View source for Helium-3

{{short description|Helium isotope with two protons and one neutron}}
{{about|the isotope| the record label|Helium 3 (record label)}}
{{Infobox isotope
| alternate_names =tralphium (obsolete)
| symbol =He
| mass_number =3
| mass =3.0160293
| num_neutrons =1
| num_protons =2
| abundance =0.000137% (% He on Earth)<br/>0.01% (% He in Solar System)
| halflife =stable
| error_halflife =
| image =Angeregtes Helium 3.jpg
| parent =Tritium
| parent_symbol =H
| parent_mass =3
| parent_decay =[[beta decay]] of tritium
| spin ={{frac|1|2}}
}}
'''Helium-3''' ('''<sup>3</sup>He'''<ref name=csaga>{{cite arXiv |last=Galli |first=D. |title=The cosmic saga of <sup>3</sup>He |date=September 2004 |eprint=astro-ph/0412380v1}}</ref><ref name="ley196610">{{Cite magazine|last=Ley |first=Willy |date=October 1966 |title=The Delayed Discovery |department=For Your Information |url=https://archive.org/stream/Galaxy_v25n01_1966-10#page/n115/mode/2up |magazine=Galaxy Science Fiction |pages=116–127 }}</ref> see also [[helion (chemistry)|helion]]) is a light, [[stable isotope|stable]] [[isotope]] of [[helium]] with two [[proton]]s and one [[neutron]]. (In contrast, the most common isotope, [[helium-4]], has two protons and two neutrons.) Helium-3 and [[Isotopes of hydrogen#Hydrogen-1 (Protium)|hydrogen-1]] are the only stable [[nuclide]]s with more protons than neutrons. It was discovered in 1939. Helium-3 atoms are [[fermionic]] and become a [[superfluid]] at the temperature of 2.491&nbsp;mK.

Helium-3 occurs as a [[primordial nuclide]], escaping from [[Earth's crust]] into its [[atmosphere]] and into [[outer space]] over millions of years. It is also thought to be a natural [[nucleogenic]] and [[cosmogenic nuclide]], one produced when [[lithium]] is bombarded by natural neutrons, which can be released by [[spontaneous fission]] and by [[nuclear reaction]]s with [[cosmic ray]]s. Some found in the terrestrial atmosphere is a remnant of atmospheric and underwater [[nuclear weapons testing]].

[[Nuclear fusion]] using helium-3 has long been viewed as a desirable future [[energy source]]. The fusion of two of its [[atom]]s would be [[Aneutronic fusion|aneutronic]], that is, it would not release the dangerous radiation of traditional fusion or require the much higher temperatures thereof.<ref>{{cite web |url=https://blogs.scientificamerican.com/news-blog/is-moons-sci-fi-vision-of-lunar-hel-2009-06-12/ |title=Is MOON's Sci-Fi Vision of Lunar Helium 3 Mining Based in Reality? |last=Matson |first=John |date=12 Jun 2009 |website=Scientific American – News Blog |access-date=29 Aug 2017 |archive-date=30 August 2017 |archive-url=https://web.archive.org/web/20170830062841/https://blogs.scientificamerican.com/news-blog/is-moons-sci-fi-vision-of-lunar-hel-2009-06-12/ |url-status=live }}</ref> The process may unavoidably create other reactions that themselves would cause the surrounding material to become radioactive.<ref>{{cite web |url=https://cds.cern.ch/record/1055767/files/CM-PRS00002036.pdf |title=Fears Over Factoids |last=Close |first=Frank |date=August 2007 |website=CERN Document Server |publisher=Physicsworld.com |access-date=8 July 2018 |archive-date=22 October 2017 |archive-url=https://web.archive.org/web/20171022191811/http://cds.cern.ch/record/1055767/files/CM-PRS00002036.pdf |url-status=live }}</ref>

Helium-3 is thought to be more abundant on the Moon than on Earth, having been deposited in the upper layer of [[regolith]] by the [[solar wind]] over billions of years,<ref>{{cite news |title=Global inventory of Helium-3 in lunar regoliths estimated by a multi-channel microwave radiometer on the Chang-E 1 lunar satellite |author1=Fa WenZhe |author2=Jin YaQiu |url=http://lunarnetworks.blogspot.in/2010/12/change-1-maps-moons-helium-3-inventory.html |date=December 2010 |access-date=2012-12-12 |archive-date=2017-10-11 |archive-url=https://web.archive.org/web/20171011021507/http://lunarnetworks.blogspot.in/2010/12/change-1-maps-moons-helium-3-inventory.html |url-status=live }}</ref> though still lower in abundance than in the Solar System's [[gas giant]]s.<ref>{{cite conference|first=E. N.|last=Slyuta|author2=Abdrakhimov, A. M.|author3=Galimov, E. M.|date=March 12–16, 2007|title=The Estimation of Helium-3 Probable Reserves in Lunar Regolith|conference=38th Lunar and Planetary Science Conference|pages=2175|url=http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf|access-date=2007-05-31|archive-date=2008-07-05|archive-url=https://web.archive.org/web/20080705122316/http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf|url-status=live}}</ref><ref name="F. H. Cocks 2010">{{cite journal|author=Cocks, F. H.|date=2010|title=<sup>3</sup>He in permanently shadowed lunar polar surfaces|journal= Icarus |volume=206 |issue=2 |pages=778–779 |doi=10.1016/j.icarus.2009.12.032 |bibcode=2010Icar..206..778C}}</ref>

== History ==
The existence of helium-3 was first proposed in 1934 by the Australian [[Nuclear physics|nuclear physicist]] [[Mark Oliphant]] while he was working at the [[University of Cambridge]] [[Cavendish Laboratory]]. Oliphant had performed experiments in which fast [[deuteron]]s collided with deuteron targets (incidentally, the first demonstration of [[nuclear fusion]]).<ref>{{cite journal|title=Transmutation Effects Observed with Heavy Hydrogen|first=M. L. E.|last=Oliphant|author2=Harteck, P. |author3=Rutherford, E. |journal=[[Proceedings of the Royal Society A]] | volume=144|issue=853|date=1934|pages=692–703|jstor=2935553| doi = 10.1098/rspa.1934.0077|bibcode=1934RSPSA.144..692O|doi-access=free}}</ref> Isolation of helium-3 was first accomplished by [[Luis Walter Alvarez|Luis Alvarez]] and [[Robert Cornog]] in 1939.<ref>{{cite journal|doi=10.1103/PhysRev.56.613|title=Helium and Hydrogen of Mass 3|date=1939|last1=Alvarez|first1=Luis|last2=Cornog|first2=Robert|journal=Physical Review|volume=56|issue=6|pages=613|bibcode = 1939PhRv...56..613A }}</ref><ref>{{cite book|url = https://archive.org/details/discoveringalvar0000alva|url-access = registration|pages =[https://archive.org/details/discoveringalvar0000alva/page/26 26]–30|title = Discovering Alvarez: selected works of Luis W. Alvarez, with commentary by his students and colleagues|publisher = University of Chicago Press|isbn = 978-0-226-81304-2|author1 = Alvarez, Luis W|author2 = Peter Trower, W|date = 1987}}</ref> Helium-3 was thought to be a [[radioactive isotope]] until it was also found in samples of natural helium, which is mostly [[helium-4]], taken both from the terrestrial atmosphere and from [[natural gas]] wells.<ref>{{cite web|date=1981|publisher=Newsmagazine Publication|url=http://www.lbl.gov/Science-Articles/Research-Review/Magazine/1981/81fepi1.html|title=Lawrence and His Laboratory: Episode: A Productive Error|access-date=2009-09-01|archive-date=2017-05-10|archive-url=https://web.archive.org/web/20170510110547/http://www2.lbl.gov/Science-Articles/Research-Review/Magazine/1981/81fepi1.html|url-status=dead}}</ref>

== Physical properties ==
Due to its low atomic mass of 3.016&nbsp;[[dalton (unit)|u]], helium-3 has some [[Physical property|physical properties]] different from those of helium-4, with a mass of 4.0026&nbsp;u. On account of the weak, induced [[dipole–dipole interaction]] between the helium atoms, their microscopic physical properties are mainly determined by their [[zero-point energy]]. Also, the microscopic properties of helium-3 cause it to have a higher zero-point energy than helium-4. This implies that helium-3 can overcome dipole–dipole interactions with less [[thermal energy]] than helium-4 can.

The [[quantum mechanical]] effects on helium-3 and helium-4 are significantly different because with two [[proton]]s, two [[neutron]]s, and two [[electron]]s, helium-4 has an overall [[Spin (physics)|spin]] of zero, making it a [[boson]], but with one fewer neutron, helium-3 has an overall spin of one half, making it a [[fermion]].

