Helium-3
Helium-3 (He-3) is a light, non-radioactive isotope of helium with two protons and one neutron, rare on Earth, sought for use in nuclear fusion research. The abundance of helium-3 is thought to be greater on the Moon (embedded in the upper layer of regolith by the solar wind over billions of years) and the solar system's gas giants (left over from the original solar nebula), though still low in quantity (28 ppm of lunar regolith is helium-4 and 0.01 ppm is helium-3).  It is proposed to be used as a second-generation fusion power source.
The helion, the nucleus of a helium-3 atom, consists of two protons but only one neutron, in contrast to two neutrons in ordinary helium. Its existence was first proposed in 1934 by the Australian nuclear physicist Mark Oliphant while based at Cambridge University's Cavendish Laboratory, in an experiment in which fast deuterons were reacted with other deuteron targets (the first demonstration of nuclear fusion). Helium-3, as an isotope, was postulated to be radioactive, until helions from it were accidentally identified as a trace "contaminant" in a sample of natural helium (which is mostly helium-4) from a gas well, by Luis W. Alvarez and Robert Cornog in a cyclotron experiment at the Lawrence Berkeley National Laboratory, in 1939. 

# Physical properties
Helium-3's atomic mass of 3.0160293, being significantly lower than Helium-4's 4.0026, causes it to have significantly different properties. Helium-3 boils at 3.19 kelvins compared to helium-4's 4.23 K, and its critical point is also lower at 3.35 K, compared to helium-4's 5.19 K. It has less than half the density when liquid at its boiling point: 0.059 g/ml compared to helium-4's 0.12473 g/ml at one atmosphere. Its latent heat of vaporization is also considerably lower at 0.026 kJ/mol compared to helium-4's 0.0829 kJ/mol.

# Fusion reactions
Some fusion processes produce highly energetic neutrons which render reactor components radioactive with their bombardment, and power generation must occur through thermal means. However, the appeal of helium-3 fusion stems from the nature of its reaction products. Helium-3 itself is non-radioactive. The lone high-energy proton produced can be contained using electric and magnetic fields, which results in direct electricity generation.
However, since both reactants need to be mixed together to fuse, side reactions (21H + 21H and 32He+ 32He) will occur, the first of which is not aneutronic. Therefore in practice this reaction is unlikely to ever be completely 'clean', thus negating some of its attraction. Also, due to the higher Coulomb barrier, the temperatures required for 21H + 32He  fusion are much higher than those of conventional 2H + 31H (deuterium + tritium) fusion.
The amounts of helium-3 needed as a replacement for conventional fuels should not be underestimated. The total amount of energy produced in the 3He + 21H+  reaction is 18.4 MeV, which corresponds to some 493 megawatt-hours (4.93x108 Wh) per three grams (one mole) of 3He. Even if that 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 thousand-megawatt electrical plant; a year's production by the same plant would require some 17.5 kilograms of helium-3.
The amount of fuel needed for large-scale applications can also be put in terms of total consumption: According to the US Energy Information Administration, "Electricity consumption by 107 million U.S. households in 2001 totaled 1,140 billion kWh" (1.114x1015 Wh). Again assuming 100% conversion efficiency, 6.7 tonnes of helium-3 would be required just for that segment of one country's energy demand, 15 to 20 tonnes given a more realistic end-to-end conversion efficiency.

# Neutron detection
Helium-3 is a most important isotope in instrumentation for neutron detection. It has a high absorption cross section for thermal neutron beams and is used as a converter gas in neutron detectors. The neutron is converted through the nuclear reaction
into charged particles tritium (T, 3H) and proton (p, 1H) which then are detected by creating a charge cloud in the stopping gas of a proportional counter or a Geiger-Muller tube. 
Furthermore, the absorption process is strongly spin-dependent, which allows a spin-polarized helium-3 volume to transmit neutrons with one spin component while absorbing the other. This effect is employed in neutron polarization analysis, a technique which probes for magnetic properties of matter. 

