Island of stability

The expected location of the island of stability

In nuclear physics, the island of stability is a set of predicted, but as-yet undiscovered, heavier isotopes of transuranium elements which are theorized to be much more stable than some of those closer in atomic number to uranium. Specifically, they are expected to have radioactive decay half-lives of minutes or days, with "some optimists" expecting half-lives of millions of years.[1] Klaus Blaum expects the island of stability to occur in the region near 300Ubn.[2]

Theory and origin

One fact should be emphasized from the outset: while the various theoretical predictions about the superheavy nuclei differ as to the expected half-lives and regions of stability, all theoretical predictions are in agreement: superheavy nuclei can exist. Thus, the search for superheavy nuclei remains as a unique, rigorous test of the predictive power of modern theories of the structure of nuclei.

— Seaborg and Loveland, 1987.[3]

The possibility of an "island of stability" was first proposed by Glenn T. Seaborg in the late 1960s.[4] The hypothesis is based upon the nuclear shell model, which implies 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.[5]

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, causing magic numbers to shift. Hassium-270 is now believed to be a doubly magic deformed nucleus, with deformed magic numbers 108 and 162.[6][7] It has a half-life of 3.6 seconds.

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). In particular, the very small critical masses of transplutonic elements (possibly as small as grams) implies that if stable elements could be found, they would enable small and compact nuclear bombs either directly or by serving as primaries to help ignite fission/fusion secondaries; this possibility motivated much of the early research and multiple nuclear tests by the United States (including Operation Plowshare) and the Soviet Union aimed at producing such elements.[8]

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. The longest-lived observed isotopes of each of the heaviest elements are shown in the following table.

Known isotopes of elements 100 through 118[9][10][11]
Number Name Longest-lived
isotope 		Half-life Article
100 Fermium 257Fm 100.5 days Isotopes of fermium
101 Mendelevium 258Md 51.5 days Isotopes of mendelevium
102 Nobelium 259No 58 minutes Isotopes of nobelium
103 Lawrencium 266Lr ~11 hours Isotopes of lawrencium
104 Rutherfordium 267Rf ~1.3 hours Isotopes of rutherfordium
105 Dubnium 268Db 1.3 days Isotopes of dubnium
106 Seaborgium 269Sg ~2.1 minutes Isotopes of seaborgium
107 Bohrium 270Bh 61 seconds Isotopes of bohrium
108 Hassium 277mHs ~12 minutes[12] Isotopes of hassium
109 Meitnerium 278Mt 7.6 seconds Isotopes of meitnerium
110 Darmstadtium 281mDs ~3.7 minutes Isotopes of darmstadtium
111 Roentgenium 281Rg 26 seconds Isotopes of roentgenium
112 Copernicium 285mCn ~8.9 minutes Isotopes of copernicium
113 Ununtrium 286Uut 19.6 seconds Isotopes of ununtrium
114 Flerovium 289mFl ~1.1 minutes 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 890 microseconds Isotopes of ununoctium

(Note that for elements 108–118, the longest-lived known isotope is always the heaviest or second-heaviest (111, 115) 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.[13]

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.[14][15][16][17][18][19] The theoretical calculations are in good agreement with the available experimental data.

A possible stronger decay mode for the heaviest superheavies was shown to be cluster decay by Dorin N. Poenaru, R.A. Gherghescu, and Walter Greiner.[20]

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 several minutes and 1 day.
  Half-lives less than several minutes.

Islands of relative stability

Region of relative stability: radium-226 to einsteinium-252
       88 89 90 91 92 93 94 95 96 97 98 99       
   
 154 
Half-life Key
  1   10  100 
  1k  10k 100k
  1M  10M 100M
  1G  10G (a)
 250Cm 252Cf  154 
 153  251Cf 252Es  153 
 152  248Cm 250Cf  152 
 151  247Cm 248Bk 249Cf  151 
 150  244Pu 246Cm 247Bk  150 
 149  245Cm  149 
 148  242Pu 243Am 244Cm  148 
 147  241Pu
⁂
 243Cm  147 
 146  238U  240Pu 241Am  146 
 145  239Pu  145 
 144  236U  237Np 238Pu  144 
 143  235U  236Np  143 
 142  232Th 234U  235Np 236Pu  142 
 141  233U   141 
 140  228Ra 230Th 231Pa 232U 
Table Axes
Neutrons (N)
Protons (Z)
  140 
 139  229Th  139 
 138  226Ra 227Ac 228Th  138 
   
       88 89 90 91 92 93 94 95 96 97 98 99       
Only isotopes with a half-life of at least one year are listed.

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. Even bismuth was found to be slightly unstable in 2003, with an α-emission half-life of 1.9×1019 years for 209
Bi. All elements beyond bismuth have relatively or very unstable isotopes: astatine, radon, and francium are extremely short-lived (and only have half-lives longer than isotopes of the heaviest elements found so far). Even thorium, with the largest known half-life in this region (1.4×1010 years for 232
Th), is still about a billion times shorter than 209
Bi, so the main periodic table ends there.