Pure helium-3 gas boils at 3.19 [[kelvin|K]] compared with helium-4 at 4.23 K, and its [[critical point (thermodynamics)|critical point]] is also lower at 3.35 K, compared with helium-4 at 5.2 K. Helium-3 has less than half the density of helium-4 when it is at its boiling point: 59 g/L compared to 125 g/L of helium-4 at a pressure of one atmosphere. Its latent heat of vaporization is also considerably lower at 0.026 [[kilojoule per mole|kJ/mol]] compared with the 0.0829 kJ/mol of helium-4.<ref>[http://www.trgn.com/database/cryogen.html Teragon's Summary of Cryogen Properties] {{Webarchive|url=https://web.archive.org/web/20170809013754/http://www.trgn.com/database/cryogen.html |date=2017-08-09 }} Teragon Research, 2005</ref><ref>{{cite journal|last=Chase|first=C. E.|author2=Zimmerman, G. O.|date=1973|title=Measurements of P-V-T and Critical Indices of He<sup>3</sup> |journal=[[Journal of Low Temperature Physics]]|volume=11|issue=5–6|pages=551|bibcode=1973JLTP...11..551C|doi=10.1007/BF00654447|s2cid=123038029}}</ref>

=== Superfluidity ===
[[File:Phase diagram of helium-3 (1975) 0.002 K region-en.svg|thumb|Phase diagram for Helium-3. "Bcc" indicates a [[body-centered cubic]] crystal lattice.]]
An important property of helium-3 atoms, which distinguishes them from the more common helium-4, is that they contain an odd number of spin {{frac|1|2}} particles, and are [[Fermion#Composite fermions|composite fermions]]. This is a direct result of the [[Angular momentum quantum number#Addition of quantized angular momenta|addition rules]] for quantized angular momentum. In contrast, helium-4 atoms are [[boson]]s, containing an even number of spin {{frac|1|2}} particles. At low temperatures (about 2.17 K), helium-4 undergoes a [[phase transition]]: A fraction of it enters a [[superfluid]] [[phase (matter)|phase]] that can be roughly understood as a type of [[Bose–Einstein condensate]]. Such a mechanism is not available for fermionic helium-3 atoms. Many speculated that helium-3 could also become a superfluid at much lower temperatures, if the atoms formed into ''pairs'' analogous to [[Cooper pair]]s in the [[BCS theory]] of [[superconductivity]]. Each Cooper pair, having integer spin, can be thought of as a boson. During the 1970s, [[David Lee (physicist)|David Lee]], [[Douglas Osheroff]] and [[Robert Coleman Richardson]] discovered two phase transitions along the melting curve, which were soon realized to be the two superfluid phases of helium-3.<ref>{{cite journal|last=Osheroff|first=D. D. |author2=Richardson, R. C. |author3=Lee, D. M. |date=1972|title=Evidence for a New Phase of Solid He<sup>3</sup> |journal=[[Physical Review Letters]]|volume=28|issue=14|pages=885–888|doi=10.1103/PhysRevLett.28.885|bibcode= 1972PhRvL..28..885O|doi-access=free}}</ref><ref>{{cite journal|last=Osheroff|first=D. D.|author2=Gully, W. J. |author3=Richardson, R. C. |author4= Lee, D. M. |date=1972|title=New Magnetic Phenomena in Liquid He<sup>3</sup> below 3 mK|journal=Physical Review Letters |volume=29|issue=14|pages=920–923|doi=10.1103/PhysRevLett.29.920|bibcode=1972PhRvL..29..920O}}</ref> The transition to a superfluid occurs at 2.491 millikelvins on the melting curve. They were awarded the 1996 [[Nobel Prize in Physics]] for their discovery. [[Alexei Alexeyevich Abrikosov|Alexei Abrikosov]], [[Vitaly Lazarevich Ginzburg|Vitaly Ginzburg]], and [[Anthony James Leggett|Tony Leggett]] won the 2003 Nobel Prize in Physics for their work on refining understanding of the superfluid phase of helium-3.<ref>{{cite journal|last=Leggett|first=A. J.|date=1972 |title=Interpretation of Recent Results on He<sup>3</sup> below 3 mK: A New Liquid Phase?|journal=Physical Review Letters |volume=29|issue=18|pages=1227–1230|doi=10.1103/PhysRevLett.29.1227|bibcode=1972PhRvL..29.1227L}}</ref>

In a zero magnetic field, there are two distinct superfluid phases of <sup>3</sup>He, the A-phase and the B-phase. The B-phase is the low-temperature, low-pressure phase which has an isotropic energy gap. The A-phase is the higher temperature, higher pressure phase that is further stabilized by a magnetic field and has two point nodes in its gap. The presence of two phases is a clear indication that <sup>3</sup>He is an unconventional superfluid (superconductor), since the presence of two phases requires an additional symmetry, other than gauge symmetry, to be broken. In fact, it is a ''p''-wave superfluid, with spin one, '''S'''=1, and angular momentum one, '''L'''=1. The ground state corresponds to total angular momentum zero, '''J'''='''S'''+'''L'''=0 (vector addition). Excited states are possible with non-zero total angular momentum, '''J'''>0, which are excited pair collective modes. These collective modes have been studied with much greater precision than in any other unconventional pairing system, because of the extreme purity of superfluid <sup>3</sup>He. This purity is due to all <sup>4</sup>He phase separating entirely and all other materials solidifying and sinking to the bottom of the liquid, making the A- and B-phases of <sup>3</sup>He the most pure condensed matter state possible.

== Natural abundance ==

=== Terrestrial abundance ===
{{Main|Isotope geochemistry}}

<sup>3</sup>He is a primordial substance in the Earth's [[mantle (geology)|mantle]], thought to have become entrapped in the Earth during planetary formation. The ratio of <sup>3</sup>He to <sup>4</sup>He within the Earth's crust and mantle is less than that of estimates of solar disk composition as obtained from meteorite and lunar samples, with terrestrial materials generally containing lower <sup>3</sup>He/<sup>4</sup>He ratios due to production of <sup>4</sup>He from radioactive decay.

<sup>3</sup>He has a cosmological ratio of 300 atoms per million atoms of <sup>4</sup>He (at. ppm),<ref name=Witt>[[#Witt|Wittenberg 1994]]</ref> leading to the assumption that the original ratio of these primordial gases in the mantle was around 200-300 ppm when Earth was formed. Over Earth's history alpha-particle decay of uranium, thorium and other radioactive isotopes has generated significant amounts of <sup>4</sup>He, such that only around 7% of the helium now in the mantle is primordial helium,<ref name=Witt /> lowering the total <sup>3</sup>He/<sup>4</sup>He ratio to around 20 ppm. Ratios of <sup>3</sup>He/<sup>4</sup>He in excess of atmospheric are indicative of a contribution of <sup>3</sup>He from the mantle. Crustal sources are dominated by the [[helium-4|<sup>4</sup>He]] produced by radioactive decay.

The ratio of helium-3 to helium-4 in natural Earth-bound sources varies greatly.<ref name=Aldrich>Aldrich, L.T.; Nier, Alfred O. Phys. Rev. 74, 1590 – 1594 (1948). The Occurrence of He3 in Natural Sources of Helium. Page 1592, Tables I and II.</ref><ref name=Holden>Holden, Normen E. 1993. Helium Isotopic Abundance Variation in Nature. [http://www.osti.gov/bridge/servlets/purl/10183304-ds0WIi/10183304.PDF copy of paper BNL-49331] "Table II. <sup>3</sup>He Abundance of Natural Gas ... <sup>3</sup>He in ppm ... Aldrich 0.05 – 0.5 ... Sano 0.46 – 22.7", "Table V. ... of Water ... <sup>3</sup>He in ppm ... 1.6 – 1.8 East Pacific ... 0.006 – 1.5 Manitoba Chalk River ... 164 Japan Sea" (Aldrich measured Helium from US wells, Sano that of Taiwan gas: {{Cite journal| doi = 10.1038/323055a0| issn = 1476-4687| volume = 323| issue = 6083| pages = 55–57| last1 = Sano| first1 = Yuji| last2 = Wakita| first2 = Hiroshi| last3 = Huang| first3 = Chin-Wang| title = Helium flux in a continental land area estimated from <sup>3</sup>He/<sup>4</sup>He ratio in northern Taiwan| journal = Nature| date = September 1986| bibcode = 1986Natur.323...55S| s2cid = 4358031}})</ref> Samples of the [[lithium]] ore [[spodumene]] from Edison Mine, South Dakota were found to contain 12 parts of helium-3 to a million parts of helium-4. Samples from other mines showed 2 parts per million.<ref name=Aldrich/>