# Cryogenics
A helium-3 refrigerator uses helium-3 to achieve temperatures of 0.2 to 0.3 kelvin. A dilution refrigerator uses a mixture of helium-3 and helium-4 to reach cryogenic temperatures as low as a few thousandths of a kelvin. 
An important property of helium-3, which distinguishes it from the more common helium-4, is that its nucleus is a fermion since it contains an odd number of spin 1/2 particles. Helium-4 nuclei are bosons, containing an even number of spin 1/2 particles. This is a direct result of the addition rules for quantized angular momentum. At low temperatures (about 2.17 K), helium-4 undergoes a phase transition: A fraction of it enters a superfluid phase that can be roughly understood as a type of Bose-Einstein condensate. Such a mechanism is not available for helium-3 atoms, which are fermions. However, it was widely speculated that helium-3 could also become a superfluid at much lower temperatures, if the atoms formed into pairs analogous to Cooper pairs in the BCS theory of superconductivity. Each Cooper pair, having integer spin, can be thought of as a boson. During the 1970s, David Morris Lee, Douglas Osheroff and Robert Coleman Richardson discovered two phase transitions along the melting curve, which was soon realized to be the two superfluid phases of helium-3. 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. Tony Leggett won the 2003 Nobel Prize in Physics for his work on refining understanding of the superfluid phase of helium-3.
In zero magnetic field, there are two distinct superfluid phases of 3He, 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 3He 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. Because of the extreme purity of superfluid 3He (since all materials except 4He have solidified and sunk to the bottom of the liquid 3He and any 4He has phase separated entirely, this is the most pure condensed matter state), these collective modes have been studied with much greater precision than in any other unconventional pairing system.

# Manufacturing
Due to the rarity of helium-3 on Earth, it is typically manufactured instead of recovered from natural deposits. Helium-3 is a byproduct of tritium decay, and tritium can be produced through neutron bombardment of lithium, boron, or nitrogen targets. Current supplies of helium-3 come, in part, from the dismantling of nuclear weapons where it accumulates; approximately 150 kilograms of it have resulted from decay of US tritium production since 1955, most of which was for warheads. However, the production and storage of huge amounts of the gas tritium is probably uneconomical, as roughly eighteen tons of tritium stock are required for each ton of helium-3 produced annually by decay (production rate dN/dt from number of moles or other unit mass of tritium N is N  = N -  where the value of t12/(ln2) is about 18 years; see radioactive decay). If commercial fusion reactors were to use helium-3 as a fuel, they would require tens of tons of helium-3 each year to produce a fraction of the world's power.  Breeding tritium with lithium-6 consumes the neutron, while breeding with lithium-7 produces a low energy neutron as a replacement for the consumed fast neutron. Note that any breeding of tritium on Earth requires the use of a high neutron flux, which proponents of helium-3 nuclear reactors hope to avoid.

# Medical lung imaging
Polarized helium-3 may be produced directly with lasers of the appropriate power, and with a thin layer of protective Cs metal on the inside of cylinders, the magnetized gas may be stored at pressures of 10 atm for up to 100 hours. When inhaled, mixtures containing the gas can be imaged with an MRI-like scanner which produces breath by breath images of lung ventilation, in real-time. Applications of this experimental technique are just beginning to be explored.

