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Island of stability

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3-dimensional rendering of the theoretical island of stability around N=178 and Z=118

The island of stability in nuclear physics describes a set of as-yet undiscovered isotopes of transuranium elements which are theorized to be much more stable than others. Specifically, they are expected to have radioactive decay half-lives of at least minutes or days as compared to seconds, with some expecting half-lives of millions of years.[1]

Theory and origin

Unsolved problem in physics:
What is the heaviest possible stable or metastable nucleus?
(more unsolved problems in physics)

The possibility of an "island of stability" was first proposed by Glenn T. Seaborg in the late 1960s[2]. The hypothesis is that the atomic nucleus is built up in "shells" in a manner similar to the structure of the much larger 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 fills 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 possess filled shells.[3]

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 flerovium-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 known stable nucleus and most stable heavy metal.)

Recent research indicates that large nuclei are deformed[how?], causing magic numbers to shift. Hassium-270 is now believed to be a doubly magic deformed nucleus, with deformed magic numbers 108 and 162.[4][5] However, it has a half-life of only 3.6 seconds.[6]

Isotopes have been produced with enough protons to plant them upon an island of stability but with too few neutrons to even place them upon the island's outer "shores". It is possible that these elements possess unusual chemical properties and, if they have isotopes with adequate lifespans, would be available for various practical applications (such as particle accelerator targets and as neutron sources as well).

Half-lives of the highest-numbered elements

All elements with an atomic number above 82 (lead) are unstable, and the "stability" (half-life of the longest-lived known isotope) of elements generally decreases with rising atomic numbers from the relatively stable uranium (92) upwards to the heaviest known element: 118. It increases very slightly in the range of elements 110 to 113, hypothesized to be at the beginning of the island of stability. The longest-lived observed isotopes of each of the heaviest elements are shown in the following table.

Known isotopes of elements 100 through 118[7][8]
Number Name Longest-lived
isotope		 Half-life 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 277mHs ~12 minutes[9] Isotopes of hassium
109 Meitnerium 278Mt 7.6 seconds Isotopes of meitnerium
110 Darmstadtium 281Ds 11 seconds Isotopes of darmstadtium
111 Roentgenium 281Rg 26 seconds Isotopes of roentgenium
112 Copernicium 285Cn 29 seconds Isotopes of copernicium
113 Ununtrium 286Uut 19.6 seconds Isotopes of ununtrium
114 Flerovium 289Fl 2.6 seconds Isotopes of flerovium
115 Ununpentium 289Uup 220 milliseconds Isotopes of ununpentium
116 Livermorium 293Lv 61 milliseconds Isotopes of livermorium
117 Ununseptium 294Uus 78 milliseconds Isotopes of ununseptium
118 Ununoctium 294Uuo 0.89 milliseconds Isotopes of ununoctium

(Note that for elements 109–110, 112–114, and 116–118 the longest-lived known isotope is always the heaviest one discovered, making it likely that there are still longer-lived isotopes among the undiscovered heavier ones)

For comparison, the shortest-lived element with atomic number below 100 is francium (element 87) with a half-life of 22 minutes.

The half-lives of nuclei in the island of stability itself are unknown since none of the isotopes that would be "on the island" have been observed. Many physicists think they are relatively short, on the order of minutes or days.[1] Some theoretical calculations indicate that their half-lives may be long, on the order of 109 years.[citation needed]

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.[10][11][12][13][14][15] The theoretical calculations are in good agreement with the available experimental data. [dubious – discuss]

A possible stronger decay mode for the heaviest superheavies was shown to be cluster decay by Dorin N Poenaru, R.A. Gherghescu, Walter Greiner. [16]

Periodic table with elements colored according to the half-life of their most stable isotope.
  Stable elements.
  Radioactive elements with half-lives of over four million years.
  Half-lives between 800 and 34,000 years.
  Half-lives between 1 day and 103 years.
  Half-lives ranging between a minute and 1 day.
  Half-lives less than a minute.

Island of relative stability

232
Th (thorium), 235
U and 238
U (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 slightly unstable in 2003, with an α-emission half-life of 1.9×1019 years for 209
Bi. All other isotopes beyond bismuth are relatively or very unstable. So the main periodic table ends there (by geographical analogy, the shore edge of a continent; a continental shelf continues however, with shallows beginning at radium that rapidly drop off again after californium, with significant islands at thorium and uranium, as well as minor ones at e.g. plutonium, all of which is surrounded by a "sea of instability"[17]) 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.[11][12][13] Also, 298
Fl is not the center of the magic island as predicted earlier.[18] On the contrary, the nucleus with Z=110, N=183 (293Ds) 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 268
Sg is predicted to have alpha-decay half-life ~3.2 hours that is greater than that (~28 s) of the deformed doubly-magic 270
Hs.[19] The superheavy nucleus 268
Sg 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.[20] Calculations in a quantum tunneling model show that such superheavy nuclei would undergo alpha decay within microseconds or less.[11][12][13]

Synthesis problems

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. For the synthesis of isotope 298 of flerovium, one could use an isotope of plutonium and one of calcium that together have a sum of at least 298 nucleons; for example, calcium-50 and plutonium-248. However, these and heavier isotopes are not available in measurable quantities, making production virtually impossible with current methods. The same problem exists for the other possible combinations of isotopes needed to generate elements on the island using target-projectile methods. It may be possible to generate the isotope 298 of flerovium, if the multi-nucleon transfer reactions would work in low-energy collisions of actinide nuclei.[21] One of these reactions may be:

