View source for Physics:Isotopes of helium

{{Short description|Nuclides with atomic number of 2 but with different mass numbers}}
{{infobox helium isotopes}}
Although there are nine known [[Physics:Isotope|isotopes]] of [[Chemistry:Helium|helium]] (<sub>2</sub>He) ([[Physics:Standard atomic weight|standard atomic weight]]: {{val|4.002602|(2)}}), only [[Physics:Helium-3|helium-3]] ({{SimpleNuclide|Helium|3}}) and [[Physics:Helium-4|helium-4]] ({{SimpleNuclide|Helium|4}}) are [[Physics:Stable isotope ratio|stable]].<ref>{{Cite web |title=helium-3 {{!}} chemical isotope {{!}} Britannica |url=https://www.britannica.com/science/helium-3 |access-date=2022-03-20 |website=www.britannica.com |language=en}}</ref> All [[Physics:Radionuclide|radioisotopes]] are short-lived, the longest-lived being {{SimpleNuclide|Helium|6}} with a [[Physics:Half-life|half-life]] of {{val|806.92|(24)|u=milliseconds}}. The least stable is {{SimpleNuclide|Helium|10}}, with a half-life of {{val|260|(40)|u=yoctoseconds}} ({{val|2.6e-22|(4)|u=s}}), although it is possible that {{SimpleNuclide|Helium|2}} may have an even shorter half-life.

In the Earth's atmosphere, the ratio of {{SimpleNuclide|Helium|3}} to {{SimpleNuclide|Helium|4}} is {{val|1.343e-6|(13)}}.<ref>{{Cite journal|last1=Sano|first1=Yuji|last2=Wakita|first2=Hiroshi|last3=Sheng|first3=Xu|date=1988|title=Atmospheric helium isotope ratio|url=https://www.jstage.jst.go.jp/article/geochemj1966/22/4/22_4_177/_article|journal=Geochemical Journal|volume=22|issue=4|pages=177–181|doi=10.2343/geochemj.22.177|bibcode=1988GeocJ..22..177S |s2cid=129104204 |doi-access=free}}</ref> However, the isotopic abundance of helium varies greatly depending on its origin. In the Local Interstellar Cloud, the proportion of {{SimpleNuclide|Helium|3}} to {{SimpleNuclide|Helium|4}} is {{val|1.62e-4|(29)}},<ref>{{Cite journal|last1=Busemann|first1=H.|last2=Bühler|first2=F.|last3=Grimberg|first3=A.|last4=Heber|first4=V. S.|last5=Agafonov|first5=Y. N.|last6=Baur|first6=H.|last7=Bochsler|first7=P.|last8=Eismont|first8=N. A.|last9=Wieler|first9=R.|last10=Zastenker|first10=G. N.|date=2006-03-01|title=Interstellar Helium Trapped with the COLLISA Experiment on the MiR Space Station—Improved Isotope Analysis by In Vacuo Etching|journal=The Astrophysical Journal|language=en|volume=639|issue=1|pages=246|doi=10.1086/499223|bibcode=2006ApJ...639..246B |s2cid=120648440 |issn=0004-637X|doi-access=free}}</ref> which is {{val|121|(22)}} times higher than that of atmospheric helium. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in [[Earth:Geology|geology]] to investigate the origin of rocks and the composition of the Earth's [[Earth:Mantle (geology)|mantle]].<ref name="heliumfundamentals"/> The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.

Equal mixtures of liquid {{SimpleNuclide|Helium|3}} and {{SimpleNuclide|Helium|4}} below {{val|0.8|u=K}} separate into two [[Chemistry:Miscibility|immiscible]] phases due to differences in [[Physics:Particle statistics#Quantum statistics|quantum statistics]]: {{SimpleNuclide|Helium|4}} atoms are [[Physics:Boson|boson]]s while {{SimpleNuclide|Helium|3}} atoms are [[Physics:Fermion|fermion]]s.<ref>
{{cite book
 |title=The Encyclopedia of the Chemical Elements
 |page=264
}}</ref> [[Physics:Dilution refrigerator|Dilution refrigerators]] take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.

