchle, M. Chelnokov, R. Dressler, Ch. E. Düllmann, K. Eberhardt, V. Gorshkov, E. Jäger, R. Krücken, A. Kuznetsov, Y. Nagame, F. Nebel,1 Z. Novackova, Z. Qin, M. Schädel, B. Schausten, E. Schimpf, A. Semchenkov, P. Thörle, A. Türler, M. Wegrzecki, B. Wierczinski, A. Yakushev, and A. Yeremin (2006). "Doubly Magic Nucleus 270108 Hs-162". Phys. Rev. Lett. 97: 242501. doi:10.1103/PhysRevLett.97.242501. http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000097000024242501000001&idtype=cvips&gifs=yes. 
  12. ^ S. Cwiok, P.-H. Heenen and W. Nazarewicz (2005). "Shape coexistence and triaxiality in the superheavy nuclei". Nature 433: 705. doi:10.1038/nature03336. http://www.phys.utk.edu/witek/fission/utk/Papers/natureSHE.pdf. 

[edit] External links

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