Helium is also present as up to 7% of some natural gas sources,<ref>[http://www.webelements.com/webelements/elements/text/He/key.html WebElements Periodic Table: Professional Edition: Helium: key information] {{Webarchive|url=https://web.archive.org/web/20080509100431/http://www.webelements.com/webelements/elements/text/He/key.html |date=2008-05-09 }}. Webelements.com. Retrieved on 2011-11-08.</ref> and large sources have over 0.5% (above 0.2% makes it viable to extract).<ref name=SmithDM>[[#Smith|Smith, D.M.]] "any concentration of helium above approximately 0.2 percent is considered worthwhile examining" ... "U.S. government still owns approximately 1 billion nm<sup>3</sup> of helium inventory", "Middle East and North Africa ... many very large, helium-rich (up to 0.5 percent) natural gas fields" (Smith uses nm<sup>3</sup> to mean "normal [[cubic metre]]", elsewhere called "cubic metre at [[normal temperature and pressure|NTP]])</ref> The fraction of <sup>3</sup>He in helium separated from natural gas in the U.S. was found to range from 70 to 242 parts per billion.<ref name=CRS/><ref name="BoM/DoI">{{cite report| first1=Thomas A. | last1=Davidson | first2=David E. | last2=Emerson| publisher=[[United States Bureau of Mines|Bureau of Mines]], [[US Department of the Interior]] | title= Method and Apparatus for Direct Determination of Helium-3 in Natural Gas and Helium | id=Report of Investigations 9302 | date=1990}}</ref> Hence the US 2002 stockpile of 1 billion normal m<sup>3</sup><ref name=SmithDM/> would have contained about {{convert|12 to 43|kg}} of helium-3.  According to American physicist [[Richard Garwin]], about {{convert|26|m3}} or almost {{convert|5|kg}} of <sup>3</sup>He is available annually for separation from the US natural gas stream.  If the process of separating out the <sup>3</sup>He could employ as feedstock the liquefied helium typically used to transport and store bulk quantities, estimates for the incremental energy cost range from {{Convert|34 to 300|$/l}} NTP, excluding the cost of infrastructure and equipment.<ref name=CRS/> Algeria's annual gas production is assumed to contain 100 million normal cubic metres<ref name=SmithDM/> and this would contain between {{convert|7 and 24| m3}} of helium-3 (about {{convert|1 to 4|kg}}) assuming a similar <sup>3</sup>He fraction.

<sup>3</sup>He is also present in the [[Earth's atmosphere]]. The natural abundance of <sup>3</sup>He in naturally occurring helium gas is 1.38{{e|-6}} (1.38 parts per million). The partial pressure of helium in the Earth's atmosphere is about {{convert|0.52|Pa}}, and thus helium accounts for 5.2 parts per million of the total pressure (101325 Pa) in the Earth's atmosphere, and <sup>3</sup>He thus accounts for 7.2 parts per trillion of the atmosphere. Since the atmosphere of the Earth has a mass of about {{convert|5.14e18|kg}},<ref>{{Cite journal|doi = 10.1175/JCLI-3299.1|title = The Mass of the Atmosphere: A Constraint on Global Analyses|year = 2005|last1 = Smith|first1 = Lesley|last2 = Trenberth|first2 = Kevin E.|journal = Journal of Climate|volume = 18|issue = 6|pages = 864–875|bibcode = 2005JCli...18..864T| s2cid=16754900 |doi-access = free}}</ref> the mass of <sup>3</sup>He in the Earth's atmosphere is the product of these numbers, or about {{convert|37,000|t}} of <sup>3</sup>He. (In fact the effective figure is ten times smaller, since the above ppm are ppmv and not ppmw. One must multiply by 3 (the molecular mass of helium-3) and divide by 29 (the mean molecular mass of the atmosphere), resulting in {{convert|3,828|t}} of helium-3 in the earth's atmosphere.)

<sup>3</sup>He is produced on Earth from three sources: lithium [[spallation]], [[cosmic rays]], and beta decay of tritium (<sup>3</sup>H). The contribution from cosmic rays is negligible within all except the oldest regolith materials, and lithium spallation reactions are a lesser contributor than the production of <sup>4</sup>He by [[alpha particle]] emissions.

The total amount of helium-3 in the mantle may be in the range of {{convert|0.1–1|Mt}}. Most mantle is not directly accessible. Some helium-3 leaks up through deep-sourced [[Hotspot (geology)|hotspot]] volcanoes such as those of the [[Hawaiian Islands]], but only {{convert|300|g}} per year is emitted to the atmosphere. [[Mid-ocean ridge]]s emit another {{convert|3|kg/year|g/day}}. Around [[subduction|subduction zone]]s, various sources produce helium-3 in [[natural gas]] deposits which possibly contain a thousand tonnes of helium-3 (although there may be 25 thousand tonnes if all ancient subduction zones have such deposits). Wittenberg estimated that United States crustal natural gas sources may have only half a tonne total.<ref>[[#Witt|Wittenberg 1994]] p. 3, Table 1; p. 9.</ref> Wittenberg cited Anderson's estimate of another {{convert|1200|t}} in [[interplanetary dust]] particles on the ocean floors.<ref>[[#Witt|Wittenberg 1994]] Page A-1 citing Anderson 1993, "1200 metric tonne"</ref> In the 1994 study, extracting helium-3 from these sources consumes more energy than fusion would release.<ref>[[#Witt|Wittenberg 1994]] Page A-4 "1 kg (<sup>3</sup>He), pumping power would be 1.13{{e|6}} MWyr ... fusion power derived ... 19 MWyr"</ref>

=== Lunar surface ===
See [[#Extraterrestrial mining|Extraterrestrial mining]] or [[Lunar resources#Helium-3|Lunar resources]]

=== Solar nebula (primordial) abundance ===
One early estimate of the primordial ratio of <sup>3</sup>He to <sup>4</sup>He in the solar nebula has been the measurement of their ratio in the atmosphere of Jupiter, measured by the mass spectrometer of the Galileo atmospheric entry probe. This ratio is about 1:10,000,<ref>{{Cite journal | bibcode = 1996Sci...272..846N | title = The Galileo Probe Mass Spectrometer: Composition of Jupiter's Atmosphere | last1 = Niemann | first1 = Hasso B. | last2 = Atreya | first2 = Sushil K. | last3 = Carignan | first3 = George R. | last4 = Donahue | first4 = Thomas M. | last5 = Haberman | first5 = John A. | last6 = Harpold | first6 = Dan N. | last7 = Hartle | first7 = Richard E. | last8 = Hunten | first8 = Donald M. | last9 = Kasprzak | first9 = Wayne T. | display-authors = 8| volume = 272 | date = 1996 | pages = 846–9 | journal = Science | doi = 10.1126/science.272.5263.846 | pmid = 8629016 | issue = 5263| s2cid = 3242002 }}</ref> or 100 parts of <sup>3</sup>He per million parts of <sup>4</sup>He. This is roughly the same ratio of the isotopes as in [[lunar regolith]], which contains 28&nbsp;ppm helium-4 and 2.8&nbsp;ppb helium-3 (which is at the lower end of actual sample measurements, which vary from about 1.4 to 15&nbsp;ppb). Terrestrial ratios of the isotopes are lower by a factor of 100, mainly due to enrichment of helium-4 stocks in the mantle by billions of years of [[alpha decay]] from [[uranium]], [[thorium]] as well as their [[decay product]]s and [[extinct radionuclide]]s.

== Human production ==
=== Tritium decay ===
{{See also|Tritium}}
Virtually all helium-3 used in industry today is produced from the radioactive decay of [[tritium]], given its very low natural abundance and its very high cost.

Production, sales and distribution of helium-3 in the United States are managed by the [[US Department of Energy]] (DOE) [[DOE Isotope Program]].<ref>{{cite web |title=Isotope Development & Production for Research and Applications (IDPRA) |url=http://science.energy.gov/np/research/idpra/ |website=US Department of Energy Office of Science |date=18 October 2018 |access-date=11 January 2019 |archive-date=19 October 2011 |archive-url=https://web.archive.org/web/20111019074945/http://science.energy.gov/np/research/idpra/ |url-status=live }}</ref>

While tritium has several different experimentally determined values of its [[half-life]], [[National Institute of Standards and Technology|NIST]] lists {{val|4500|8|u=days|fmt=commas}} ({{val|12.32|0.02|u=years}}).<ref>
{{Cite journal
 |author          = Lucas, L. L.
 |author2         = Unterweger, M. P.
 |name-list-style = amp
 |date            = 2000
 |title           = Comprehensive Review and Critical Evaluation of the Half-Life of Tritium
 |doi             = 10.6028/jres.105.043
 |pmid = 27551621
 |journal         = [[Journal of Research of the National Institute of Standards and Technology]]
 |volume          = 105
 |issue           = 4
 |pages = 541–549
 |pmc= 4877155
 }}</ref> It decays into helium-3 by [[beta decay]] as in this nuclear equation:

:{| border="0"
|- style="height:2em;"
|{{nuclide|Hydrogen|3|}}&nbsp;||→&nbsp;||{{nuclide|helium|3|charge=1+}}&nbsp;||+&nbsp;||{{math|{{SubatomicParticle|link=yes|Electron}}}}&nbsp;||+&nbsp;||{{math|{{SubatomicParticle|link=yes|Electron Antineutrino}}}}
|}
<!-- simpler
:<chem>^3_1H \to ^3_2{He^+} + {e^-} + \bar{\nu}_e + 18.6 keV</chem>
-->
Among the total released energy of {{val|18.6|u=keV}}, the part taken by [[electron]]'s kinetic energy varies, with an average of {{val|5.7|u=keV}}, while the remaining energy is carried off by the nearly undetectable [[electron antineutrino]]. 
[[Beta particles]] from tritium can penetrate only about {{convert|6.0|mm}} of air, and they are incapable of passing through the dead outermost layer of human skin.<ref>[https://web.archive.org/web/20130520184942/http://www.ehso.emory.edu/content-forms/3anuclidedatasafetysheets.pdf Nuclide safety data sheet: Hydrogen-3]. ehso.emory.edu</ref> The unusually low energy released in the tritium beta decay makes the decay (along with that of [[Isotopes of rhenium|rhenium-187]]) appropriate for absolute neutrino mass measurements in the laboratory (the most recent experiment being [[KATRIN]]).