== List of isotopes ==
{{Isotopes table
|symbol=He
|refs=NUBASE2020, AME2020 II
|notes=resonance, unc(), var[], daughter-st, spin#, spin(), n, p
}}
|-
| rowspan=2|{{SimpleNuclide|Helium|2}}<ref group="n">Intermediate in the [[Astronomy:Proton–proton chain|proton–proton chain]]reaction</ref>
| rowspan=2 style="text-align:right" | 2
| rowspan=2 style="text-align:right" | 0
| rowspan="2" | {{val|2.015894|(2)}}<!--not in AME2020-->
| rowspan=2 | ≪ {{val|e=-9|u=s}}<ref name="Schewe"/>
| [[Physics:Proton emission|p]] (> {{val|99.99|u=%}})
| '''[[Physics:Hydrogen atom|{{SimpleNuclide|Hydrogen|1}}]]'''
| rowspan=2 | 0+#
| rowspan=2 |
| rowspan=2 | 
|-
| [[Physics:Positron emission|β<sup>+</sup>]] (< {{val|0.01|u=%}})
| '''[[Physics:Deuterium|{{SimpleNuclide|Hydrogen|2}}]]'''
|-
| [[Physics:Helium-3|{{SimpleNuclide|Helium|3}}]]<ref group="n" name="PN">Produced during [[Astronomy:Big Bang nucleosynthesis|Big Bang nucleosynthesis]]</ref><ref group="n">This and {{SimpleNuclide|Hydrogen|1}} are the only stable nuclides with more protons than neutrons</ref>
| style="text-align:right" | 2
| style="text-align:right" | 1
| {{val|3.016029321967|(60)}}
| colspan=3 align=center|'''Stable'''
| 1/2+
| {{val|0.000002|(2)}}<ref name="Atomic Weight of Helium">{{Cite web|title=Atomic Weight of Helium|url=https://ciaaw.org/helium.htm|url-status=live|access-date=6 October 2021 |publisher=[[Organization:Commission on Isotopic Abundances and Atomic Weights|Commission on Isotopic Abundances and Atomic Weights]] |archive-url=https://web.archive.org/web/20230504051114/https://ciaaw.org/helium.htm |archive-date=4 May 2023 }}</ref>
| [{{val|4.6|e=-10}}, {{val|0.000041}}]<ref name="Isotopic Compositions of Elements 2013">{{Cite journal |last1=Meija |first1=Juris |last2=Coplen |first2=Tyler B. |last3=Berglund |first3=Michael |last4=Brand |first4=Willi A. |last5=Bièvre |first5=Paul De |last6=Gröning |first6=Manfred |last7=Holden |first7=Norman E. |last8=Irrgeher |first8=Johanna |last9=Loss |first9=Robert D. |last10=Walczyk |first10=Thomas |last11=Prohaska |first11=Thomas |date=2016-03-01 |title=Isotopic compositions of the elements 2013 (IUPAC Technical Report) |journal=Pure and Applied Chemistry |language=en |volume=88 |issue=3 |pages=293–306 |doi=10.1515/pac-2015-0503 |s2cid=104472050 |issn=1365-3075|doi-access=free |hdl=11858/00-001M-0000-0029-C408-7 |hdl-access=free }}</ref>
|-
| [[Physics:Helium-4|{{SimpleNuclide|Helium|4}}]]<ref group="n" name="PN" />
| style="text-align:right" | 2
| style="text-align:right" | 2
| {{val|4.002603254130|(158)}}
| colspan=3 align=center|'''Stable'''
| 0+
| {{val|0.999998|(2)}}<ref name="Atomic Weight of Helium"/>
| [{{val|0.999959}}, {{val|1.000000}}]<ref name="Isotopic Compositions of Elements 2013" />
|-
| {{SimpleNuclide|Helium|5

