View source for Beryllium-8

{{Short description|Isotope of beryllium}}
{{Infobox isotope
 | image =Beryllium-8.svg
 | alternate_names =
 | mass_number = 8
 | symbol = Be
 | num_neutrons = 4
 | num_protons = 4
 | abundance = 0 (extinct){{efn|<sup>8</sup>Be does not occur naturally on Earth, but it exists in [[secular equilibrium]] in the cores of helium-burning stars.<ref name=otheru/>}}
 | halflife = {{val|8.19|0.37|e=−17 |u=s}}
 | decay_product =
 | decay_mass = 4
 | decay_symbol = He
 | mass = 8.00530510(4)
 | spin = 0
 | excess_energy =
 | binding_energy =
 | decay_mode1 = [[Alpha decay|α]]
 | decay_energy1 = {{val|91.84|4|e=-3}}{{AME2016 II|ref}}
 | decay_mode2 =
 | decay_energy2 =
 | decay_mode3 =
 | decay_energy3 =
 | decay_mode4 =
 | decay_energy4 =
    }}
'''Beryllium-8''' ('''<sup>8</sup>Be''', '''Be-8''') is a [[radionuclide]] with 4 [[neutron]]s and 4 [[proton]]s. It is an unbound [[resonance (particle physics)|resonance]] and nominally an [[isotopes of beryllium|isotope of beryllium]]. It has a half-life on the order of 8.19{{e|−17}}&nbsp;seconds, decaying into two [[alpha particle]]s. This has important ramifications in [[stellar nucleosynthesis]] as it creates a bottleneck in the creation of heavier [[chemical element]]s. The properties of <sup>8</sup>Be have also led to speculation on the [[fine-tuned universe|fine tuning]] of the [[universe]], and theoretical investigations on cosmological evolution had <sup>8</sup>Be been stable.

== 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, England|Cambridge]], in which they irradiated [[lithium-7]] with [[proton]]s. They reported that this populated a nucleus with ''A''&nbsp;=&nbsp;8 that near-instantaneously decays into two alpha particles. This activity was observed again several months later, and was inferred to originate from <sup>8</sup>Be.<ref name=isodisc>{{Thoennessen2016|pages=45–48}}</ref>

== Properties ==
[[File:Triple-Alpha Process.svg|left|thumb|300px|Triple-alpha process]]
Beryllium-8 is [[nuclear drip line|unbound]] with respect to alpha emission by 92&nbsp;keV; it is a resonance having a width of 6&nbsp;eV.<ref name=a58>{{cite journal |last1=Coc |first1=A. |last2=Olive |first2=K. A. |last3=Uzan |first3=J.-P. |last4=Vangioni |first4=E. |title=Variation of fundamental constants and the role of ''A''&nbsp;=&nbsp;5 and ''A''&nbsp;=&nbsp;8 nuclei on primordial nucleosynthesis |date=2012 |journal=Physical Review D |volume=86 |issue=4 |pages=043529 |doi=10.1103/PhysRevD.86.043529 |arxiv=1206.1139|bibcode=2012PhRvD..86d3529C |s2cid=119230483 }}</ref> The nucleus of helium-4 is particularly stable, having a [[doubly magic]] configuration and larger [[nuclear binding energy|binding energy per nucleon]] than <sup>8</sup>Be. As the total energy of <sup>8</sup>Be is greater than that of two [[alpha particle]]s, the decay into two alpha particles is energetically favorable,<ref name=schatz>{{cite journal |last1=Schatz |first1=H. |last2=Blaum |first2=K. |title=Nuclear masses and the origin of the elements |date=2006 |journal=Europhysics News |volume=37 |issue=5 |pages=16–21 |doi=10.1051/epn:2006502 |bibcode=2006ENews..37e..16S |url=https://www.europhysicsnews.org/articles/epn/pdf/2006/05/epn06502.pdf |doi-access=free }}</ref> and the synthesis of <sup>8</sup>Be from two <sup>4</sup>He nuclei is endothermic. The decay of <sup>8</sup>Be is facilitated by the structure of the <sup>8</sup>Be nucleus; it is highly deformed, and is believed to be a molecule-like cluster of two alpha particles that are very easily separated.<ref name=exb1>{{cite book |last=Freer |first=M. |editor-last=Scheidenberger |editor-first=C. |editor-last2=Pfützner |editor-first2=M. |title=The Euroschool on Exotic Beams: Lecture Notes in Physics |chapter=Clustering in Light Nuclei; from the Stable to the Exotic |volume=4 |pages=1–37 |publisher=Springer |date=2014 |issn=0075-8450 |isbn=978-3-642-45140-9 |doi=10.1007/978-3-642-45141-6 |chapter-url=https://www.euroschoolonexoticbeams.be/site/files/nlp/LNP879_Chapter1.pdf|series=Lecture Notes in Physics |url=https://cds.cern.ch/record/1969486 }}</ref><ref name=cluster>{{cite journal |last1=Zhou |first1=B. |last2=Ren |first2=Z. |title=Nonlocalized clustering in nuclei |date=2017 |journal=Advances in Physics |volume=2 |issue=2 |pages=359–372 |doi=10.1080/23746149.2017.1294033 |bibcode=2017AdPhX...2..359Z |doi-access=free }}</ref> Furthermore, while other [[alpha nuclide]]s have similar short-lived resonances, <sup>8</sup>Be is exceptionally already in the [[ground state]]. The unbound system of two α-particles has a low energy of the [[Coulomb barrier]], which enables its existence for any significant length of time.<ref name=8gap/> Namely, <sup>8</sup>Be decays with a half-life of 8.19{{e|−17}}&nbsp;seconds.{{NUBASE2016|ref}}

