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Beryllium-8

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Beryllium-8,8Be
General
Symbol8Be
Namesberyllium-8, 8Be, Be-8
Protons(Z)4
Neutrons(N)4
Nuclide data
Natural abundance0 (extinct)[a]
Half-life(t1/2)(8.19±0.37)×10−17s
Isotope mass8.00530510(4)Da
Spin0
Decay products4He
Decay modes
Decay modeDecay energy(MeV)
α(91.84±4)×10−3[2]
Isotopes of beryllium
Complete table of nuclides

Beryllium-8(8Be,Be-8) is aradionuclidewith 4neutronsand 4protons.It is an unboundresonanceand nominally anisotope of beryllium.It decays into twoAlpha particleswith a half-life on the order of 8.19×10−17seconds. This has important ramifications instellar nucleosynthesisas it creates a bottleneck in the creation of heavierchemical elements.The properties of8Be have also led to speculation on thefine tuningof theUniverse,and theoretical investigations on cosmological evolution had8Be been stable.

Discovery

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The discovery of beryllium-8 occurred shortly after the construction of the firstparticle acceleratorin 1932. PhysicistsJohn Douglas CockcroftandErnest Waltonperformed their first experiment with their accelerator at theCavendish LaboratoryinCambridge,in which they irradiatedlithium-7withprotons.They reported that this populated a nucleus withA= 8 that near-instantaneously decays into two Alpha particles. This activity was observed again several months later, and was inferred to originate from8Be.[3]

Properties

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Triple- Alpha process

Beryllium-8 isunboundwith respect to Alpha emission by 92 keV; it is a resonance having a width of 6 eV.[4]The nucleus of helium-4 is particularly stable, having adoubly magicconfiguration and largerbinding energy per nucleonthan8Be. As the total energy of8Be is greater than that of twoAlpha particles,the decay into two Alpha particles is energetically favorable,[5]and the synthesis of8Be from two4He nuclei is endothermic. The decay of8Be is facilitated by the structure of the8Be nucleus; it is highly deformed, and is believed to be a molecule-like cluster of two Alpha particles that are very easily separated.[6][7]Furthermore, while otherAlpha nuclideshave similar short-lived resonances,8Be is exceptionally already in theground state.The unbound system of two α-particles has a low energy of theCoulomb barrier,which enables its existence for any significant length of time.[8]Namely,8Be decays with a half-life of 8.19×10−17seconds.[9]

Beryllium-8 is the only unstable nuclide with the sameeven number≤ 20 ofprotonsandneutrons.It is also one of the only two unstable nuclides (the other ishelium-5) withmass number≤ 143 which arestableto bothbeta decayanddouble beta decay.

There are also several excited states of8Be, all short-lived resonances – having widths up to several MeV and varyingisospins– that quickly decay to the ground state or into two Alpha particles.[10]

Decay anomaly and possible fifth force

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Main article:X17 particle

A 2015 experiment by Attila Krasznahorkay et al. at theHungarian Academy of Sciences's Institute for Nuclear Researchfound anomalous decays in the 17.64 and 18.15 MeV excited states of8Be, populated by proton irradiation of7Li. An excess of decays creatingelectron-positronpairs at a 140° angle with a combined energy of 17 MeV was observed. Jonathan Feng et al. attribute this 6.8-σanomaly to a 17 MeV protophobic X-bosondubbed theX17 particle.This boson would mediate afifth fundamental forceacting over a short range (12fm) and perhaps explain the decay of these8Be excited states.[10]A 2018 rerun of this experiment found the same anomalous particle scattering and set a narrower mass range of the proposed fifth boson,17.01±0.16MeV/c2.[11]While further experiments are needed to corroborate these observations, the influence of a fifth boson has been proposed as "the most straightforward possibility".[12]

Role in stellar nucleosynthesis

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Instellar nucleosynthesis,twohelium-4nuclei may collide andfuseinto a single beryllium-8 nucleus. Beryllium-8 has an extremely short half-life (8.19×10−17seconds), anddecaysback into two helium-4 nuclei. This, along with the unbound nature of5He and5Li, creates a bottleneck inBig Bang nucleosynthesisandstellar nucleosynthesis,[8]for it necessitates a very fast reaction rate.[13]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 acarbon-12nucleus. This reaction was first theorized independently by Öpik[14]and Salpeter[15]in the early 1950s.

Owing to the instability of8Be, thetriple- Alpha processis the only reaction in which12C and heavier elements may be produced in observed quantities. The triple- Alpha process, despite being a three-body reaction, is facilitated when8Be production increases such that its concentration is approximately 10−8relative to4He;[16]this occurs when8Be is produced faster than it decays.[17]However, this alone is insufficient, as the collision between8Be and4He is more likely to break apart the system rather than enable fusion;[18]the reaction rate would still not be fast enough to explain the observed abundance of12C.[1]In 1954,Fred Hoylethus postulated the existence of aresonancein 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.[19]The existence of this resonance (theHoyle state) was confirmed experimentally shortly thereafter; its discovery has been cited in formulations of theanthropic principleand the fine-tuned Universe hypothesis.[20][21]