}}
| style="text-align:right" | 2
| style="text-align:right" | 3
| {{val|5.012057|(21)}}
| {{val|602|(22)|u=ys}}<br/>[{{val|758|(28)|u=keV}}]
| [[Physics:Neutron emission|n]]
| '''[[Physics:Helium-4|{{SimpleNuclide|Helium|4}}]]'''
| 3/2−
|
|
|-
| rowspan=2|{{SimpleNuclide|Helium|6}}<ref group="n">Has 2 [[Physics:Halo nucleus|halo]] neutrons</ref>
| rowspan=2 style="text-align:right" | 2
| rowspan=2 style="text-align:right" | 4
| rowspan=2|{{val|6.018885889|(57)}}
| rowspan=2|{{val|806.92|(24)|u=ms}}
| [[Physics:Beta decay|β<sup>−</sup>]] ({{val|99.999722|(18)}}%)
| '''{{SimpleNuclide|Lithium|6}}'''
| rowspan=2|0+
| rowspan=2|
| rowspan=2|
|-
| β<sup>−</sup>d<ref group="n">d: [[Physics:Deuterium|Deuteron]] emission</ref> ({{val|0.000278|(18)}}%)
| '''{{SimpleNuclide|Helium|4}}'''
|-
| {{SimpleNuclide|Helium|7}}
| style="text-align:right" | 2
| style="text-align:right" | 5
| {{val|7.027991|(8)}}
| {{val|2.51|(7)|u=zs}}<br/>[{{val|182|(5)|u=keV}}]
| n
| {{SimpleNuclide|Helium|6}}
| (3/2)−
|
|
|-
| rowspan=3|{{SimpleNuclide|Helium|8}}<ref group="n">Has 4 halo neutrons</ref>
| rowspan=3 style="text-align:right" | 2
| rowspan=3 style="text-align:right" | 6
| rowspan=3|{{val|8.033934388|(95)}}
| rowspan=3|{{val|119.5|(1.5)|u=ms}}
| β<sup>−</sup> ({{val|83.1|(1.0)|u=%}})
| {{SimpleNuclide|Lithium|8}}
| rowspan=3|0+
| rowspan=3|
| rowspan=3|
|-
| β<sup>−</sup>n ({{val|16|(1)|u=%}})
| '''{{SimpleNuclide|Lithium|7}}'''
|-
| β<sup>−</sup>t<ref group="n">t: [[Physics:Tritium|Triton]] emission</ref> ({{val|0.9|(1)|u=%}})
| {{SimpleNuclide|Helium|5}}
|-
| {{SimpleNuclide|Helium|9}}
| style="text-align:right" | 2
| style="text-align:right" | 7
| {{val|9.043946|(50)}}
| {{val|2.5|(2.3)|u=zs}}
| n
| {{SimpleNuclide|Helium|8}}
| 1/2(+)
|
|
|-
| {{SimpleNuclide|Helium|10}}
| style="text-align:right" | 2
| style="text-align:right" | 8
| {{val|10.05281531|(10)}}
| {{val|260|(40)|u=ys}}<br/>[{{val|1.76|(27)|u=MeV}}]
| 2n
| {{SimpleNuclide|Helium|8}}
| 0+
|
|
{{Isotopes table/footer}}

== Helium-2 (diproton) ==
Helium-2, or {{SimpleNuclide|Helium|2}}, is an extremely unstable isotope of helium. Its nucleus, a '''diproton''', consists of two [[Physics:Proton|protons]] with no [[Physics:Neutron|neutrons]]. According to theoretical calculations, it would have been much more stable (although still undergoing [[Physics:Positron emission|β<sup>+</sup> decay]] to [[Physics:Deuterium|deuterium]]) if the [[Physics:Strong interaction|strong interaction]] had been 2% greater.<ref name="Bradford">{{cite journal|last1=Bradford|first1=R. A. W.|title=The effect of hypothetical diproton stability on the universe|journal=Journal of Astrophysics and Astronomy|date=27 August 2009|volume=30|issue=2|pages=119–131|url=http://rickbradford.co.uk/Diprotons.pdf|doi=10.1007/s12036-009-0005-x|bibcode=2009JApA...30..119B|citeseerx=10.1.1.495.4545|s2cid=122223720 }}</ref> Its instability is due to spin–spin interactions in the nuclear force and to the quantum mechanics described by the [[Physics:Pauli exclusion principle|Pauli exclusion principle]], which states that within a given quantum system two or more identical particles with the same half-integer spins (that is, fermions) cannot simultaneously occupy the same quantum state{{mdash}}all which presents for helium-2 that its two protons (of the diproton) have opposite-aligned spins and the diproton itself has a negative [[Physics:Nuclear binding energy|binding energy]].<ref>''Nuclear Physics in a Nutshell'', C. A. Bertulani, Princeton University Press, Princeton, N.J., 2007, Chapter 1, {{ISBN|978-0-691-12505-3}}.</ref>