Beryllium-8 is the only unstable nuclide with the same [[even number]] ≤&nbsp;20 of [[proton]]s and [[neutron]]s. It is also one of the only two unstable nuclides (the other is [[helium-5]]) with [[mass number]] ≤&nbsp;143 which are [[Beta-decay stable isobars|stable]] to both [[beta decay]] and [[double beta decay]]. 

There are also several excited states of <sup>8</sup>Be, all short-lived resonances – having widths up to several MeV and varying [[nuclear spin|isospins]] – that quickly decay to the ground state or into two alpha particles.<ref name=5force>{{cite journal |last1=Feng |first1=J. L. |last2=Fornal |first2=B. |last3=Galon |first3=I. |last4=Gardner |first4=S. |last5=Smolinsky |first5=J. |last6=Tait |first6=T. M. P. |last7=Tanedo |first7=P. |display-authors=3 |date=2016 |title=Evidence for a protophobic fifth force from <sup>8</sup>Be nuclear transitions |journal=Physical Review Letters |volume=117 |issue=7 |pages=071803 |doi=10.1103/PhysRevLett.117.071803 |pmid=27563952 |arxiv=1604.07411|s2cid=206279817 }}</ref>
{{clear}}

=== Decay anomaly and possible fifth force ===
{{main|X17 particle}}
A 2015 experiment by Attila Krasznahorkay et al. at the [[ATOMKI|Hungarian Academy of Sciences's Institute for Nuclear Research]] found anomalous decays in the 17.64 and 18.15&nbsp;MeV excited states of <sup>8</sup>Be, populated by proton irradiation of <sup>7</sup>Li. An excess of decays creating [[electron]]-[[positron]] pairs at a 140° angle with a combined energy of 17&nbsp;MeV was observed. Jonathan Feng et al. attribute this 6.8-[[standard deviation|σ]] anomaly to a 17&nbsp;MeV protophobic X-[[gauge boson|boson]] dubbed the [[X17 particle]]. This boson would mediate a [[fifth force|fifth fundamental force]] acting over a short range (12&nbsp;[[femtometer|fm]]) and perhaps explain the decay of these <sup>8</sup>Be excited states.<ref name=5force/> A 2018 rerun of this experiment found the same anomalous particle scattering and set a narrower mass range of the proposed fifth boson, {{val|17.01|0.16}} [[Electronvolt (mass)|MeV/c<sup>2</sup>]].<ref name=expt18>{{cite journal |last1=Krasznahorkay |first1=A. J. |last2=Csatlós |first2=M. |last3=Csige |first3=L. |last4=Gácsi |first4=Z. |last5=Gulyás |first5=J. |last6=Nagy |first6=Á. |last7=Sas |first7=N. |last8=Timár |first8=J. |last9=Tornyi |first9=T. G. |last10=Vajda |first10=I. |display-authors=3 |title=New results on the <sup>8</sup>Be anomaly |journal=Journal of Physics: Conference Series |date=2018 |volume=1056 |issue=1 |pages=012028 |doi=10.1088/1742-6596/1056/1/012028|bibcode=2018JPhCS1056a2028K |url=https://cds.cern.ch/record/2678071/files/pdf.pdf |doi-access=free }}</ref> While further experiments are needed to corroborate these observations, the influence of a fifth boson has been proposed as "the most straightforward possibility".<ref name=5fnews>{{cite news |title=Has a Hungarian physics lab found a fifth force of nature? |last=Cartlidge |first=E. |date=25 May 2016 |accessdate=14 July 2019 |publisher=Nature |url=https://www.nature.com/news/has-a-hungarian-physics-lab-found-a-fifth-force-of-nature-1.19957}}</ref>

