View source for Isotopes of helium

{{Short description|none}}
{{Infobox helium isotopes}}
[[Helium]] ({{sub|2}}He) ([[standard atomic weight]]: {{val|4.002602|(2)}}) has nine known [[isotope]]s, but only [[helium-3]] ({{sup|3}}He) and [[helium-4]] ({{sup|4}}He) are [[stable isotope|stable]].<ref>{{cite web |title=helium-3 {{!}} chemical isotope |language=en |website=britannica.com |url=https://www.britannica.com/science/helium-3 |access-date=2022-03-20}}</ref> All [[radioisotope]]s are short-lived; the longest-lived is {{sup|6}}He with [[half-life]] {{val|806.92|(24)|u=milliseconds}}. The least stable is {{sup|10}}He, with half-life {{val|260|(40)|u=[[yoctoseconds]]}} ({{val|2.6e-22|(4)|u=s}}), though {{sup|2}}He may have an even shorter half-life.

In Earth's atmosphere, the ratio of {{sup|3}}He to {{sup|4}}He is {{val|1.343e-6|(13)}}.<ref>{{cite journal |last1=Sano |first1=Yuji |last2=Wakita |first2=Hiroshi |last3=Sheng |first3=Xu |title=Atmospheric helium isotope ratio |date=1988 |journal=Geochemical Journal |volume=22 |issue=4 |pages=177–181 |bibcode=1988GeocJ..22..177S |doi=10.2343/geochemj.22.177 |doi-access=free |s2cid=129104204 |url=https://www.jstage.jst.go.jp/article/geochemj1966/22/4/22_4_177/_article}}</ref> However, the isotopic abundance of helium varies greatly depending on its origin. In the [[Local Interstellar Cloud]], the proportion of {{sup|3}}He to {{sup|4}}He 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. |title=Interstellar Helium Trapped with the COLLISA Experiment on the MiR Space Station—Improved Isotope Analysis by In Vacuo Etching |date=2006-03-01 |journal=The Astrophysical Journal |language=en |volume=639 |issue=1 |pages=246 |bibcode=2006ApJ...639..246B |issn=0004-637X |doi=10.1086/499223 |doi-access=free |s2cid=120648440}}</ref> which is ~121 times higher than in Earth's atmosphere. Rocks from Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in [[geology]] to investigate the origin of rocks and the composition of the Earth's [[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 {{sup|3}}He and {{sup|4}}He below {{val|0.8|u=K}} separate into two [[immiscible]] phases due to differences in [[quantum statistics]]: {{sup|4}}He atoms are [[boson]]s while {{sup|3}}He atoms are [[fermion]]s.<ref>{{cite book |title=The Encyclopedia of the Chemical Elements |page=264}}</ref> [[Dilution refrigerator]]s take advantage of the immiscibility of these two isotopes to achieve temperatures of a few milli[[kelvin]].

A mix of the two isotopes spontaneously separates into {{sup|3}}He-rich and {{sup|4}}He-rich regions.<ref>{{cite book |last=Pobell |first=Frank |title=Matter and methods at low temperatures |date=2007 |publisher=Springer |isbn=978-3-540-46356-6 |edition=3rd rev. and expanded |location=Berlin |oclc=122268227}}</ref> Phase separation also exists in [[ultracold atom|ultracold gas]] systems.<ref>{{cite journal |last1=Carlson |first1=J. |last2=Reddy |first2=Sanjay |title=Asymmetric Two-Component Fermion Systems in Strong Coupling |date=2005-08-02 |journal=Physical Review Letters |volume=95 |issue=6 |pages=060401 |arxiv=cond-mat/0503256 |bibcode=2005PhRvL..95f0401C |pmid=16090928 |doi=10.1103/PhysRevLett.95.060401 |s2cid=448402}}</ref> It has been shown experimentally in a two-component ultracold [[Fermi gas]] case.<ref>{{cite journal |last1=Shin |first1=Y. |last2=Zwierlein |first2=M. W. |last3=Schunck |first3=C. H. |last4=Schirotzek |first4=A. |last5=Ketterle |first5=W. |title=Observation of Phase Separation in a Strongly Interacting Imbalanced Fermi Gas |date=2006-07-18 |journal=Physical Review Letters |volume=97 |issue=3 |pages=030401 |arxiv=cond-mat/0606432 |bibcode=2006PhRvL..97c0401S |pmid=16907486 |doi=10.1103/PhysRevLett.97.030401 |s2cid=11323402}}</ref><ref>{{cite journal |last1=Zwierlein |first1=Martin W. |last2=Schirotzek |first2=André |last3=Schunck |first3=Christian H. |last4=Ketterle |first4=Wolfgang |title=Fermionic Superfluidity with Imbalanced Spin Populations |date=2006-01-27 |journal=Science |language=en |volume=311 |issue=5760 |pages=492–496 |arxiv=cond-mat/0511197 |bibcode=2006Sci...311..492Z |issn=0036-8075 |pmid=16373535 |doi=10.1126/science.1122318 |s2cid=13801977}}</ref> The phase separation can compete with other phenomena as [[quantum vortex|vortex lattice]] formation or an exotic [[Fulde–Ferrell–Larkin–Ovchinnikov phase]].<ref>{{cite journal |last1=Kopyciński |first1=Jakub |last2=Pudelko |first2=Wojciech R. |last3=Wlazłowski |first3=Gabriel |title=Vortex lattice in spin-imbalanced unitary Fermi gas |date=2021-11-23 |journal=Physical Review A |volume=104 |issue=5 |pages=053322 |arxiv=2109.00427 |bibcode=2021PhRvA.104e3322K |doi=10.1103/PhysRevA.104.053322 |s2cid=237372963}}</ref>