There may have been observations of {{SimpleNuclide|Helium|2}}. In 2000, physicists first observed a new type of radioactive decay in which a nucleus [[Physics:Proton emission|emits two protons]] at once—perhaps a {{SimpleNuclide|Helium|2}} nucleus.<ref>[http://physicsworld.com/cws/article/news/2762 Physicists discover new kind of radioactivity] {{Webarchive|url=https://web.archive.org/web/20110423211717/http://physicsworld.com/cws/article/news/2762 |date=2011-04-23 }}, in [http://physicsworld.com/cws/home physicsworld.com] Oct 24, 2000.</ref><ref>{{cite journal |author=J. Gómez del Campo |author2=A. Galindo-Uribarri|display-authors=0 |date=2001 |title=Decay of a Resonance in <sup>18</sup>Ne by the Simultaneous Emission of Two Protons |journal= Physical Review Letters  |volume= 86 |issue= 2001 |pages= 43–46 |doi=10.1103/PhysRevLett.86.43|pmid=11136089|bibcode=2001PhRvL..86...43G }}</ref> The team led by Alfredo Galindo-Uribarri of the Oak Ridge National Laboratory announced that the discovery will help scientists understand the strong nuclear force and provide fresh insights into the [[Physics:Stellar nucleosynthesis|creation of elements inside stars]]. Galindo-Uribarri and co-workers chose an isotope of neon with an energy structure that prevents it from emitting protons one at a time. This means that the two protons are ejected simultaneously. The team fired a beam of fluorine ions at a proton-rich target to produce {{SimpleNuclide|Neon|18}}, which then decayed into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. There are two ways in which the two-proton emission may proceed. The neon nucleus might eject a "diproton"—a pair of protons bundled together as a {{SimpleNuclide|Helium|2}} nucleus—which then decays into separate protons. Alternatively, the protons may be emitted separately but simultaneously—so-called "democratic decay". The experiment was not sensitive enough to establish which of these two processes was taking place.

More evidence of {{SimpleNuclide|Helium|2}} was found in 2008 at the Istituto Nazionale di Fisica Nucleare, in Italy.<ref name="Schewe">{{cite journal |last=Schewe |first=Phil |date=2008-05-29 |title=New Form of Artificial Radioactivity |url=http://www.aip.org/pnu/2008/split/865-2.html |url-status=dead |journal=Physics News Update |number=865&nbsp;#2 |archive-url=https://web.archive.org/web/20081014143925/http://www.aip.org/pnu/2008/split/865-2.html |archive-date=2008-10-14 }}</ref><ref>{{cite journal |last1=Raciti |first1=G. |last2=Cardella |first2=G. |last3=De Napoli |first3=M. |last4=Rapisarda |first4=E. |last5=Amorini |first5=F. |last6=Sfienti |first6=C. |date=2008 |title=Experimental Evidence of {{SimpleNuclide|Helium|2}} Decay from {{SimpleNuclide|Neon|18}} Excited States |journal=[[Physics:Physical Review Letters|Phys. Rev. Lett.]] |volume=100 |issue= 19|pages=192503–192506 |doi=10.1103/PhysRevLett.100.192503 |pmid=18518446 |bibcode=2008PhRvL.100s2503R }}</ref> A beam of {{SimpleNuclide|Neon|20}} ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into {{SimpleNuclide|Neon|18}} nuclei. These nuclei then collided with a foil of lead. The second collision had the effect of exciting the {{SimpleNuclide|Neon|18}} nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the {{SimpleNuclide|Neon|18}} nucleus decayed into an {{SimpleNuclide|Oxygen|16}} nucleus, plus two protons detected exiting from the same direction. The new experiment showed that the two protons were initially ejected together, correlated in a quasibound [[Physics:Term symbol|<sup>1</sup>S configuration]], before decaying into separate protons much less than a nanosecond later.