The low energy of tritium's radiation makes it difficult to detect tritium-labeled compounds except by using [[liquid scintillation counting]].

Tritium is a radioactive isotope of hydrogen and is typically produced by bombarding lithium-6 with neutrons in a nuclear reactor. The lithium nucleus absorbs a neutron and splits into helium-4 and tritium. Tritium decays into helium-3 with a half-life of {{val|12.3|u=years}}, so helium-3 can be produced by simply storing the tritium until it undergoes radioactive decay. As tritium forms a stable compound with oxygen ([[tritiated water]]) while helium-3 does not, the storage and collection process could [[continuous process|continuously]] collect the material that [[outgas]]ses from the stored material.

Tritium is a critical component of [[nuclear weapons]] and historically it was produced and stockpiled primarily for this application. The decay of tritium into helium-3 reduces the explosive power of the fusion warhead, so periodically the accumulated helium-3 must be removed from warhead reservoirs and tritium in storage. Helium-3 removed during this process is marketed for other applications.

For decades this has been, and remains, the principal source of the world's helium-3.<ref>{{Cite web |url=http://www.srs.gov/general/news/factsheets/tritium_esrs.pdf |title=Savannah River Tritium Enterprise: Fact Sheet |access-date=2016-03-01 |archive-date=2016-12-22 |archive-url=https://web.archive.org/web/20161222012847/http://www.srs.gov/general/news/factsheets/tritium_esrs.pdf |url-status=live }}</ref> Since the signing of the [[START I]] Treaty in 1991 the number of nuclear warheads that are kept ready for use has decreased.<ref>Charmian Schaller [https://web.archive.org/web/20061029124748/http://afci.lanl.gov/aptnews/aptnews.mar1_98.html Accelerator Production of Tritium – That Could Mean 40 Years of Work]. Los Alamos Monitor. March 1, 1998</ref><ref>[http://www.ieer.org/sdafiles/vol_5/5-1/tritium.html Science for Democratic Action Vol. 5 No. 1] {{Webarchive|url=https://web.archive.org/web/20060927015706/http://www.ieer.org/sdafiles/vol_5/5-1/tritium.html |date=2006-09-27 }}. IEER. Retrieved on 2011-11-08;</ref> This has reduced the quantity of helium-3 available from this source. Helium-3 stockpiles have been further diminished by increased demand,<ref name=CRS>{{cite report | first1=Dana A. | last1=Shea | first2=Daniel | last2=Morgan | publisher=[[Congressional Research Service]] | title=The Helium-3 Shortage: Supply, Demand, and Options for Congress | id=7-5700 | url=https://www.fas.org/sgp/crs/misc/R41419.pdf | date=22 December 2010 | access-date=23 December 2015 | archive-date=4 March 2016 | archive-url=https://web.archive.org/web/20160304003156/http://www.fas.org/sgp/crs/misc/R41419.pdf | url-status=live }}</ref> primarily for use in neutron radiation detectors and medical diagnostic procedures. US industrial demand for helium-3 reached a peak of {{convert|70,000|L}} (approximately {{convert|8|kg}}) per year in 2008. Price at auction, historically about {{convert|100|$/l}}, reached as high as {{Convert|2000|$/l}}.<ref>[https://spectrum.ieee.org/physics-projects-deflate-for-lack-of-helium3 Physics Projects Deflate for Lack of Helium-3] . Spectrum.ieee.org. Retrieved on 2011-11-08.</ref>  Since then, demand for helium-3 has declined to about {{convert|6000|L}} per year due to the high cost and efforts by the DOE to recycle it and find substitutes. Assuming a density of {{Convert|114|g/m3}} at $100/l helium-3 would be about a thirtieth as expensive as tritium (roughly {{convert|880|$/g}} vs roughly {{convert|30000|$/g}}) while at $2000/l helium-3 would be about half as expensive as tritium ({{convert|17540|$/g}} vs {{convert|30000|$/g}}).

The DOE recognized the developing shortage of both tritium and helium-3, and began producing tritium by lithium irradiation at the [[Tennessee Valley Authority]]'s [[Watts Bar Nuclear Generating Station]] in 2010.<ref name=CRS/> In this process tritium-producing burnable absorber rods (TPBARs) containing lithium in a ceramic form are inserted into the reactor in place of the normal boron control rods<ref>[http://pbadupws.nrc.gov/docs/ML0325/ML032521359.pdf Tritium Production] {{Webarchive|url=https://web.archive.org/web/20160827030415/http://pbadupws.nrc.gov/docs/ML0325/ML032521359.pdf |date=2016-08-27 }} Nuclear Regulatory Commission, 2005.</ref> Periodically the TPBARs are replaced and the tritium extracted.

Currently only two commercial nuclear reactors (Watts Bar Nuclear Plant Units 1 and 2) are being used for tritium production but the process could, if necessary, be vastly scaled up to meet any conceivable demand simply by utilizing more of the nation's power reactors{{Citation needed|reason=It is not obvious that any reactor could use TPBARs|date=February 2024}}. 
Substantial quantities of tritium and helium-3 could also be extracted from the heavy water moderator in [[CANDU]] nuclear reactors.<ref name=CRS/><ref name=D2O>{{cite patent
 | inventor1-last = Sur
 | inventor1-first = Bhaskar
 | inventor2-last = Rodrigo
 | inventor2-first = Lakshman
 | inventor3-last = Didsbury
 | inventor3-first = Richard
 | title = System and method for collecting <sup>3</sup>He gas from heavy water nuclear reactors
 | issue-date = 2013
 | patent-number = 2810716
 | country-code = CA
 | publication-date = 30 September 2013
 | url = http://www.ic.gc.ca/opic-cipo/cpd/eng/patent/2810716/summary.html?type=number_search&tabs1Index=tabs1_1
 }} {{Webarchive|url=https://web.archive.org/web/20151223142855/http://www.ic.gc.ca/opic-cipo/cpd/eng/patent/2810716/summary.html?type=number_search&tabs1Index=tabs1_1 |date=23 December 2015 }}</ref> India and Canada, the two countries with the largest [[heavy water reactor]] fleet, are both known to extract tritium from moderator/coolant heavy water, but those amounts are not nearly enough to satisfy global demand of either tritium or helium-3.

As tritium is also produced inadvertently in various processes in [[light water reactor]]s (see the article on tritium for details), extraction from those sources could be another source of helium-3. If the annual discharge of tritium (per 2018 figures) at [[La Hague reprocessing facility]] is taken as a basis, the amounts discharged ({{convert|31.2|g}} at La Hague) are not nearly enough to satisfy demand, even if 100% recovery is achieved.
{{Annual discharge of tritium from nuclear facilities}}

==Uses==
===Helium-3 spin echo===
Helium-3 can be used to do [[Helium-3 surface spin echo|spin echo experiments of surface dynamics]], which are underway at the Surface Physics Group at [[Cavendish Laboratory|the Cavendish Laboratory]] in Cambridge and in the Chemistry Department at [[Swansea University]].

===Neutron detection===
Helium-3 is an important isotope in instrumentation for [[neutron detection]]. It has a high absorption cross section for thermal [[neutron radiation|neutron]] beams and is used as a converter gas in neutron detectors. The neutron is converted through the nuclear reaction
:n + <sup>3</sup>He → <sup>3</sup>H + <sup>1</sup>H + 0.764 MeV
into charged particles [[tritium]] ions (T, <sup>3</sup>H) and [[Hydrogen ions]], or protons (p, <sup>1</sup>H) which then are detected by creating a charge cloud in the stopping gas of a [[proportional counter]] or a [[Geiger–Müller tube]].<ref>[http://www.lanl.gov/quarterly/q_sum03/neutron_detect.shtml A Modular Neutron Detector | Summer 2003| Los Alamos National Laboratory] {{Webarchive|url=https://web.archive.org/web/20080503051236/http://www.lanl.gov/quarterly/q_sum03/neutron_detect.shtml |date=2008-05-03 }}. Lanl.gov. Retrieved on 2011-11-08.</ref>