== Role in stellar nucleosynthesis ==
{{Main|Triple-alpha process}}
In [[stellar nucleosynthesis]], two [[helium-4]] nuclei may collide and [[Nuclear fusion|fuse]] into a single beryllium-8 nucleus. Beryllium-8 has an extremely short half-life (8.19{{e|−17}}&nbsp;seconds), and [[Radioactive decay|decays]] back into two helium-4 nuclei. This, along with the unbound nature of <sup>5</sup>He and <sup>5</sup>Li, creates a bottleneck in [[Big Bang nucleosynthesis]] and [[stellar nucleosynthesis]],<ref name=8gap/> for it necessitates a very fast reaction rate.<ref name=exft>{{cite arXiv |last=Landsman |first=K. |title=The Fine-Tuning Argument |date=2015 |eprint=1505.05359|class=physics.hist-ph }}</ref> 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<ref>{{cite journal |last=Öpik |first=E. J. |author-link=Ernst Öpik|date=1951 |title=Stellar Models with Variable Composition. II. Sequences of Models with Energy Generation Proportional to the Fifteenth Power of Temperature |journal=Proceedings of the Royal Irish Academy, Section A |volume=54 |pages=49–77 |jstor=20488524}}</ref> and Salpeter<ref>{{cite journal |last=Salpeter |first=E. E. |date=1952 |title=Nuclear Reactions in the Stars. I. Proton-Proton Chain". |journal=[[Physical Review]] |volume=88 |issue=3 |pages=547–553 |doi=10.1103/PhysRev.88.547|bibcode=1952PhRv...88..547S }}</ref> in the early 1950s.

Owing to the instability of <sup>8</sup>Be, the [[triple-alpha process]] is the only reaction in which <sup>12</sup>C and heavier elements may be produced in observed quantities. The triple-alpha process, despite being a three-body reaction, is facilitated when <sup>8</sup>Be production increases such that its concentration is approximately 10<sup>−8</sup> relative to <sup>4</sup>He;<ref name=bld>{{cite web |last=Piekarewicz |first=J. |title=The Birth, Life, and Death of Stars |date=2014 |url=https://web2.physics.fsu.edu/~piekarewicz/OLLI-Lecture2.pdf |accessdate=13 July 2019 |publisher=Florida State University}}</ref> this occurs when <sup>8</sup>Be is produced faster than it decays.<ref name=rcap>{{cite journal |last1=Sadeghi |first1=H. |last2=Pourimani |first2=R. |last3=Moghadasi |first3=A. |title=Two-helium radiative capture process and the <sup>8</sup>Be nucleus at settler energies |journal=Astrophysics and Space Science |date=2014 |volume=350 |issue=2 |pages=707–712 |doi=10.1007/s10509-014-1806-1 |bibcode=2014Ap&SS.350..707S |s2cid=123444620 |url=https://www.researchgate.net/publication/260518982}}</ref> However, this alone is insufficient, as the collision between <sup>8</sup>Be and <sup>4</sup>He is more likely to break apart the system rather than enable fusion;<ref name=gizmodo>{{cite web |last1=Inglis-Arkell |first1=E. |title=This Unbelievable Coincidence Is Responsible For Life In The Universe |url=https://io9.gizmodo.com/this-unbelievable-coincidence-is-responsible-for-life-i-1709731652 |publisher=Gizmodo |accessdate=14 July 2019}}</ref> the reaction rate would still not be fast enough to explain the observed abundance of <sup>12</sup>C.<ref name=otheru>{{cite journal |last1=Adams |first1=F. C. |last2=Grohs |first2=E. |title=Stellar helium burning in other universes: A solution to the triple alpha fine-tuning problem |date=2017 |journal=Astroparticle Physics |volume=7 |pages=40–54 |doi=10.1016/j.astropartphys.2016.12.002 |arxiv=1608.04690|bibcode=2017APh....87...40A |s2cid=119287629 }}</ref> In 1954, [[Fred Hoyle]] thus postulated the existence of a [[resonance (particle physics)|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.<ref>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}}</ref> 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.<ref name=hoyles>{{cite journal |last1=Epelbaum |first1=E. |last2=Krebs |first2=H. |last3=Lee |first3=D. |last4=Meißner |first4=Ulf-G. |s2cid=33827991 |date=2011 |title=''Ab initio'' calculation of the Hoyle state |journal=Physical Review Letters |volume=106 |issue=19 |pages=192501–1–192501–4 |doi=10.1103/PhysRevLett.106.192501 |pmid=21668146 |arxiv=1101.2547 |bibcode=2011PhRvL.106s2501E |doi-access=free }}</ref><ref>{{Cite web|last1=Jenkins|first1=David|last2=Kirsebom|first2=Oliver|date=2013-02-07|title=The secret of life|url=https://physicsworld.com/a/the-secret-of-life/|url-status=live|access-date=2021-08-21|website=Physics World|language=en-GB|archive-url=https://web.archive.org/web/20210213104419/https://physicsworld.com/a/the-secret-of-life/ |archive-date=2021-02-13 }}</ref>