Further evidence comes from [[Organization:Riken|RIKEN]] in Japan{{Citation needed|date=December 2011}} and the [[Organization:Joint Institute for Nuclear Research|Joint Institute for Nuclear Research]] in [[Organization:Dubna|Dubna]], Russia,{{Citation needed|date=December 2011}} where beams of {{SimpleNuclide|Helium|6}} nuclei were directed at a cryogenic hydrogen target to produce {{SimpleNuclide|Hydrogen|5}}. It was discovered that the {{SimpleNuclide|Helium|6}} nucleus can donate all four of its neutrons to the hydrogen.{{Citation needed|date=January 2012}} The two remaining protons could be simultaneously ejected from the target as a {{SimpleNuclide|Helium|2}} nucleus, which quickly decayed into two protons. A similar reaction has also been observed from {{SimpleNuclide|Helium|8}} nuclei colliding with hydrogen.<ref>{{cite journal |title=Experimental Evidence for the Existence of {{SimpleNuclide|Hydrogen|7}} and for a Specific Structure of {{SimpleNuclide|Helium|8}} |url=http://fy.chalmers.se/~f2bmz/papers/korsheninnikov_2003_7h.pdf |author=Korsheninnikov A. A. |date=2003-02-28 |journal=Physical Review Letters |volume=90 |issue=8 |pages=082501 |doi=10.1103/PhysRevLett.90.082501 |pmid=12633420 |bibcode=2003PhRvL..90h2501K |display-authors=0}}</ref>

Under the influence of electromagnetic interactions, the Jaffe-Low primitives 
<ref name=Jaffe1979>
{{cite journal
 |last1=Jaffe |first1=R. L.
 |last2=Low  |first2=F. E.
|year=1979
 |title=Connection between quark-model eigenstates and low-energy scattering
 |journal=Physical Review D
 |volume=19 |pages=2105-2118 
 |doi=10.1103/PhysRevD.19.2105
 |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.19.2105
 }}</ref>
may leave the unitary cut, creating narrow two-nucleon resonances, like a diproton resonance 
with a mass of 2000 MeV and a width of a few hundred keV.
<ref name=Krivoruchenko2011>
{{cite journal
 |last1=Krivoruchenko |first=M. I.
 |year=2011
 |title=Possibility of narrow resonances in nucleon-nucleon channels
 |journal=Physical Review C
 |volume=84 |pages=015206 
 |doi=10.1103/PhysRevC.84.015206
 |url=https://journals.aps.org/prc/abstract/10.1103/PhysRevC.84.015206
 }}</ref>
To search for this resonance, a beam of protons with kinetic energy T = 250 MeV 
and an energy spread below 100 keV is required, which is feasible considering electron cooling of the beam. 