248
Cm
 + 238
U
 → 298
Fl
 + 186
W
 + 2 1
0n

See also

References

  1. ^ a b "Superheavy Element 114 Confirmed: A Stepping Stone to the Island of Stability". Retrieved 11 October 2009.
  2. ^ "The Island of Stability?". Retrieved 2012-07-24.
  3. ^ "Shell Model of Nucleus". HyperPhysics. Department of Physics and Astronomy, Georgia State University. Retrieved 22 January 2007.
  4. ^ Dvořák, Jan (2007-07-12). "PhD. Thesis: Decay properties of nuclei close to Z = 108 and N = 162" (PDF). Technische Universität München.
  5. ^ Dvorak, J.; Brüchle, W.; Chelnokov, M.; Dressler, R.; Düllmann, Ch.; Eberhardt, K.; Gorshkov, V.; Jäger, E.; Krücken, R. (2006). "Doubly Magic Nucleus Hs162-108-270". Physical Review Letters. 97 (24): 242501. Bibcode:2006PhRvL..97x2501D. doi:10.1103/PhysRevLett.97.242501. PMID 17280272.
  6. ^ See reference in Hassium.
  7. ^ Emsley, John (2001). Nature's Building Blocks ((Hardcover, First Edition) ed.). Oxford University Press. pp. (pages 143, 144, 458). ISBN 0-19-850340-7.
  8. ^ Alexandra Witze (April 6, 2010). "Superheavy element 117 makes debut". Retrieved April 6, 2010.
  9. ^ http://www.rsc.org/chemistryworld/podcast/Interactive_Periodic_Table_Transcripts/Hassium.asp
  10. ^ P. Roy Chowdhury, C. Samanta, and D. N. Basu (January 26, 2006). "α decay half-lives of new superheavy elements". Phys. Rev. C. 73: 014612. arXiv:nucl-th/0507054. Bibcode:2006PhRvC..73a4612C. doi:10.1103/PhysRevC.73.014612.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ 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. arXiv:nucl-th/0703086. Bibcode:2007NuPhA.789..142S. doi:10.1016/j.nuclphysa.2007.04.001.
  12. ^ 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 (4): 044603. arXiv:0802.3837. Bibcode:2008PhRvC..77d4603C. doi:10.1103/PhysRevC.77.044603.{{cite journal}}: CS1 maint: multiple names: authors list (link) Cite error: The named reference "longlived" was defined multiple times with different content (see the help page).
  13. ^ 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 (6): 781. arXiv:0802.4161. Bibcode:2008ADNDT..94..781C. doi:10.1016/j.adt.2008.01.003.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  14. ^ P. Roy Chowdhury, D. N. Basu and C. Samanta (January 26, 2007). "α decay chains from element 113". Phys. Rev. C. 75 (4): 047306. arXiv:0704.3927. Bibcode:2007PhRvC..75d7306C. doi:10.1103/PhysRevC.75.047306.
  15. ^ 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 (12): 124201–124204. arXiv:0708.4355. Bibcode:2007JPSJ...76l4201S. doi:10.1143/JPSJ.76.124201.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  16. ^ Dorin N Poenaru, R.A. Gherghescu, Walter Greiner (2011). "Heavy-Particle Radioactivity of Superheavy Nuclei". Phys. Rev. Lett. 107 (6): 062503. arXiv:1106.3271. Bibcode:2011PhRvL.107f2503P. doi:10.1103/PhysRevLett.107.062503. PMID 21902317.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  17. ^ Note graphic: Known and predicted regions of nuclear stability, surrounded by a “sea” of instability. cf. the Chart of Nuclides by half-life.
  18. ^ Sven Gösta Nilsson, Chin Fu Tsang, Adam Sobiczewski, Zdzislaw Szymaski, Slawomir Wycech, Christer Gustafson, Inger-Lena Lamm, Peter Möller and Björn Nilsson (February 14, 1969). "On the nuclear structure and stability of heavy and superheavy elements". Nuclear Physics A. 131 (1): 1–66. Bibcode:1969NuPhA.131....1N. doi:10.1016/0375-9474(69)90809-4.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  19. ^ 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". Physical Review Letters. 97 (24): 242501. Bibcode:2006PhRvL..97x2501D. doi:10.1103/PhysRevLett.97.242501. PMID 17280272.{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  20. ^ S. Cwiok, P.-H. Heenen and W. Nazarewicz (2005). "Shape coexistence and triaxiality in the superheavy nuclei" (PDF). Nature. 433 (7027): 705. Bibcode:2005Natur.433..705C. doi:10.1038/nature03336. PMID 15716943. {{cite journal}}: More than one of |pages= and |page= specified (help)
  21. ^ Zagebraev, V; Greiner, W (2008). "Synthesis of superheavy nuclei: A search for new production reactions". Physical Review C. 78 (3): 034610. arXiv:0807.2537. Bibcode:2008PhRvC..78c4610Z. doi:10.1103/PhysRevC.78.034610.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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