==List of isotopes==
{{Isotopes table
|symbol=He
|refs=NUBASE2020, AME2020 II
|notes=resonance, unc(), var[], daughter-st, spin#, spin(), n, p
}}
|-
| rowspan=2|{{sup|2}}He<ref group="n" name="PP">Intermediate in the [[proton–proton chain]]</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"/>
| [[Proton emission|p]] (> {{val|99.99|u=%}})
| '''{{sup|1}}H'''
| rowspan=2 | 0+#
| rowspan=2 |
| rowspan=2 |
|-
| [[Positron emission|β{{sup|+}}]] (< {{val|0.01|u=%}})
| '''[[Deuterium|{{sup|2}}H]]'''
|-
| [[Helium-3|{{sup|3}}He]]<ref group="n" name="PP"/><ref group="n" name="PN">Produced in [[Big Bang nucleosynthesis]]</ref><ref group="n">This and {{sup|1}}H are the only stable nuclei 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 |publisher=[[Commission on Isotopic Abundances and Atomic Weights]] |url=https://ciaaw.org/helium.htm |access-date=6 October 2021 |url-status=live |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 |title=Isotopic compositions of the elements 2013 (IUPAC Technical Report) |date=2016-03-01 |journal=Pure and Applied Chemistry |language=en |volume=88 |issue=3 |pages=293–306 |issn=1365-3075 |doi=10.1515/pac-2015-0503 |doi-access=free |hdl=11858/00-001M-0000-0029-C408-7 |hdl-access=free |s2cid=104472050}}</ref>
|-
| [[Helium-4|{{sup|4}}He]]<ref group="n" name="PN"/><ref group="n">Produced in [[alpha decay]]</ref>
| 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"/>
|-
| {{sup|5}}He
| style="text-align:right" | 2
| style="text-align:right" | 3
| {{val|5.012057|(21)}}
| {{val|6.02|(22)|e=-22|u=s}}<br/>[{{val|758|(28)|u=keV}}]
| [[neutron emission|n]]
| '''{{sup|4}}He'''
| 3/2−
|
|
|-
| rowspan=2|{{sup|6}}He<ref group="n">Has 2 [[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}}
| [[Beta decay|β{{sup|−}}]] ({{val|99.999722|(18)}}%)
| '''{{sup|6}}Li'''
| rowspan=2|0+
| rowspan=2|
| rowspan=2|
|-
| β{{sup|−}}d<ref group="n">d: Deuteron emission</ref> ({{val|0.000278|(18)}}%)
| '''{{sup|4}}He'''
|-id=Helium-7
| {{sup|7}}He
| style="text-align:right" | 2
| style="text-align:right" | 5
| {{val|7.027991|(8)}}
| {{val|2.51|(7)|e=-21|u=s}}<br/>[{{val|182|(5)|u=keV}}]
| n
| {{sup|6}}He
| (3/2)−
|
|
|-
| rowspan=3|{{sup|8}}He<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|−}} ({{val|83.1|(1.0)|u=%}})
| {{sup|8}}Li
| rowspan=3|0+
| rowspan=3|
| rowspan=3|
|-
| β{{sup|−}}n ({{val|16|(1)|u=%}})
| '''{{sup|7}}Li'''
|-
| β{{sup|−}}t<ref group="n">t: [[Tritium|Triton]] emission</ref> ({{val|0.9|(1)|u=%}})
| {{sup|5}}He
|-id=Helium-9
| {{sup|9}}He
| style="text-align:right" | 2
| style="text-align:right" | 7
| {{val|9.043946|(50)}}
| {{val|2.5|(2.3)|e=-21|u=s}}
| n
| {{SimpleNuclide|Helium|8}}
| 1/2(+)
|
|
|-
| {{sup|10}}He
| style="text-align:right" | 2
| style="text-align:right" | 8
| {{val|10.05281531|(10)}}
| {{val|2.60|(40)|e=-22|u=s}}<br/>[{{val|1.76|(27)|u=MeV}}]
| 2n
| {{sup|8}}He
| 0+
|
|
{{Isotopes table/footer}}