== Hypothetical universes with stable <sup>8</sup>Be ==
As beryllium-8 is unbound by only 92&nbsp;keV, it is theorized that very small changes in [[nuclear potential]] and the fine tuning of certain constants (such as α, the [[fine structure constant]]), could sufficiently increase the binding energy of <sup>8</sup>Be to prevent its alpha decay, thus making it [[stable nuclide|stable]]. This has led to investigations of hypothetical scenarios in which <sup>8</sup>Be is stable and speculation about [[multiverse|other universes]] with different fundamental constants.<ref name=otheru/> These studies suggest that the disappearance of the bottleneck<ref name=hoyles/> created by <sup>8</sup>Be would result in a very different reaction mechanism in [[Big Bang nucleosynthesis]] and the triple-alpha process, as well as alter the abundances of heavier chemical elements.<ref name=a58/> 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 if <sup>8</sup>Be were stable.<ref name=8gap>{{cite journal |last1=Coc |first1=A. |last2=Vangioni |first2=E. |title=The triple-alpha reaction and the ''A''&nbsp;=&nbsp;8 gap in BBN and Population III stars |journal=Memorie della Società Astronomica Italiana |volume=85 |pages=124–129 |date=2014 |bibcode=2014MmSAI..85..124C |url=http://inspirehep.net/record/1338211/files/2014MmSAI..85..124C.pdf?version=1}}</ref> However, stable <sup>8</sup>Be would enable alternative reaction pathways in helium burning (such as <sup>8</sup>Be + <sup>4</sup>He and <sup>8</sup>Be + <sup>8</sup>Be; constituting a "beryllium burning" phase) and possibly affect the abundance of the resultant <sup>12</sup>C, <sup>16</sup>O, and heavier nuclei, though <sup>1</sup>H and <sup>4</sup>He would remain the most abundant nuclides. This would also affect [[stellar evolution]] through an earlier onset and faster rate of helium burning (and beryllium burning), and result in a different [[main sequence]] than our Universe.<ref name=otheru/>

== Notes ==
{{Notelist}}

== References ==
{{Reflist}}

{{Isotope sequence
|element=beryllium
|lighter=[[beryllium-7]]
|heavier=[[beryllium-9]]
|before=[[carbon-9]] '''([[Beta decay|β<sup>+</sup>]], [[proton emission|p]])<br />'''[[boron-9]] '''([[proton emission|p]])<br />'''[[lithium-8]] '''([[beta decay|β<sup>−</sup>]])'''
|after=[[helium-4]] '''(α)'''
}}

{{Authority control}}

[[Category:Isotopes of beryllium]]
[[Category:Nucleosynthesis]]