Furthermore, the absorption process is strongly [[Spin (physics)|spin]]-dependent, which allows a [[Spin polarization|spin-polarized]] helium-3 volume to transmit neutrons with one spin component while absorbing the other. This effect is employed in [[Polarized neutron scattering|neutron polarization analysis]], a technique which probes for magnetic properties of matter.<ref>[http://www.ncnr.nist.gov/AnnualReport/FY2002_html/pages/neutron_spin.htm NCNR Neutron Spin Filters] {{Webarchive|url=https://web.archive.org/web/20070520100431/http://www.ncnr.nist.gov/AnnualReport/FY2002_html/pages/neutron_spin.htm |date=2007-05-20 }}. Ncnr.nist.gov (2004-04-28). Retrieved on 2011-11-08.</ref><ref>[http://www.ill.eu/science-technology/neutron-technology-at-ill/optics/<sup>3</sup>He-spin-filters/ ILL <sup>3</sup>He spin filters]{{Dead link|date=September 2023 |bot=InternetArchiveBot |fix-attempted=yes }}. Ill.eu (2010-10-22). Retrieved on 2011-11-08.</ref><ref>{{cite journal|url= http://www.ncnr.nist.gov/staff/hammouda/publications/2000_gentile_j_appl_cryst.pdf|title= SANS polarization analysis with nuclear spin-polarized <sup>3</sup>He|doi= 10.1107/S0021889800099817|journal= J. Appl. Crystallogr.|date= 2000|volume= 33|issue= 3|pages= 771–774|last1= Gentile|first1= T.R.|last2= Jones|first2= G.L.|last3= Thompson|first3= A.K.|last4= Barker|first4= J.|last5= Glinka|first5= C.J.|last6= Hammouda|first6= B.|last7= Lynn|first7= J.W.|bibcode= 2000JApCr..33..771G|access-date= 2011-11-08|archive-date= 2012-04-02|archive-url= https://web.archive.org/web/20120402004050/http://www.ncnr.nist.gov/staff/hammouda/publications/2000_gentile_j_appl_cryst.pdf|url-status= live}}</ref><ref>[http://www.ncnr.nist.gov/equipment/he3nsf/index.html Neutron Spin Filters: Polarized <sup>3</sup>He] {{Webarchive|url=https://web.archive.org/web/20111016142515/http://ncnr.nist.gov/equipment/he3nsf/index.html |date=2011-10-16 }}. NIST.gov</ref>

The United States [[Department of Homeland Security]] had hoped to deploy detectors to spot smuggled plutonium in shipping containers by their neutron emissions, but the worldwide shortage of helium-3 following the drawdown in nuclear weapons production since the [[Cold War]] has to some extent prevented this.<ref>{{Cite news |last=Wald |first=Matthew L. |date=2009-11-22 |title=Shortage Slows a Program to Detect Nuclear Bombs |url=https://www.nytimes.com/2009/11/23/us/23helium.html |access-date=2025-05-11 |work=The New York Times |language=en-US |issn=0362-4331}}</ref>  As of 2012, DHS determined the commercial supply of [[boron-10]] would support converting its neutron detection infrastructure to that technology.<ref>{{Cite web |url=http://science.energy.gov/~/media/np/pdf/research/idpra/workshop-on-isotope-federal-supply-and-demand/presentations/Slovik_He3_Alternative_Isotopes_DOE_IP_Workshop_Jan_11_2012.pdf |title=Office of Science |access-date=2014-07-18 |archive-url=https://web.archive.org/web/20140726214826/http://science.energy.gov/~/media/np/pdf/research/idpra/workshop-on-isotope-federal-supply-and-demand/presentations/Slovik_He3_Alternative_Isotopes_DOE_IP_Workshop_Jan_11_2012.pdf |archive-date=2014-07-26 |url-status=dead }}</ref>

===Cryogenics===
Helium-3 refrigerators are devices used in experimental physics for obtaining temperatures down to about 0.2 [[kelvin]].<ref name="liquidseas">{{cite web |title=Workings of a 3He refrigerator |website=Harvard X-Ray Group Home Page |url=https://liquids.seas.harvard.edu/penanen/workings.html |access-date=2025-04-17}}</ref> By [[evaporative cooling]] of helium-4, a [[1-K pot]] liquefies a small amount of helium-3 in a small vessel called a helium-3 pot. Evaporative cooling at low pressure of the liquid helium-3, usually driven by [[adsorption]] since due to its high price the helium-3 is usually contained in a closed system to avoid losses, cools the helium-3 pot to a fraction of a kelvin.

A [[dilution refrigerator]] uses a mixture of helium-3 and helium-4 to reach [[cryogenics|cryogenic]] temperatures as low as a few thousandths of a kelvin.<ref name="cerndilution">{{cite web |title=Dilution Refrigeration |website=na47sun05.cern.ch |date=2009-02-07 |url=http://na47sun05.cern.ch/target/outline/dilref.html |archive-url=https://web.archive.org/web/20100208194054/http://na47sun05.cern.ch/target/outline/dilref.html |archive-date=2010-02-08 |url-status=dead |access-date=2025-04-17}}</ref>

===Nuclear magnetic resonance===
Helium-3 nuclei have an intrinsic [[nuclear spin]] of {{frac|1|2}}, and a relatively high [[gyromagnetic ratio]]. Because of this, it is possible to use [[Nuclear magnetic resonance]] (NMR) to observe Helium-3. This analytical technique, usually called <sup>3</sup>He-NMR, can be  used to identify helium-containing compounds. It is however limited by the low abundance of helium-3 in comparison to helium-4, which is itself not NMR-active.

Helium-3 can be [[Hyperpolarization (physics)|hyperpolarized]] using non-equilibrium means such as spin-exchange optical pumping.<ref>{{cite journal|title = Hyperpolarized <sup>3</sup>He Gas Production and MR Imaging of the Lung|first1 = Jason C.|last1 = Leawoods|first2 = Dmitriy A.|last2 = Yablonskiy|first3 = Brian|last3 = Saam|first4 = David S.|last4 = Gierada|first5 = Mark S.|last5 = Conradi|date =2001|journal =Concepts in Magnetic Resonance|volume =13|issue = 5|pages =277–293|doi=10.1002/cmr.1014|citeseerx = 10.1.1.492.8128}}</ref> During this process, [[circular polarization|circularly polarized]] infrared laser light, tuned to the appropriate wavelength, is used to excite electrons in an [[alkali metal]], such as [[caesium]] or [[rubidium]] inside a sealed glass vessel. The [[angular momentum]] is transferred from the alkali metal electrons to the noble gas nuclei through collisions. In essence, this process effectively aligns the nuclear spins with the magnetic field in order to enhance the NMR signal.

The hyperpolarized gas may then be stored at pressures of 10 atm, for up to 100 hours. Following inhalation, gas mixtures containing the hyperpolarized helium-3 gas can be imaged with an MRI scanner to produce anatomical and functional images of lung ventilation. This technique is also able to produce images of the airway tree, locate unventilated defects, measure the [[pulmonary gas pressures|alveolar oxygen partial pressure]], and measure the [[ventilation/perfusion ratio]]. This technique may be critical for the diagnosis and treatment management of chronic respiratory diseases such as [[chronic obstructive pulmonary disease|chronic obstructive pulmonary disease (COPD)]], [[emphysema]], [[cystic fibrosis]], and [[asthma]].<ref>{{cite journal|title = Hyperpolarized Gas Imaging of the Lung|first1 = Talissa|last1 = Altes|first2 = Michael|last2 = Salerno|date =2004|journal =J Thorac Imaging|volume =19|issue = 4|pages =250–258|doi=10.1097/01.rti.0000142837.52729.38|pmid = 15502612}}</ref>

Because a helium atom, or even [[Helium dimer#Cage|two helium atoms]], can be encased in [[fullerene]]-like cages, the NMR spectroscopy of this element can be a sensitive probe for changes of the carbon framework around it.<ref>{{cite journal |title= <sup>3</sup>He NMR: A Powerful New Tool for Following Fullerene Chemistry |first1=  Martin |last1= Saunders |first2= Hugo A. |last2= Jimenez-Vazquez |first3= Benedict W. |last3= Bangerter |first4= R. James |last4= Cross |first5= Stanley |last5= Mroczkowski |first6= Daron I. |last6= Freedberg |first7= Frank A. L. |last7= Anet |journal= Journal of the American Chemical Society |year= 1994 |volume= 116 |issue= 8 |pages= 3621–3622 |doi= 10.1021/ja00087a067 }}</ref><ref>{{cite web |url= http://chem.ch.huji.ac.il/nmr/techniques/1d/row1/he.html |title= (3He) Helium NMR |author= Institute of Chemistry, The [[Hebrew University of Jerusalem]] }}</ref> Using [[carbon-13 NMR]] to analyze fullerenes themselves is complicated by so many subtle differences among the carbons in anything but the simplest, highly-symmetric structures.