{{anchor|Helium-2}}

==Helium-2 (diproton)==
{{Redirect-distinguish|Helium 2|Helium II|Helium dimer}}
Helium-2, {{sup|2}}He, is extremely unstable. Its nucleus, a '''diproton''', consists of two [[proton]]s with no [[neutron]]s. According to theoretical calculations, it would be much more stable (but still [[positron emission|β{{sup|+}} decay]] to [[deuterium]]) if the [[strong force]] were 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 |bibcode=2009JApA...30..119B |citeseerx=10.1.1.495.4545 |doi=10.1007/s12036-009-0005-x |s2cid=122223720 |url=http://rickbradford.co.uk/Diprotons.pdf}}</ref> Its instability is due to [[Spin–spin interaction|spin–spin interactions]] in the [[nuclear force]] and the [[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; so {{sup|2}}He's two protons have opposite-aligned spins and the diproton itself has negative [[nuclear binding energy|binding energy]].<ref>''Nuclear Physics in a Nutshell'', C. A. Bertulani, Princeton University Press, Princeton, NJ, 2007, Chapter 1, {{ISBN|978-0-691-12505-3}}.</ref>

{{sup|2}}He may have been observed. In 2000, physicists first observed a new type of radioactive decay in which a nucleus [[proton emission|emits two protons]] at once—perhaps {{sup|2}}He.<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=etal |title=Decay of a Resonance in {{sup|date=2001|18}}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 Oak Ridge National Laboratory announced that the discovery will help understand the strong nuclear force and provide fresh insights into [[stellar nucleosynthesis]]. 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 the two protons are ejected simultaneously. The team fired a beam of [[fluorine]] ions at a proton-rich target to produce {{sup|18}}Ne, which then decayed into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. The two-proton emission may proceed in two ways: the neon might eject a diproton, which then decays into separate protons, or the protons may be emitted separately but simultaneously in a "democratic decay". The experiment was not sensitive enough to establish which of these two processes was taking place.

More evidence of {{sup|2}}He was found in 2008 at [[Istituto Nazionale di Fisica Nucleare]], in Italy.<ref name="Schewe">{{cite journal |last=Schewe |first=Phil |title=New Form of Artificial Radioactivity |date=2008-05-29 |journal=Physics News Update |number=865&nbsp;#2 |url=http://www.aip.org/pnu/2008/split/865-2.html |url-status=dead |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. |title=Experimental Evidence of {{sup|date=2008 |2}}He Decay from {{sup|18}}Ne Excited States |journal=[[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 {{sup|20}}Ne ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into {{sup|18}}Ne nuclei. These nuclei then collided with a foil of lead. The second collision excited the {{sup|18}}Ne nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the {{sup|18}}Ne nucleus decayed into an {{sup|16}}O 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 [[term symbol|{{sup|1}}S configuration]], before decaying into separate protons much less than a nanosecond later.

