Beryllium-8

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Beryllium-8
Beryllium-8.svg
General
Symbol 8Be
Names beryllium-8
Protons (Z)4
Neutrons (N)4
Nuclide data
Natural abundance 0 [a]
Half-life (t1/2)(8.19±0.37)×10−17 s [2]
Isotope mass 8.00530510(4) [3] Da
Spin 0
Decay products 4He
Decay modes
Decay mode Decay energy (MeV)
α 0.09184(4)
Isotopes of beryllium
Complete table of nuclides

Beryllium-8 (8Be, Be-8) is a radionuclide with 4 neutrons and 4 protons. It is an unbound resonance of two alpha particles and nominally an isotope of beryllium. This has important ramifications in stellar nucleosynthesis as it creates a bottleneck in the creation of heavier chemical elements.

Contents

Discovery

The discovery of beryllium-8 occurred shortly after the construction of the first particle accelerator in 1932. Physicists John Douglas Cockcroft and Ernest Walton performed their first experiment with their accelerator at the Cavendish Laboratory in Cambridge, in which they irradiated lithium-7 with protons. They reported that this populated a nucleus with A = 8 that near-instantaneously decays into two alpha particles. This activity was observed again several months later, and was inferred to originate from 8Be. [4]

Properties

Triple-alpha process Triple-Alpha Process.svg
Triple-alpha process

Beryllium-8 is unbound with respect to alpha emission by 92 keV; it is a resonance having a width of 6 eV. [5] The nucleus of helium-4 is particularly stable, having a doubly magic configuration and larger binding energy per nucleon than 8Be. As the total energy of 8Be is greater than that of two alpha particles, the decay into two alpha particles is energetically favorable, [6] and the synthesis of 8Be from two 4He nuclei is endothermic. The decay of 8Be is facilitated by the structure of the 8Be nucleus; it is highly deformed, and is believed to be a molecule-like cluster of two alpha particles that are very easily separated. [7] [8] Furthermore, while other alpha nuclides have similar short-lived resonances, 8Be is exceptionally already in the ground state. The unbound system of two α-particles has a low-energy Coulomb barrier, which makes it unable to exist for any significant length of time: [9] its half-life is about 8.2×10−17 seconds.

Beryllium-8 is the only unstable nuclide with the same even number ≤ 20 of protons and neutrons. It is also one of the only two unstable nuclides (the other is helium-5) with mass number ≤ 143 which are stable to both beta decay and double beta decay.

There are also several excited states of 8Be, all short-lived resonances – having widths up to several MeV and varying isospins – that quickly decay to the ground state or into two alpha particles. [10]

Role in stellar nucleosynthesis

In stellar nucleosynthesis, two helium-4 nuclei may collide and fuse into a single beryllium-8 nucleus. Beryllium-8 has an extremely short life before reverting to two helium-4 nuclei, but can still exist in a useful equilibrium concentration. This, along with the unbound nature of 5He and 5Li, creates a bottleneck in Big Bang nucleosynthesis and stellar nucleosynthesis, [9] for it necessitates a very fast reaction rate. [11] This impedes formation of heavier elements in the former, and limits the yield in the latter process. If the beryllium-8 collides with a helium-4 nucleus before decaying, they can fuse into a carbon-12 nucleus. This reaction was first theorized independently by Öpik [12] and Salpeter [13] in the early 1950s.

Owing to the instability of 8Be, the triple-alpha process is the only reaction in which 12C and heavier elements may be produced in observed quantities. The triple-alpha process, despite being a three-body reaction, is facilitated when 8Be production increases such that its concentration is approximately 10−8 relative to 4He; [14] this occurs when 8Be is produced faster than it decays. [15] However, this alone is insufficient, as the collision between 8Be and 4He will very seldom result in fusion [16] and the reaction rate would still not be fast enough to explain the observed abundance of 12C. [1] In 1954, Fred Hoyle thus postulated the existence of a resonance in carbon-12 within the stellar energy region of the triple-alpha process, enhancing the creation of carbon-12 despite the extremely short half-life of beryllium-8. [17] The existence of this resonance (the Hoyle state) was confirmed experimentally shortly thereafter; its discovery has been cited in formulations of the anthropic principle and the fine-tuned Universe hypothesis. [18] [19]

Notes

  1. ↑ 8Be does not occur naturally on Earth, but it exists in secular equilibrium in the cores of helium-burning stars. [1]

