Helium-3 occurs as a primordial nuclide, escaping from Earth's crust into its atmosphere and into outer space over millions of years.

[6][7] The existence of helium-3 was first proposed in 1934 by the Australian nuclear physicist Mark Oliphant while he was working at the University of Cambridge Cavendish Laboratory.

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.

[12][13] 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.

Alexei Abrikosov, Vitaly Ginzburg, and Tony Leggett won the 2003 Nobel Prize in Physics for their work on refining understanding of the superfluid phase of helium-3.

These collective modes have been studied with much greater precision than in any other unconventional pairing system, because of the extreme purity of superfluid 3He.

This purity is due to all 4He phase separating entirely and all other materials solidifying and sinking to the bottom of the liquid, making the A- and B-phases of 3He the most pure condensed matter state possible.

According to American physicist Richard Garwin, about 26 cubic metres (920 cu ft) or almost 5 kilograms (11 lb) of 3He is available annually for separation from the US natural gas stream.

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.

Some helium-3 leaks up through deep-sourced hotspot volcanoes such as those of the Hawaiian Islands, but only 300 grams (11 oz) per year is emitted to the atmosphere.

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.

Beta particles from tritium can penetrate only about 6.0 millimetres (0.24 in) of air, and they are incapable of passing through the dead outermost layer of human skin.

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

Helium-3 stockpiles have been further diminished by increased demand,[22] primarily for use in neutron radiation detectors and medical diagnostic procedures.

Substantial quantities of tritium and helium-3 could also be extracted from the heavy water moderator in CANDU nuclear reactors.

[40] 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.

[41][42][43][44] 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.

[48] During this process, 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.

In essence, this process effectively aligns the nuclear spins with the magnetic field in order to enhance the NMR signal.

[49] 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.

The conventional deuterium + tritium ("D–T") fusion process produces energetic neutrons which render reactor components radioactive with activation products.

The momentum energy of this proton (created in the fusion process) will interact with the containing electromagnetic field, resulting in direct net electricity generation.

High speed protons, as positively charged particles, can have their kinetic energy converted directly into electricity, through use of 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 electrical generator.

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.

[7] A number of people, starting with Gerald Kulcinski in 1986,[66] have proposed to explore the Moon, mine lunar regolith and use the helium-3 for fusion.

[67] 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.

[69][70] 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 which operation "each year, three space shuttle missions could bring enough fuel for all human beings across the world".

[76] The British Interplanetary Society's hypothetical Project Daedalus interstellar probe design was fueled by helium-3 mines in the atmosphere of Jupiter, for example.