# Terrestrial occurrence
3He is a primordial substance in the Earth's mantle, considered to have become entrapped within the Earth during planetary formation. The ratio of 3He to 4He within the Earth's crust and mantle is less than that for assumptions of solar disk composition as obtained from meteorite and lunar samples, with terrestrial materials generally containing lower 3He/4He ratios due to ingrowth of 4He from radioactive decay.
3He is present within the mantle, in the ratio of 200-300 parts of 3He to a million parts of 4He. Ratios of 3He/4He in excess of atmospheric are indicative of a contribution of 3He from the mantle. Crustal sources are dominated by the 4He which is produced by the decay of radioactive elements in the crust and mantle.
3He is also present in the Earth's atmosphere. The natural abundance of 3He in naturally occurring helium gas is 1.38Template:E. The partial pressure of helium in the Earth's atmosphere is about 4 millitorr, and thus 5.2 parts per million of helium. It has been proven that the Earth's atmosphere contains approximately 4000 tons of 3He.
3He is produced on Earth from three sources: lithium spallation, cosmic rays, and decay of tritium (3H). 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 4He by alpha particle emissions.
The total amount of helium-3 in the mantle may be in the range of 100 thousand to a million tonnes. However, this mantle helium is not directly accessible. Some of it leaks up through deep-sourced hotspot volcanoes such as those of the Hawaiian islands, but only 300 grams per year is emitted to the atmosphere. Mid-ocean ridges emit another 3 kilogram per year. Around subduction zones, 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). Crustal natural gas sources may have only half a tonne total. There may be another four thousand tonnes in interplanetary dust particles on the ocean floors. Extracting helium-3 from these sources consumes more energy than fusion would release.  Extraction from the most efficient source, natural gas, consumes ten times the energy available from fusion reactions. 

# Extraterrestrial supplies
The Moon's surface contains helium-3 at concentrations on the order of 0.01 ppm.  A number of people, starting with Gerald Kulcinski in 1986, have proposed to explore the moon, mine lunar regolith and using the helium-3 for fusion. Because of the low concentrations of helium-3, any mining equipment would need to process large amounts of regolith, and some proposals have suggested that helium-3 extraction be piggybacked onto a larger mining and development operation.
Cosmochemist and geochemist Ouyang Ziyuan from the Chinese Academy of Sciences  who is now in charge of the 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 where "each year three space shuttle missions could bring enough fuel for all human beings across the world."
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, if funding can be found.
Mining gas giants for helium-3 has also been proposed. The British Interplanetary Society's hypothetical Project Daedalus interstellar probe design was fueled by helium-3 mines on the planet Jupiter, for example. Jupiter's high gravity makes this a less energetically favorable operation than extracting helium-3 from the other gas giants of the solar system, however.

# Power generation
A second-generation approach to controlled fusion power involves combining helium-3 (32He) and deuterium (21H). This reaction produces an helium-4 ion (42He) (like an alpha particle, but of different origin) and a high-energy proton (positively charged hydrogen ion) (11p). 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 ions and the fusion protons. Protons, as positively charged particles, can be converted directly into electricity, through use of solid-state conversion materials as well as other techniques. Potential conversion efficiencies of 70 percent may be possible, as there is no need to convert proton energy to heat in order to drive turbine-powered generators.
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 costs 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 capital will be required to develop and construct the first helium-3 fusion power plant. Financial breakeven at today's wholesale electricity prices (5 cents per kilowatt-hour) would occur after five 1000-megawatt plants were on line, replacing old conventional plants or meeting new demand.
The reality is not so clean-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 other tokamaks). 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. 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+He3, the barrier is much higher, and He3-He3 higher still. 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-He3 fusion, plus the higher conversion efficiency, means that more electricity is obtained per kilogram than with D-T fusion (17.6 MeV), but not that much more. As a further downside, the rates of reaction for He3 fusion reactions are not particularly high, requiring a reactor that is larger still or more reactors to produce the same amount of electricity.
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-He3 fusion, a number of other reactors have been proposed -- the Fusor, Polywell, Focus fusion, and many more. These generally attempt to achieve fusion in thermal disequilibrium, something that could potentially prove impossible, and consequently, these long-shot programs tend to have trouble garnering funding despite their low budgets.  Unlike the "big", "hot" fusion systems, however, if such systems were to work, they could scale to the higher barrier "aneutronic" fuels.  However, these systems would scale well enough that their proponents tend to promote p-B fusion, which requires no exotic fuels like He-3.