===Radio energy absorber for tokamak plasma experiments===
Both MIT's [[Alcator C-Mod]] tokamak and the [[Joint European Torus]] (JET) have experimented with adding a little helium-3 to a H–D plasma to increase the absorption of radio-frequency (RF) energy to heat the hydrogen and deuterium ions, a "three-ion" effect.<ref>{{Cite web |url=https://www.popularmechanics.com/science/energy/a27961/mit-nuclear-fusion-experiment-increases-efficiency/ |title=''MIT Achieves Breakthrough in Nuclear Fusion'' Aug 2017 |date=28 August 2017 |access-date=2020-07-18 |archive-date=2020-08-01 |archive-url=https://web.archive.org/web/20200801123209/https://www.popularmechanics.com/science/energy/a27961/mit-nuclear-fusion-experiment-increases-efficiency/ |url-status=live }}</ref><ref>{{cite journal|url= https://www.nature.com/articles/nphys4167.epdf?author_access_token=kPYxN3CzZYD2LT4Si32eOdRgN0jAjWel9jnR3ZoTv0MmuIUEPmNcONMVzXjNf2zbVw9w-V0n8MdnZGKP1E4gbnnf8HWpVEg2srMePcKJH7P-Epjuig2d7CKPHhckCLXI|title= Efficient generation of energetic ions in multi-ion plasmas by radio-frequency heating|journal= Nature Physics|date= 19 June 2017|doi= 10.1038/nphys4167|last1= Kazakov|first1= Ye. O.|last2= Ongena|first2= J.|last3= Wright|first3= J. C.|last4= Wukitch|first4= S. J.|last5= Lerche|first5= E.|last6= Mantsinen|first6= M. J.|last7= Van Eester|first7= D.|last8= Craciunescu|first8= T.|last9= Kiptily|first9= V. G.|last10= Lin|first10= Y.|last11= Nocente|first11= M.|last12= Nabais|first12= F.|last13= Nave|first13= M. F. F.|last14= Baranov|first14= Y.|last15= Bielecki|first15= J.|last16= Bilato|first16= R.|last17= Bobkov|first17= V.|last18= Crombé|first18= K.|last19= Czarnecka|first19= A.|last20= Faustin|first20= J. M.|last21= Felton|first21= R.|last22= Fitzgerald|first22= M.|last23= Gallart|first23= D.|last24= Giacomelli|first24= L.|last25= Golfinopoulos|first25= T.|last26= Hubbard|first26= A. E.|last27= Jacquet|first27= Ph.|last28= Johnson|first28= T.|last29= Lennholm|first29= M.|last30= Loarer|first30= T.|volume= 13|issue= 10|pages= 973–978|bibcode= 2017NatPh..13..973K|hdl= 1721.1/114949|s2cid= 106402331|display-authors= 1|hdl-access= free|access-date= 18 July 2020|archive-date= 1 August 2020|archive-url= https://web.archive.org/web/20200801122526/https://www.nature.com/articles/nphys4167.epdf?author_access_token=kPYxN3CzZYD2LT4Si32eOdRgN0jAjWel9jnR3ZoTv0MmuIUEPmNcONMVzXjNf2zbVw9w-V0n8MdnZGKP1E4gbnnf8HWpVEg2srMePcKJH7P-Epjuig2d7CKPHhckCLXI|url-status= live}}</ref>

===Nuclear fuel===
{{See also|Aneutronic fusion|Fusion rocket}}
{| class="wikitable"
|+ Comparison of [[neutronicity]] for different reactions<ref>{{cite web|url=http://members.tm.net/lapointe/IEC_Fusion.html|title=Inertial Electrostatic Confinement Fusion|access-date=2007-05-06|archive-date=2021-01-26|archive-url=https://web.archive.org/web/20210126172744/https://members.tm.net/lapointe/IEC_Fusion.html|url-status=live}}</ref><ref>{{cite web|url=http://www.lancs.ac.uk/ug/suttond1/#fusion|title = Nuclear Fission and Fusion|access-date=2007-05-06|archive-url=https://web.archive.org/web/20070404153838/http://www.lancs.ac.uk/ug/suttond1/#fusion <!-- Bot retrieved archive -->|archive-date=2007-04-04}}</ref><ref>{{cite web|url=http://library.thinkquest.org/28383/nowe_teksty/htmla/2_37a.html|title=The Fusion Reaction|access-date=2007-05-06|archive-date=2013-07-31|archive-url=https://web.archive.org/web/20130731134644/http://library.thinkquest.org/28383/nowe_teksty/htmla/2_37a.html|url-status=dead}}</ref><ref>{{cite web|url=http://fti.neep.wisc.edu/pdf/fdm1291.pdf|title=A Strategy for D – {{SimpleNuclide|Helium|3}} Development|author=John Santarius|date=June 2006|access-date=2007-05-06|archive-date=2007-07-03|archive-url=https://web.archive.org/web/20070703200058/http://fti.neep.wisc.edu/pdf/fdm1291.pdf|url-status=dead}}</ref><ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nucrea.html|title=Nuclear Reactions|access-date=2007-05-06|archive-date=2000-02-01|archive-url=https://web.archive.org/web/20000201210035/http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/nucrea.html|url-status=live}}</ref>
|-
! scope="col" |
! scope="col" | Reactants
! scope="col" | Products
! scope="col" | ''Q''
! scope="col" | n/MeV
|-
! scope="row" rowspan="3" |First-generation fusion fuels
| {{chem2|[[Deuterium|^{2}D]] + ^{2}D}}
| {{chem2|^{3}He}} + {{Physics particle|n|TL=1|BL=0}}
| style="text-align: right;" | 3.268 [[MeV]]
| 0.306
|- 
| {{chem2|^{2}D + ^{2}D}}
| {{chem2|^{3}T|link=Tritium}} + {{Physics particle|p|TL=1|BL=1}}
| style="text-align: right;" | 4.032 [[MeV]]
| 0
|- 
| {{chem2|^{2}D + ^{3}T}}
| {{chem2|^{4}He}} + {{Physics particle|n|TL=1|BL=0}}
| style="text-align: right;" |17.571 [[MeV]]
| 0.057
|- 
! scope="row" | Second-generation fusion fuel
| {{chem2|^{2}D + ^{3}He}}
| {{chem2|^{4}He}} + {{Physics particle|p|TL=1|BL=1}}
| style="text-align: right;" |18.354 [[MeV]]
| 0
|- 
! scope="row" | Net result of <sup>2</sup>D burning (sum of first 4 rows)
| {{chem2|6 ^{2}D}}
| class="nowrap" | 2({{chem2|^{4}He}} + n + p)
| class="nowrap" style="text-align: right;" |43.225 [[MeV]]
| 0.046
|- 
! scope="row" rowspan="2" | Third-generation fusion fuels
| class="nowrap" | {{chem2|^{3}He + ^{3}He}}
| {{chem2|^{4}He}} + 2 {{Physics particle|p|TL=1|BL=1}}
| style="text-align: right;" |12.86 [[MeV]]
| 0
|- 
| {{chem2|^{11}B|link=Boron-11}} + {{Physics particle|p|TL=1|BL=1}}
| 3 {{chem2|^{4}He}}
| style="text-align: right;" |8.68 [[MeV]]
| 0
|-
! scope="row" | Current nuclear fuel
| {{chem2|[[Uranium 235|^{235}U]] + n}}
| 2 [[Fission product|FP]]+ 2.5n
| style="text-align: right;" |~200 [[MeV]]
| 0.0075
|}
{{chem2|^{3}He}} can be produced by the low temperature fusion of  {{overset|(D-p)|<sup>2</sup>H + <sup>1</sup>p}}  → {{chem2|^{3}He}} + γ + 4.98 MeV. If the fusion temperature is below that for the helium nuclei to fuse, the reaction produces a high energy alpha particle which quickly acquires an electron producing a stable light helium ion which can be utilized directly as a source of electricity without producing dangerous neutrons.  [[File:Fusion rxnrate.svg|right|300px|thumb|The fusion [[reaction rate]] increases rapidly with temperature until it maximizes and then gradually drops off. The DT rate peaks at a lower temperature (about 70&nbsp;keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.]]

{{chem2|^{3}He}} can be used in fusion reactions by either of the reactions {{chem2|^{2}H + ^{3}He -> ^{4}He + ^{1}p}} + 18.3 [[electronvolt|MeV]], or {{chem2|^{3}He + ^{3}He -> ^{4}He + 2 ^{1}p}} + 12.86 MeV.

The conventional [[deuterium]] + [[tritium]] ("[[deuterium–tritium fusion|D–T]]") fusion process produces energetic neutrons which render reactor components [[radioactive]] with [[activation product]]s. The appeal of helium-3 fusion stems from the [[aneutronic fusion|aneutronic]] nature of its reaction products. Helium-3 itself is non-radioactive. The lone high-energy by-product, the [[proton]], can be contained by means of electric and magnetic fields. The momentum energy of this proton (created in the fusion process) will interact with the containing electromagnetic field, resulting in direct net electricity generation.<ref>{{cite web|url=http://fti.neep.wisc.edu/presentations/jfs_ieee0904.pdf|title=Lunar {{SimpleNuclide|Helium|3}} and Fusion Power|author=John Santarius|date=September 28, 2004|access-date=2007-05-06|archive-date=2007-07-03|archive-url=https://web.archive.org/web/20070703200103/http://fti.neep.wisc.edu/presentations/jfs_ieee0904.pdf|url-status=dead}}</ref>

Because of the higher [[Coulomb barrier]], the temperatures required for {{chem2|^{2}H + ^{3}He}} fusion are much higher than those of conventional [[deuterium–tritium fusion|D–T fusion]]. Moreover, since both reactants need to be mixed together to fuse, reactions between nuclei of the same reactant will occur, and the D–D reaction ({{chem2|^{2}H + ^{2}H}}) does produce a [[neutron]]. Reaction rates vary with temperature, but the D–{{chem2|^{3}He}} reaction rate is never greater than 3.56 times the D–D reaction rate (see graph). Therefore, fusion using D–{{chem2|^{3}He}} fuel at the right temperature and a D-lean fuel mixture, can produce a much lower neutron flux than D–T fusion, but is not clean, negating some of its main attraction.