{{SimpleNuclide|Helium|2}} is an intermediate in the first step of the [[Astronomy:Proton–proton chain|proton–proton chain]]reaction. The first step of the proton–proton chain reaction is a two-stage process; first, two protons fuse to form a diproton:
:[[Physics:Hydrogen atom|{{nuclide|hydrogen|1}}]] + {{nuclide|hydrogen|1}} + {{val|1.25|ul=MeV}}{{cn|date=January 2023}} → {{nuclide|helium|2}},
followed by the immediate [[Physics:Positron emission|beta-plus decay]] of the diproton to deuterium:
:{{nuclide|helium|2}} → [[Physics:Deuterium|{{nuclide|deuterium|2}}]] + {{math|{{SubatomicParticle|link=yes|Positron}} + {{SubatomicParticle|link=yes|Electron Neutrino}}}} + {{val|1.67|u=MeV}}{{cn|date=January 2023}},
with the overall formula
:{{nuclide|hydrogen|1}} + {{nuclide|hydrogen|1}} → {{nuclide|deuterium|2}} + {{math|{{SubatomicParticle|Positron}} + {{SubatomicParticle|Electron Neutrino}}}} + {{val|0.42|u=MeV}}{{cn|date=January 2023}}.
The hypothetical effect of the binding of the diproton on [[Astronomy:Big Bang nucleosynthesis|Big Bang]] and [[Physics:Stellar nucleosynthesis|stellar nucleosynthesis]] has been investigated.<ref name="Bradford"/> Some models suggest that variations in the strong force allowing the existence of a bound diproton would enable the conversion of all primordial hydrogen to helium in the Big Bang, with catastrophic consequences on the development of stars and life. This proposition is used as an example of the [[Astronomy:Anthropic principle|anthropic principle]]. However, a 2009 study suggests that such a conclusion cannot be drawn, as the formed diprotons would still decay to deuterium, whose binding energy would also increase. In some scenarios, it is postulated that hydrogen (in the form of deuterium) could still survive in relatively large quantities, rebutting arguments that the strong force is [[Astronomy:Fine-tuned universe|tuned]] within a precise anthropic limit.<ref name="weakanthropic">{{cite journal |last1=MacDonald |first1=J. |last2=Mullan |first2=D.J. |title=Big Bang Nucleosynthesis: The strong nuclear force meets the weak anthropic principle |date=2009 |arxiv=0904.1807 |journal=Physical Review D |volume=80 |issue=4 |pages=043507 |doi=10.1103/PhysRevD.80.043507|bibcode=2009PhRvD..80d3507M |s2cid=119203730 }}</ref>

== Helium-3 ==
{{main|Physics:Helium-3}}
{{SimpleNuclide|Helium|3}} is stable and is the only stable isotope other than {{SimpleNuclide|Hydrogen|1}} with more protons than neutrons. (There are many such unstable isotopes, the lightest being {{SimpleNuclide|Beryllium|7}} and {{SimpleNuclide|Boron|8}}.<!--please don't change this; the proton-rich Li isotopes are not bound and fall apart immediately!-->) There is only a trace amount ({{val|0.000002|(2)}})<ref name="Atomic Weight of Helium"/> of {{SimpleNuclide|Helium|3}} on Earth, primarily present since the formation of the Earth, although some falls to Earth trapped in cosmic dust.<ref name="heliumfundamentals">
{{cite web
 |title=Helium Fundamentals
 |url=http://www.mantleplumes.org/HeliumFundamentals.html
}}</ref> Trace amounts are also produced by the [[Physics:Beta decay|beta decay]] of [[Physics:Tritium|tritium]].<ref>
{{cite web
 |author=K. L. Barbalace
 |title=Periodic Table of Elements: Li—Lithium
 |url=http://environmentalchemistry.com/yogi/periodic/Li-pg2.html
 |work=EnvironmentalChemistry.com
 |access-date=2010-09-13
}}</ref> In [[Astronomy:Star|stars]], however, {{SimpleNuclide|Helium|3}} is more abundant, a product of [[Physics:Nuclear fusion|nuclear fusion]]. Extraplanetary material, such as [[Astronomy:Moon|lunar]] and [[Astronomy:Asteroid|asteroid]] regolith, has trace amounts of {{SimpleNuclide|Helium|3}} from [[Astronomy:Solar wind|solar wind]] bombardment.

For helium-3 to form a [[Physics:Superfluidity|superfluid]], it must be cooled to a temperature of {{val|0.0025|ul=K}}, or almost a thousand times lower than helium-4 ({{val|2.17|u=K}}). This difference is explained by [[Physics:Particle statistics#Quantum statistics|quantum statistics]], since helium-3 atoms are [[Physics:Fermion|fermions]], while helium-4 atoms are [[Physics:Boson|bosons]], which condense to a superfluid more easily.