Hypothetical universes with stable8Be

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As beryllium-8 is unbound by only 92 keV, it is theorized that very small changes innuclear potentialand the fine tuning of certain constants (such as α, thefine structure constant), could sufficiently increase the binding energy of8Be to prevent its Alpha decay, thus making itstable.This has led to investigations of hypothetical scenarios in which8Be is stable and speculation aboutother universeswith different fundamental constants.[1]These studies suggest that the disappearance of the bottleneck[20]created by8Be would result in a very different reaction mechanism inBig Bang nucleosynthesisand the triple- Alpha process, as well as alter the abundances of heavier chemical elements.[4]As Big Bang nucleosynthesis only occurred within a short period having the necessary conditions, it is thought that there would be no significant difference in carbon production even if8Be were stable.[8]However, stable8Be would enable alternative reaction pathways in helium burning (such as8Be +4He and8Be +8Be; constituting a "beryllium burning" phase) and possibly affect the abundance of the resultant12C,16O, and heavier nuclei, though1H and4He would remain the most abundant nuclides. This would also affectstellar evolutionthrough an earlier onset and faster rate of helium burning (and beryllium burning), and result in a differentmain sequencethan our Universe.[1]

Notes

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  1. ^It does not occur naturally on Earth, but it exists insecular equilibriumin the cores of helium-burning stars.[1]

References

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  1. ^abcdAdams, 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.S2CID119287629.
  2. ^Wang, M.; Audi, G.; Kondev, F. G.; Huang, W. J.; Naimi, S.; Xu, X. (2017)."The AME2016 atomic mass evaluation (II). Tables, graphs, and references"(PDF).Chinese Physics C.41(3): 030003-1–030003-442.doi:10.1088/1674-1137/41/3/030003.
  3. ^Thoennessen, M. (2016).The Discovery of Isotopes: A Complete Compilation.Springer. pp. 45–48.doi:10.1007/978-3-319-31763-2.ISBN978-3-319-31761-8.LCCN2016935977.
  4. ^abCoc, A.; Olive, K. A.; Uzan, J.-P.; Vangioni, E. (2012). "Variation of fundamental constants and the role ofA= 5 andA= 8 nuclei on primordial nucleosynthesis ".Physical Review D.86(4): 043529.arXiv:1206.1139.Bibcode:2012PhRvD..86d3529C.doi:10.1103/PhysRevD.86.043529.S2CID119230483.
  5. ^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.
  6. ^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.Lecture Notes in Physics. Vol. 4. Springer. pp. 1–37.doi:10.1007/978-3-642-45141-6.ISBN978-3-642-45140-9.ISSN0075-8450.
  7. ^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.
  8. ^abcCoc, A.; Vangioni, E. (2014)."The triple- Alpha reaction and theA= 8 gap in BBN and Population III stars "(PDF).Memorie della Società Astronomica Italiana.85:124–129.Bibcode:2014MmSAI..85..124C.
  9. ^Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017)."The NUBASE2016 evaluation of nuclear properties"(PDF).Chinese Physics C.41(3): 030001.Bibcode:2017ChPhC..41c0001A.doi:10.1088/1674-1137/41/3/030001.
  10. ^abFeng, J. L.; Fornal, B.; Galon, I.; et al. (2016). "Evidence for a protophobic fifth force from8Be nuclear transitions ".Physical Review Letters.117(7): 071803.arXiv:1604.07411.doi:10.1103/PhysRevLett.117.071803.PMID27563952.S2CID206279817.
  11. ^Krasznahorkay, A. J.; Csatlós, M.; Csige, L.; et al. (2018)."New results on the8Be anomaly "(PDF).Journal of Physics: Conference Series.1056(1): 012028.Bibcode:2018JPhCS1056a2028K.doi:10.1088/1742-6596/1056/1/012028.
  12. ^Cartlidge, E. (25 May 2016)."Has a Hungarian physics lab found a fifth force of nature?".Nature.Retrieved14 July2019.
  13. ^Landsman, K. (2015). "The Fine-Tuning Argument".arXiv:1505.05359[physics.hist-ph].
  14. ^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.JSTOR20488524.
  15. ^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.
  16. ^Piekarewicz, J. (2014)."The Birth, Life, and Death of Stars"(PDF).Florida State University.Retrieved13 July2019.
  17. ^Sadeghi, H.; Pourimani, R.; Moghadasi, A. (2014)."Two-helium radiative capture process and the8Be nucleus at settler energies ".Astrophysics and Space Science.350(2): 707–712.Bibcode:2014Ap&SS.350..707S.doi:10.1007/s10509-014-1806-1.S2CID123444620.
  18. ^Inglis-Arkell, E."This Unbelievable Coincidence Is Responsible For Life In The Universe".Gizmodo.Retrieved14 July2019.
  19. ^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
  20. ^abEpelbaum, E.; Krebs, H.; Lee, D.; Meißner, Ulf-G. (2011)."Ab initiocalculation 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.PMID21668146.S2CID33827991.
  21. ^Jenkins, David; Kirsebom, Oliver (2013-02-07)."The secret of life".Physics World.Archivedfrom the original on 2021-02-13.Retrieved2021-08-21.


Lighter:
beryllium-7 		Beryllium-8 is an
isotopeofberyllium 		Heavier:
beryllium-9
Decay productof:
carbon-9(β+,p)
boron-9(p)
lithium-8(β−) 		Decay chain
of beryllium-8 		Decaysto:
helium-4(α)
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