Further evidence comes from [[Riken]] in [[Japan]] and [[Joint Institute for Nuclear Research]] in [[Dubna]], [[Russia]], where beams of {{sup|6}}He nuclei were directed at a cryogenic [[hydrogen]] target to produce {{sup|5}}H. It was discovered that the {{sup|6}}He 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 diproton, which quickly decayed into two protons. A similar reaction has also been observed from {{sup|8}}He nuclei colliding with hydrogen.<ref>{{cite journal |title=Experimental Evidence for the Existence of {{sup|7}}H and for a Specific Structure of {{sup|8}}He |author=Korsheninnikov A. A. |display-authors=etal |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 |url=http://fy.chalmers.se/~f2bmz/papers/korsheninnikov_2003_7h.pdf}}</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 |issue=7
|pages=2105–2118
|bibcode=1979PhRvD..19.2105J
|doi=10.1103/PhysRevD.19.2105
|url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.19.2105
|url-access=subscription
}}</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 |first1=M. I.
|year=2011
|title=Possibility of narrow resonances in nucleon-nucleon channels
|journal=Physical Review C
|volume=84 |issue=1
|pages=015206
|arxiv=1102.2718
|bibcode=2011PhRvC..84a5206K
|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 250 MeV and an energy spread below 100 keV is required, which is feasible considering the electron cooling of the beam.

{{sup|2}}He is an intermediate in the first step of the [[proton–proton chain]]. The first step of the proton-proton chain is a two-stage process: first, two protons fuse to form a diproton:
:{{sup|1}}H + {{sup|1}}H + {{val|1.25|ul=MeV}} → {{sup|2}}He;
in a low-probability branch, the diproton [[beta-plus decay]]s into [[deuterium]]:
:{{sup|2}}He → {{sup|2}}H + [[positron|e{{sup|+}}]] + [[electron neutrino|ν{{sub|e}}]] + {{val|1.67|u=MeV}};
with the overall formula
:{{sup|1}}H + {{sup|1}}H → {{sup|2}}H + e{{sup|+}} + ν{{sub|e}} + {{val|0.42|u=MeV}}.
More than 99.99% of the time the diproton fissions back to two protons. The hypothetical effect of a bound diproton on [[Big Bang nucleosynthesis|Big Bang]] and [[stellar nucleosynthesis]], has been investigated.<ref name="Bradford"/> Some models suggest that variations in the strong force allowing a bound diproton would enable the conversion of all primordial hydrogen to helium in the Big Bang, which would be catastrophic for the development of stars and life. This notion is an example of the [[anthropic principle]]. However, a 2009 study suggests that such a conclusion can't be drawn, as the formed diproton would still decay to deuterium, whose binding energy would also increase. In some scenarios, it is postulated that hydrogen (in the form of {{sup|2}}H) could still survive in large amounts, rebutting arguments that the strong force is [[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 |bibcode=2009PhRvD..80d3507M |doi=10.1103/PhysRevD.80.043507 |s2cid=119203730}}</ref>

==Helium-3==
{{Main|Helium-3}}
{{sup|3}}He is the only stable isotope other than {{sup|1}}H with more protons than neutrons. (There are many such unstable isotopes; the lightest are {{sup|7}}Be and {{sup|8}}B.<!--please don't change this; the proton-rich Li isotopes are not bound and fall apart immediately!-->) There is only a trace (~2ppm)<ref name="Atomic Weight of Helium"/> of {{sup|3}}He on Earth, mainly 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 [[beta decay]] of [[tritium]].<ref>
{{cite web
|author=K. L. Barbalace
|title=Periodic Table of Elements: Li—Lithium
|work=EnvironmentalChemistry.com
|url=http://environmentalchemistry.com/yogi/periodic/Li-pg2.html
|access-date=2010-09-13
}}</ref> In [[star]]s, however, {{sup|3}}He is more abundant, a product of [[nuclear fusion]]. Extraplanetary material, such as [[Moon|lunar]] and [[asteroid]] [[regolith]], has traces of {{sup|3}}He from [[solar wind]] bombardment.

To become [[superfluid]], {{sup|3}}He must be cooled to 2.5 milli[[kelvin]], ~900 times lower than {{sup|4}}He ({{val|2.17|u=K}}). This difference is explained by [[quantum statistics]]: {{sup|3}}He atoms are [[fermion]]s, while {{sup|4}}He atoms are [[boson]]s, which condense to a superfluid more easily.

==Helium-4==
{{Main|Helium-4}}
The most common isotope, {{sup|4}}He, is produced on Earth by [[alpha decay]] of heavier elements; the [[alpha particle]]s that emerge are fully ionized {{sup|4}}He nuclei. {{sup|4}}He is an unusually stable nucleus because it is [[doubly magic]]. It was formed in enormous quantities in [[Big Bang nucleosynthesis]].