The second possibility, fusing {{chem2|^{3}He}} with itself ({{chem2|^{3}He + ^{3}He}}), requires even higher temperatures (since now both reactants have a +2 charge), and thus is even more difficult than the D-{{chem2|^{3}He}} reaction. It offers a theoretical reaction that produces no neutrons; the charged protons produced can be contained in electric and magnetic fields, which in turn directly generates electricity. {{chem2|^{3}He + ^{3}He}} fusion is feasible as demonstrated in the laboratory and has immense advantages, but commercial viability is many years in the future.<ref>{{cite journal|url=http://www.technologyreview.com/energy/19296/|title=Mining the Moon: Lab experiments suggest that future fusion reactors could use helium-3 gathered from the moon|author=Mark Williams|journal=MIT Technology Review|date=August 23, 2007|access-date=2011-01-25|archive-date=2010-12-30|archive-url=https://web.archive.org/web/20101230224725/http://www.technologyreview.com/Energy/19296/|url-status=live}}</ref>

The amounts of helium-3 needed as a replacement for [[fossil fuel|conventional fuel]]s are substantial by comparison to amounts currently available. The total amount of energy produced in the {{chem2|^{2}D + ^{3}He}} reaction is 18.4 M[[electronvolt|eV]], which corresponds to some 493 [[watt-hour|megawatt-hour]]s (4.93×10<sup>8</sup> W·h) per three [[gram]]s (one [[mole (chemistry)|mole]]) of {{chem2|^{3}He}}. If the total amount of energy could be converted to electrical power with 100% efficiency (a physical impossibility), it would correspond to about 30 minutes of output of a gigawatt electrical plant per mole of {{chem2|^{3}He}}.  Thus, a year's production (at 6 grams for each operation hour) would require 52.5 kilograms of helium-3. The amount of fuel needed for large-scale applications can also be put in terms of total consumption: electricity consumption by 107 million U.S. households in 2001<ref>Date from the US Energy Information Administration</ref> totaled 1,140 billion kW·h (1.14×10<sup>15</sup> W·h). Again assuming 100% conversion efficiency, 6.7 [[tonne]]s per year of helium-3 would be required for that segment of the energy demand of the United States, 15 to 20 tonnes per year given a more realistic end-to-end conversion efficiency.{{citation needed|date=January 2011}}

A second-generation approach to controlled [[nuclear fusion|fusion]] power involves combining helium-3 and [[deuterium|deuterium, {{chem2|^{2}D}}]]. This reaction produces an [[alpha particle]] and a high-energy [[proton]]. The most important potential advantage of this fusion reaction for power production as well as other applications lies in its compatibility with the use of [[electrostatic]] fields to control fuel [[ion]]s and the fusion protons. High speed protons, as positively charged particles, can have their kinetic energy converted directly into [[electricity]], through use of [[Solid-state chemistry|solid-state]] conversion materials as well as other techniques. Potential conversion efficiencies of 70% may be possible, as there is no need to convert proton energy to heat in order to drive a [[turbine]]-powered [[Electric generator|electrical generator]].{{Citation needed|date=April 2012}}

====He-3 power plants====
There have been many claims about the capabilities of helium-3 power plants. According to proponents, fusion power plants operating on [[deuterium]] and helium-3 would offer lower capital and [[operating cost]]s than their competitors due to less technical complexity, higher conversion efficiency, smaller size, the absence of radioactive fuel, no air or water [[pollution]], and only low-level [[radioactive]] waste disposal requirements. Recent estimates suggest that about $6 billion in [[Investment (macroeconomics)|investment]] [[Capital (economics)|capital]] will be required to develop and construct the first helium-3 fusion [[power plant]]. Financial break even at today's wholesale [[electricity]] prices (5 US cents per [[kilowatt-hour]]) would occur after five 1-[[gigawatt]] plants were on line, replacing old conventional plants or meeting new demand.<ref>{{cite news|url=http://www.popularmechanics.com/science/air_space/1283056.html?page=4|title=Mining The Moon|author=Paul DiMare|date=October 2004|work=Popular Mechanics|access-date=2007-05-06|archive-url=https://web.archive.org/web/20070814162104/http://www.popularmechanics.com/science/air_space/1283056.html?page=4|archive-date=2007-08-14|url-status=dead}}</ref>

The reality is not so clear-cut. The most advanced fusion programs in the world are [[inertial confinement fusion]] (such as [[National Ignition Facility]]) and [[magnetic confinement fusion]] (such as [[ITER]] and [[Wendelstein 7-X]]). In the case of the former, there is no solid roadmap to power generation. In the case of the latter, commercial power generation is not expected until around 2050.<ref>{{cite news|url=http://www.iter.org/proj/Pages/ITERAndBeyond.aspx |title=ITER & Beyond |access-date=2009-08-04 |url-status=dead |archive-url=https://web.archive.org/web/20090520151601/http://www.iter.org/PROJ/Pages/ITERAndBeyond.aspx |archive-date=2009-05-20 }}</ref> In both cases, the type of fusion discussed is the simplest: D–T fusion. The reason for this is the very low [[Coulomb barrier]] for this reaction; for D+<sup>3</sup>He, the barrier is much higher, and it is even higher for <sup>3</sup>He–<sup>3</sup>He. The immense cost of reactors like [[ITER]] and [[National Ignition Facility]] are largely due to their immense size, yet to scale up to higher plasma temperatures would require reactors far larger still. The 14.7 MeV proton and 3.6 MeV alpha particle from D–<sup>3</sup>He fusion, plus the higher conversion efficiency, means that more electricity is obtained per kilogram than with [[deuterium–tritium fusion|D–T fusion]] (17.6 MeV), but not that much more. As a further downside, the rates of reaction for [[Aneutronic fusion#Candidate reactions|helium-3 fusion reactions]] are not particularly high, requiring a reactor that is larger still or more reactors to produce the same amount of electricity.

In 2022, [[Helion Energy]] claimed that their 7th fusion prototype (Polaris; fully funded and under construction as of September 2022) will demonstrate "net electricity from fusion", and will demonstrate "helium-3 production through deuterium–deuterium fusion" by means of a "patented high-efficiency closed-fuel cycle".<ref>{{cite web |url=https://www.helionenergy.com/faq/ |title=Helion FAQ |accessdate=29 September 2022}}</ref>

====Alternatives to He-3====
To attempt to work around this problem of massively large power plants that may not even be economical with D–T fusion, let alone the far more challenging D–<sup>3</sup>He fusion, a number of other reactors have been proposed&nbsp;– the [[Fusor]], [[Polywell]], [[Focus fusion]], and many more, though many of these concepts have fundamental problems with achieving a net energy gain, and generally attempt to achieve fusion in thermal disequilibrium, something that could potentially prove impossible,<ref>{{cite news|title=A general critique of inertial-electrostatic confinement fusion systems|author= Todd Rider|hdl = 1721.1/29869}}</ref> and consequently, these long-shot programs tend to have trouble garnering funding despite their low budgets. Unlike the "big" and "hot" fusion systems, if such systems worked, they could scale to the higher barrier [[aneutronic fusion|aneutronic]] fuels, and so their proponents tend to promote [[Aneutronic fusion#Boron|p-B fusion]], which requires no exotic fuel such as helium-3.