== Helium-4 ==
{{main|Physics:Helium-4}}
The most common isotope, {{SimpleNuclide|Helium|4}}, is produced on Earth by [[Physics:Alpha decay|alpha decay]] of heavier radioactive elements; the [[Physics:Alpha particle|alpha particles]] that emerge are fully ionized {{SimpleNuclide|Helium|4}} nuclei. {{SimpleNuclide|Helium|4}} is an unusually stable nucleus because its [[Physics:Nucleon|nucleons]] are arranged into [[Physics:Nuclear shell model|complete shells]]. It was also formed in enormous quantities during [[Astronomy:Big Bang nucleosynthesis|Big Bang nucleosynthesis]].

Terrestrial helium consists almost exclusively ({{val|0.999998|(2)}})<ref name="Atomic Weight of Helium"/> of this isotope. Helium-4's boiling point of {{val|4.2|u=K}} is the second lowest of all known substances, second only to helium-3. When cooled further to {{val|2.17|u=K}}, it transforms to a unique [[Physics:Superfluidity|superfluid]] state of zero [[Physics:Viscosity|viscosity]]. It solidifies only at pressures above 25 atmospheres, where its melting point is {{val|0.95|u=K}}.

== Heavier helium isotopes ==
Although all heavier helium isotopes [[Physics:Radioactive decay|decay]] with a [[Physics:Half-life|half-life]] of less than one [[Second|second]], researchers have used [[Physics:Particle accelerator|particle accelerator]] collisions to create unusual [[Physics:Atomic nucleus|atomic nuclei]] for [[Chemistry:Chemical element|elements]] such as helium, [[Chemistry:Lithium|lithium]] and [[Chemistry:Nitrogen|nitrogen]]. The unusual nuclear structures of such isotopes may offer insights into the isolated properties of [[Physics:Neutron|neutrons]] and physics [[Physics:Physics beyond the Standard Model|beyond]] the Standard Model.<ref>{{Cite web |date=2008-01-25 |title=Helium-8 study gives insight into nuclear theory, neutron stars {{!}} Argonne National Laboratory |url=https://www.anl.gov/article/helium8-study-gives-insight-into-nuclear-theory-neutron-stars |access-date=2023-09-10 |website=www.anl.gov |language=en}}</ref><ref>{{Cite web |date=1999-11-29 |title=Radioactive beams drive physics forward |url=https://cerncourier.com/a/radioactive-beams-drive-physics-forward/ |access-date=2023-09-10 |website=CERN Courier |language=en-GB}}</ref>

The shortest-lived isotope is helium-10 with a [[Physics:Half-life|half-life]] of {{val|260|(40)|u=yoctoseconds}}. Helium-6 decays by emitting a [[Physics:Beta particle|beta particle]] and has a half-life of {{val|806.92|(24)|u=milliseconds}}. The most widely studied heavy helium isotope is helium-8. This isotope, as well as helium-6, is thought to consist of a normal helium-4 nucleus surrounded by a neutron "halo" (containing two neutrons in {{SimpleNuclide|Helium|6}} and four neutrons in {{SimpleNuclide|Helium|8}}). [[Physics:Halo nucleus|Halo nuclei]] have become an area of intense research. Isotopes up to helium-10, with two protons and eight neutrons, have been confirmed. {{SimpleNuclide|Helium|10}}, despite being a [[Physics:Magic number#Doubly magic|doubly magic]] isotope, has a very short half-life; it is not particle-bound and near-instantaneously [[Physics:Nuclear drip line|drips out two neutrons]].<ref name="Hampel1968">
{{cite book
 |author=Clifford A. Hampel
 |year=1968
 |title=The Encyclopedia of the Chemical Elements
 |url=https://archive.org/details/encyclopediaofch00hamp
 |url-access=registration
 |page=[https://archive.org/details/encyclopediaofch00hamp/page/260 260]
 |publisher=Reinhold Book Corporation
 |isbn=978-0278916432
}}</ref>

== References ==
{{reflist}}

== External links ==
*[https://web.archive.org/web/20191222181359/https://tunl.duke.edu/nucldata/General_Tables/General_Tables.shtml General Tables] &mdash; abstracts for helium and other exotic light nuclei

{{Navbox element isotopes}}

[[Category:Isotopes of helium| ]]
[[Category:Helium]]
[[Category:Lists of isotopes by element|Helium]]

{{Sourceattribution|Isotopes of helium}}