Terrestrial helium consists almost exclusively (all but ~2ppm)<ref name="Atomic Weight of Helium"/> of {{sup|4}}He. {{sup|4}}He's boiling point of {{val|4.2|u=K}} is the lowest of all known substances except {{sup|3}}He. When cooled further to {{val|2.17|u=K}}, it becomes a unique [[superfluid]] with zero [[viscosity]]. It solidifies only at pressures above 25 atmospheres, where it melts at {{val|0.95|u=K}}.

== Helium-5 ==
[[File:1987 CPA 5891.jpg|thumb|A 1987 Soviet stamp celebrating the [[T-15 (reactor)|T-15]] tokamak depicts the helium-5 nucleus during [[deuterium–tritium fusion]]|left]]
[[File:Cross Section for Fusion reactions.png|thumb|Fusion cross sections of major reactions. Without the resonance in helium-5, the DT reaction would be similar to the DD reaction.]]
Helium-5 is extremely unstable, decaying to helium-4 with a half-life of 602 yoctoseconds. It is briefly produced in the favorable fusion reaction:

<math>{}^2\mathrm{H} + {}^3\mathrm{H} \longrightarrow {}^5\mathrm{He}^{*} \longrightarrow {}^4\mathrm{He} + n + 17.6\ \mathrm{MeV} </math>

The reaction is greatly enhanced by the existence of a resonance. Helium-5, which has a natural spin state of -3/2 at the 0 MeV ground state, has a +3/2 excited spin state at 16.84 MeV. Because the reaction creates helium-5 nuclei with an energy level close to this state, it happens more frequently. This was discovered by [[Egon Bretscher]], who was investigating weaponization of fusion reactions for the [[Manhattan Project]]. 

The DT reaction specifically is 100 times more likely than the DD reaction at relevant energies, but would be similar without the resonance. The {{sup|2}}H-{{sup|3}}He reaction benefits from a similar resonance in [[lithium-5]], but is [[Coulomb barrier|Coulomb]]-suppressed i.e. the +2 helium nucleus charge increases the electrostatic repulsion for fusing nuclei.<ref name="a707">{{cite arXiv|eprint=2305.00647 |title=DT fusion through the {{sup|5}}He 3/2{{sup|+}} "Bretscher state" accounts for ≥25% of our existence via nucleosynthesis and for the possibility of fusion energy |last1=Chadwick |first1=Mark B. |last2=Paris |first2=Mark W. |last3=Haines |first3=Brian M. |date=2023 |class=physics.hist-ph }}</ref>

{{anchor|Helium-6|Helium-8}}

==Helium-6 and helium-8==
Helium-6 is the longest-lived radioactive isotope of helium; it [[beta decay]]s with a half-life of 806.92 milliseconds. The most widely studied heavy helium isotope is {{sup|8}}He,{{cn|date=April 2025}} which beta decays with a half-life of 119.5 milliseconds. {{sup|6}}He and {{sup|8}}He are thought to consist of a normal {{sup|4}}He nucleus surrounded by a neutron [[halo nuclei|"halo"]] (of two neutrons in {{sup|6}}He and four neutrons in {{sup|8}}He). The unusual structures of halo nuclei may offer insights into the isolated properties of [[neutron]]s and [[physics beyond the Standard Model]].<ref>{{cite web |title=Helium-8 study gives insight into nuclear theory, neutron stars |date=2008-01-25 |language=en |publisher=Argonne National Laboratory |website=anl.gov |url=https://www.anl.gov/article/helium8-study-gives-insight-into-nuclear-theory-neutron-stars |access-date=2023-09-10}}</ref><ref>{{cite web |title=Radioactive beams drive physics forward |date=1999-11-29 |language=en-GB |website=CERN Courier |url=https://cerncourier.com/a/radioactive-beams-drive-physics-forward/ |access-date=2023-09-10}}</ref>

==Helium-10==
The shortest-lived and heaviest known helium isotope is {{sup|10}}He. Despite being a [[doubly magic]] isotope, {{sup|10}}He is not particle-bound and near-instantly [[nuclear drip line|drips out two neutrons]] (half-life ~260 yoctoseconds).<ref name="Hampel1968">{{cite book |author=Clifford A. Hampel |year=1968 |title=The Encyclopedia of the Chemical Elements |page=[https://archive.org/details/encyclopediaofch00hamp/page/260 260] |publisher=Reinhold Book Corporation |isbn=978-0278916432 |url=https://archive.org/details/encyclopediaofch00hamp |url-access=registration}}</ref>

==See also==
'''Daughter products other than helium'''
* [[Isotopes of lithium]]
* [[Isotopes of hydrogen]]

==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]]