== Extraterrestrial ==

=== Moon ===
{{Further|Lunar resources#Helium-3|Changesite-(Y)}}

Materials on the [[Moon]]'s surface contain helium-3 at concentrations between 1.4 and 15 [[Parts per billion|ppb]] in sunlit areas,<ref>[http://fti.neep.wisc.edu/Research/he3_pubs.html FTI Research Projects :: <sup>3</sup>He Lunar Mining] {{Webarchive|url=https://web.archive.org/web/20060904144943/http://fti.neep.wisc.edu/Research/he3_pubs.html |date=2006-09-04 }}. Fti.neep.wisc.edu. Retrieved on 2011-11-08.</ref><ref>{{cite journal|url=http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf|title=The estimation of helium-3 probable reserves in lunar regolith|issue=1338|pages=2175|author1=E. N. Slyuta|author2=A. M. Abdrakhimov|author3=E. M. Galimov|journal=Lunar and Planetary Science XXXVIII|date=2007|bibcode=2007LPI....38.2175S|access-date=2007-05-31|archive-date=2008-07-05|archive-url=https://web.archive.org/web/20080705122316/http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf|url-status=live}}</ref> and may contain concentrations as much as 50 [[Parts per billion|ppb]] in permanently shadowed regions.<ref name="F. H. Cocks 2010"/> A number of people, starting with Gerald Kulcinski in 1986,<ref>{{cite news|url=http://www.thespacereview.com/article/536/1|title=A fascinating hour with Gerald Kulcinski|author=Eric R. Hedman|date=January 16, 2006|work=The Space Review|access-date=August 30, 2007|archive-date=January 9, 2011|archive-url=https://web.archive.org/web/20110109082500/http://thespacereview.com/article/536/1|url-status=live}}</ref> have proposed to [[Exploration of the Moon|explore the Moon]], mine lunar [[regolith]] and use the helium-3 for [[Nuclear fusion|fusion]]. Because of the low concentrations of helium-3, any mining equipment would need to process extremely large amounts of regolith (over 150 tonnes of regolith to obtain one gram of helium-3).<ref>{{cite web|title=The challenge of mining He-3 on the lunar surface: how all the parts fit together|author=I.N. Sviatoslavsky|date=November 1993|url=http://fti.neep.wisc.edu/pdf/wcsar9311-2.pdf|access-date=2008-03-04|archive-date=2019-01-20|archive-url=https://web.archive.org/web/20190120035522/http://fti.neep.wisc.edu/pdf/wcsar9311-2.pdf|url-status=dead}} Wisconsin Center for Space Automation and Robotics Technical Report WCSAR-TR-AR3-9311-2.</ref>

The primary objective of [[Indian Space Research Organisation]]'s first lunar probe called [[Chandrayaan-1]], launched on October 22, 2008, was reported in some sources to be mapping the Moon's surface for helium-3-containing minerals.<ref>{{cite news|url= http://economictimes.indiatimes.com/News/News_By_Industry/ET_Cetera/With_He-3_on_mind_India_gets_ready_for_lunar_mission/articleshow/3500270.cms|title= With He-3 on mind, India gets ready for lunar mission|work= The Times Of India|date= 2008-09-19|access-date= 2008-09-21|archive-date= 2008-09-21|archive-url= https://web.archive.org/web/20080921212710/http://economictimes.indiatimes.com/News/News_By_Industry/ET_Cetera/With_He-3_on_mind_India_gets_ready_for_lunar_mission/articleshow/3500270.cms|url-status= live}}</ref> No such objective is mentioned in the project's official list of goals, though many of its scientific payloads have held helium-3-related applications.<ref>[http://www.isro.org/chandrayaan/htmls/objective_scientific.htm Scientific] {{webarchive|url=https://web.archive.org/web/20091012110215/http://www.isro.org/Chandrayaan/htmls/objective_scientific.htm |date=2009-10-12 }}. Isro.org (2008-11-11). Retrieved on 2011-11-08.</ref><ref>[http://luna-ci.blogspot.com/2008/11/chandrayaan-1-payload-feature-2-sub-kev.html Luna C/I:: Chandrayaan-1 Payload Feature #2: Sub KeV Atom Reflecting Analyser (SARA)] {{Webarchive|url=https://web.archive.org/web/20190720000332/http://luna-ci.blogspot.com/2008/11/chandrayaan-1-payload-feature-2-sub-kev.html |date=2019-07-20 }}. Luna-ci.blogspot.com (2008-11-12). Retrieved on 2011-11-08.</ref>

[[Cosmochemistry|Cosmochemist]] and [[geochemist]] [[Ouyang Ziyuan]] from the [[Chinese Academy of Sciences]] who is now in charge of the [[Chang'e program|Chinese Lunar Exploration Program]] has already stated on many occasions that one of the main goals of the program would be the mining of helium-3, from which operation "each year, three space shuttle missions could bring enough fuel for all human beings across the world".<ref>[http://www.chinadaily.com.cn/cndy/2006-07/26/content_649325.htm He asked for the moon-and got it] {{Webarchive|url=https://web.archive.org/web/20230615211000/http://www.chinadaily.com.cn/cndy/2006-07/26/content_649325.htm |date=2023-06-15 }}. Chinadaily.com.cn (2006-07-26). Retrieved on 2011-11-08.</ref>
 
In January 2006, the Russian space company [[RKK Energiya]] announced that it considers lunar helium-3 a potential economic resource to be mined by 2020,<ref>[https://web.archive.org/web/20080809210848/http://www.space.com/news/ap_060126_russia_moon.html Russian Rocket Builder Aims for Moon Base by 2015, Reports Say]. Associated Press (via space.com). 26 January 2006</ref> if funding can be found.<ref>{{cite web|url=http://www.thespacereview.com/article/551/1|title=Moonscam: Russians try to sell the Moon for foreign cash|author=James Oberg|date=February 6, 2006|access-date=August 30, 2007|archive-date=June 15, 2023|archive-url=https://web.archive.org/web/20230615211006/https://www.thespacereview.com/article/551/1|url-status=live}}</ref><ref>{{cite web|url=http://www.thespacereview.com/article/824/1|title=Death throes and grand delusions|author=Dwayne A. Day|author-link=Dwayne A. Day|work=[[The Space Review]]|date=March 5, 2007|access-date=August 30, 2007|archive-date=June 15, 2023|archive-url=https://web.archive.org/web/20230615211002/https://www.thespacereview.com/article/824/1|url-status=live}}</ref>

Not all writers feel the extraction of lunar helium-3 is feasible, or even that there will be a demand for it for fusion. [[Dwayne A. Day|Dwayne Day]], writing in ''[[The Space Review]]'' in 2015, characterises helium-3 extraction from the Moon for use in fusion as magical thinking about an unproven technology, and questions the feasibility of lunar extraction, as compared to production on Earth.<ref>{{cite news |last1=Day |first1=Dwayne |author-link=Dwayne A. Day |title=The helium-3 incantation |url=http://www.thespacereview.com/article/2834/1 |access-date=11 January 2019 |work=The Space Review |date=September 28, 2015 |quote=The belief in helium-3 mining is a great example of a myth that has been incorporated into the larger enthusiasm for human spaceflight, a magical incantation that is murmured, but rarely actually discussed. |archive-date=27 December 2018 |archive-url=https://web.archive.org/web/20181227075502/http://www.thespacereview.com/article/2834/1 |url-status=live }}</ref>

=== Gas giants ===
Mining [[gas giant]]s for helium-3 has also been proposed.<ref>{{cite web|title=Atmospheric Mining in the Outer Solar System |author=Bryan Palaszewski |url=http://gltrs.grc.nasa.gov/reports/2006/TM-2006-214122.pdf |url-status=dead |archive-url=https://web.archive.org/web/20090327051914/http://gltrs.grc.nasa.gov/reports/2006/TM-2006-214122.pdf |archive-date=2009-03-27 }} NASA Technical Memorandum 2006-214122. AIAA–2005–4319. Prepared for the 41st Joint Propulsion Conference and Exhibit cosponsored by AIAA, ASME, SAE, and ASEE, Tucson, Arizona, July 10–13, 2005.</ref> The [[British Interplanetary Society]]'s hypothetical [[Project Daedalus]] interstellar probe design was fueled by helium-3 mines in the atmosphere of [[Jupiter]], for example.

== See also ==
* [[List of elements facing shortage]]

== Notes and references ==
{{reflist|30em}}

=== Bibliography ===
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 |pages=119–138
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* {{cite conference
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 |date=July 1994
 |orig-date=presented July 1993, revised July 1994
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 |title=Non-Lunar <sup>3</sup>He Resources
 |conference=Second Wisconsin Symposium on Helium-3 and Fusion Power
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* {{cite book
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* {{cite book
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* {{cite book
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* {{cite book
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== External links ==
* [http://nobelprize.org/physics/laureates/2003/presentation-speech.html The Nobel Prize in Physics 2003, presentation speech] {{Webarchive|url=https://web.archive.org/web/20080723225703/http://nobelprize.org/physics/laureates/2003/presentation-speech.html |date=2008-07-23 }}
* [http://www.bbc.co.uk/sn/tvradio/programmes/horizon/broadband/tx/moonsale/ ''Moon for Sale'': A BBC Horizon documentary on the possibility of lunar mining of Helium-3] {{Webarchive|url=https://web.archive.org/web/20230615212019/https://www.bbc.co.uk/sn/tvradio/programmes/horizon/broadband/tx/moonsale/ |date=2023-06-15 }}

{{Isotope sequence
|element=helium
|lighter=[[diproton]]
|heavier=[[helium-4]]
|before=[[lithium-4]] '''([[proton emission|p]])<br />'''[[hydrogen-3]] '''([[beta decay|β−]])'''
|after=Stable
}}
{{Authority control}}

[[Category:Isotopes of helium|Helium-03]]
[[Category:Nuclear fusion fuels]]
[[Category:Superfluidity]]
[[Category:Nuclear magnetic resonance]]
[[Category:MRI contrast agents]]
[[Category:Cryogenics]]
[[Category:Cooling technology]]