By geographical analogy, bismuth is the shore edge of a continent. A continental shelf continues though, with shallows beginning at radium (see 'map' at right) that rapidly drop off again after californium. Significant islands appear at thorium and uranium, and with minor ones (i.e. plutonium and curium) form an archipelago. All of this is surrounded by a "sea of instability".[21]

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.[15][16][17] Also, 298
Fl is not the center of the magic island as predicted earlier.[22] 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.[23] 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.[24] Calculations in a quantum tunneling model show that such superheavy nuclei would undergo alpha decay within microseconds or less.[15][16][17]

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. 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.[25] One of these reactions may be:

248Cm + 238U → 298Fl + 186W + 2 1
0n

Hypothetical second island

Recently, interest has been expressed over the possibility of a second island of stability. The idea of a second island of stability was presented by Yuri Oganessian at the 235th national meeting of the American Chemical Society. This new island would be centered around element 164 (unhexquadium), especially the isotope 482Uhq, with a stability similar to that of flerovium.[26] It is thought that to be able to synthesize these elements, a new, stronger particle accelerator would be needed.[27]

Concept criticism

Norman D. Cook notes the fact that despite the unanimity of [mainstream] theoretical models in predicting the existence of super-heavies, decades of searching for such isotopes, in both nature and through the collision of large nuclei, have come up empty. He argues that this is because the island is "a mirage" due to improper "extrapolation from conventional ideas about the nuclear force".[13] Cook advocates for a nuclear structure model that predicts decreasing stability in the super-heavy region.[28]

See also

References

  1. ↑ 1.0 1.1 "Superheavy Element 114 Confirmed: A Stepping Stone to the Island of Stability". Retrieved 11 October 2009.
  2. ↑ "Superheavy, and yet stable". Max Planck Institute for Nuclear Physics. 23 August 2012. Retrieved 23 June 2013.
    Quoting: “We expect [the island of stability] at around element 120,” says Blaum, “and to be more precise, in a nucleus with around 180 neutrons.”
  3. ↑ Seaborg, G. T. (1987). "Superheavy elements". Contemporary Physics 28: 33. doi:10.1080/00107518708211038.
  4. ↑ "The Island of Stability?". Retrieved 2012-07-24.
  5. ↑ "Shell Model of Nucleus". HyperPhysics. Department of Physics and Astronomy, Georgia State University. Retrieved 22 January 2007.
  6. ↑ Dvořák, Jan (2007-07-12). "PhD. Thesis: Decay properties of nuclei close to Z = 108 and N = 162". Technische Universität München.
  7. ↑ Dvorak, J.; Brüchle, W.; Chelnokov, M.; Dressler, R.; Düllmann, Ch.; Eberhardt, K.; Gorshkov, V.; Jäger, E. et al. (2006). "Doubly Magic Nucleus Hs162-108-270". Physical Review Letters 97 (24): 242501. Bibcode:2006PhRvL..97x2501D. doi:10.1103/PhysRevLett.97.242501. PMID 17280272.
  8. ↑ pp. 129–133, The physical principles of thermonuclear explosives, inertial confinement fusion, and the quest for fourth generation nuclear weapons (Andre Gsponer and Jean-Pierre Hurni 2009)
  9. ↑ Emsley, John (2001). Nature's Building Blocks ((Hardcover, First Edition) ed.). Oxford University Press. pp. (pages 143, 144,458). ISBN 0-19-850340-7.
  10. ↑ Khuyagbaatar, J. (2014). "Ca48+Bk249 Fusion Reaction Leading to Element Z=117: Long-Lived α-Decaying Db270 and Discovery of Lr266". Phys. Rev. Lett. 112: 172501. Bibcode:2014PhRvL.112b2501S. doi:10.1103/PhysRevLett.112.022501. Retrieved May 8, 2014.
  11. ↑ Alexandra Witze (April 6, 2010). "Superheavy element 117 makes debut". Retrieved April 6, 2010.
  12. ↑ "Hassium". Rsc.org. Retrieved 2014-05-02.
  13. ↑ 13.0 13.1 Cook, N. D. (2010). "Models of the Atomic Nucleus". Springer. doi:10.1007/978-3-642-14737-1. ISBN 978-3-642-14736-4. Available to freely download as a PDF from the author's website:
  14. ↑ 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.
  15. ↑ 15.0 15.1 15.2 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.
  16. ↑ 16.0 16.1 16.2 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.
  17. ↑ 17.0 17.1 17.2 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.
  18. ↑ 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.
  19. ↑ 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.
  20. ↑ 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.
  21. ↑ Note graphic: Known and predicted regions of nuclear stability, surrounded by a “sea” of instability. cf. the Chart of Nuclides by half-life.
  22. ↑ 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.
  23. ↑ 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.
  24. ↑ S. Cwiok, P.-H. Heenen and W. Nazarewicz (2005). "Shape coexistence and triaxiality in the superheavy nuclei" (PDF). Nature 433 (7027): 705–9. Bibcode:2005Natur.433..705C. doi:10.1038/nature03336. PMID 15716943.
  25. ↑ 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.
  26. ↑ http://link.springer.com/article/10.1007%2FBF01406719/lookinside/000.png
  27. ↑ "Nuclear scientists eye future landfall on a second 'island of stability'". Eurekalert.org. 2008-04-06. Retrieved 2014-05-02.
  28. ↑ Cook, N. D. (1990). "The Attenuation of the Periodic Table". Modern Physics Letters A 05 (17): 1321. doi:10.1142/S0217732390001505.

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