References

  1. 1 2 Adams, F. C.; Grohs, E. (2017). "Stellar helium burning in other universes: A solution to the triple alpha fine-tuning problem". Astroparticle Physics. 7: 40–54. arXiv: 1608.04690 . Bibcode:2017APh....87...40A. doi:10.1016/j.astropartphys.2016.12.002. S2CID   119287629.
  2. ↑ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3) 030001. doi:10.1088/1674-1137/abddae.
  3. ↑ Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3) 030003. doi:10.1088/1674-1137/abddaf.
  4. ↑ Thoennessen, M. (2016). The Discovery of Isotopes: A Complete Compilation. Springer. pp. 45–48. doi:10.1007/978-3-319-31763-2. ISBN   978-3-319-31761-8. LCCN   2016935977.
  5. ↑ Coc, A.; Olive, K. A.; Uzan, J.-P.; Vangioni, E. (2012). "Variation of fundamental constants and the role of A = 5 and A = 8 nuclei on primordial nucleosynthesis". Physical Review D. 86 (4) 043529. arXiv: 1206.1139 . Bibcode:2012PhRvD..86d3529C. doi:10.1103/PhysRevD.86.043529. S2CID   119230483.
  6. ↑ Schatz, H.; Blaum, K. (2006). "Nuclear masses and the origin of the elements" (PDF). Europhysics News. 37 (5): 16–21. Bibcode:2006ENews..37e..16S. doi: 10.1051/epn:2006502 .
  7. ↑ Freer, M. (2014). "Clustering in Light Nuclei; from the Stable to the Exotic" (PDF). In Scheidenberger, C.; Pfützner, M. (eds.). The Euroschool on Exotic Beams: Lecture Notes in Physics. Vol. 4. Springer. pp. 1–37. doi:10.1007/978-3-642-45141-6. ISBN   978-3-642-45140-9. ISSN   0075-8450.
  8. ↑ Zhou, B.; Ren, Z. (2017). "Nonlocalized clustering in nuclei". Advances in Physics. 2 (2): 359–372. Bibcode:2017AdPhX...2..359Z. doi: 10.1080/23746149.2017.1294033 .
  9. 1 2 Coc, A.; Vangioni, E. (2014). "The triple-alpha reaction and the A = 8 gap in BBN and Population III stars" (PDF). Memorie della Società Astronomica Italiana. 85: 124–129. Bibcode:2014MmSAI..85..124C.
  10. ↑ Feng, J. L.; Fornal, B.; Galon, I.; et al. (2016). "Evidence for a protophobic fifth force from 8Be nuclear transitions". Physical Review Letters. 117 (7) 071803. arXiv: 1604.07411 . doi:10.1103/PhysRevLett.117.071803. PMID   27563952. S2CID   206279817.
  11. ↑ Landsman, K. (2015). "The Fine-Tuning Argument". arXiv: 1505.05359 [physics.hist-ph].
  12. ↑ Öpik, E. J. (1951). "Stellar Models with Variable Composition. II. Sequences of Models with Energy Generation Proportional to the Fifteenth Power of Temperature". Proceedings of the Royal Irish Academy, Section A. 54: 49–77. JSTOR   20488524.
  13. ↑ Salpeter, E. E. (1952). "Nuclear Reactions in the Stars. I. Proton-Proton Chain"". Physical Review . 88 (3): 547–553. Bibcode:1952PhRv...88..547S. doi:10.1103/PhysRev.88.547.
  14. ↑ Piekarewicz, J. (2014). "The Birth, Life, and Death of Stars" (PDF). Florida State University. Retrieved 13 July 2019.
  15. ↑ Sadeghi, H.; Pourimani, R.; Moghadasi, A. (2014). "Two-helium radiative capture process and the 8Be nucleus at settler energies". Astrophysics and Space Science. 350 (2): 707–712. Bibcode:2014Ap&SS.350..707S. doi:10.1007/s10509-014-1806-1. S2CID   123444620.
  16. ↑ Inglis-Arkell, E. (8 June 2015). "This Unbelievable Coincidence Is Responsible For Life In The Universe". Gizmodo. Retrieved 14 July 2019.
  17. ↑ Hoyle, F. (1954). "On Nuclear Reactions Occurring in Very Hot STARS. I. the Synthesis of Elements from Carbon to Nickel". Astrophysical Journal Supplement.1: 121–146, doi : 10.1086/190005
  18. ↑ Epelbaum, E.; Krebs, H.; Lee, D.; Meißner, Ulf-G. (2011). "Ab initio calculation of the Hoyle state". Physical Review Letters. 106 (19): 192501–1–192501–4. arXiv: 1101.2547 . Bibcode:2011PhRvL.106s2501E. doi: 10.1103/PhysRevLett.106.192501 . PMID   21668146. S2CID   33827991.
  19. ↑ Jenkins, David; Kirsebom, Oliver (2013-02-07). "The secret of life". Physics World. Archived from the original on 2021-02-13. Retrieved 2021-08-21.
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