Helium-3
Helium-3 is a light, stable isotope of helium with two protons and one neutron. Helium-3 and hydrogen-1 are the only stable nuclides 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 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 reactions with cosmic rays. 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 atoms would be aneutronic, that is, it would not release the dangerous radiation of traditional fusion or require the much higher temperatures thereof. The process may unavoidably create other reactions that themselves would cause the surrounding material to become radioactive.
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, though still lower in abundance than in the Solar System's gas giants.
History
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. Oliphant had performed experiments in which fast deuterons collided with deuteron targets. Isolation of helium-3 was first accomplished by Luis Alvarez and Robert Cornog in 1939. 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.Physical properties
Due to its low atomic mass of 3.016 Da, helium-3 has some physical properties different from those of helium-4, with a mass of 4.0026 Da. 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 protons, two neutrons, and two electrons, helium-4 has an overall 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 K compared with helium-4 at 4.23 K, and its 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 kJ/mol compared with the 0.0829 kJ/mol of helium-4.
Superfluidity
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 particles, and therefore are composite fermions. This is a direct result of the addition rules for quantized angular momentum. In contrast, helium-4 atoms are bosons, containing an even number of spin- particles. At low temperatures, 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 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 pairs in the BCS theory of superconductivity. Each Cooper pair, having integer spin, can be thought of as a boson. During the 1970s, 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. 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 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.In a 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, 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,, and angular momentum one,. The ground state corresponds to total angular momentum zero, . Excited states are possible with non-zero total angular momentum,, 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 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.
Natural abundance
Terrestrial abundance
3He is a primordial substance in the Earth's mantle, thought to have been trapped during the planet's initial formation. The ratio of 3He to 4He within the Earth's crust and mantle is less than that in the solar disk, with terrestrial materials generally containing lower 3He/4He ratios due to production of 4He from radioactive decay.3He has a cosmological ratio of 300 atoms per million atoms of 4He, 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 the course of Earth's history, a significant amount of 4He has been generated by the alpha decay of uranium, thorium and other radioactive isotopes, to the point that only around 7% of the helium now in the mantle is primordial helium, thus lowering the total 3He:4He ratio to around 20 ppm. Ratios of 3He:4He in excess of the atmospheric ratio are indicative of a contribution of 3He from the mantle. Crustal sources are dominated by the 4He produced by radioactive decay.
The ratio of helium-3 to helium-4 in natural Earth-bound sources varies greatly. 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.
Helium itself is present as up to 7% of some natural gas sources, and large sources have over 0.5%. The fraction of 3He in helium separated from natural gas in the U.S. was found to range from 70 to 242 parts per billion. Hence the US 2002 stockpile of 1 billion normal m3 would have contained about of helium-3. According to American physicist Richard Garwin, about or almost of 3He is available annually for separation from the US natural gas stream. If the process of separating out the 3He could employ as feedstock the liquefied helium typically used to transport and store bulk quantities, estimates for the incremental energy cost range from NTP, excluding the cost of infrastructure and equipment. Algeria's annual gas production is assumed to contain 100 million normal cubic metres and this would contain between of helium-3 assuming a similar 3He fraction.
3He is also present in the Earth's atmosphere. The natural abundance of 3He in atmospheric helium is . The partial pressure of helium in the Earth's atmosphere is about, and thus helium accounts for 5.2 parts per million of the total pressure in the Earth's atmosphere, and 3He thus accounts for 7.2 parts per trillion of the atmosphere. Since the atmosphere of the Earth has a mass of about, the mass of 3He in the Earth's atmosphere is the product of these numbers and the molecular weight ratio of helium-3 to air, giving a mass of 3815 tonnes of helium-3 in the earth's atmosphere.
3He is produced on Earth from three sources: lithium spallation, cosmic rays, and beta decay of tritium. 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 0.1–1 megatonnes. Some helium-3 finds its way up through deep-sourced hotspot volcanoes such as those of the Hawaiian Islands, but only per year is emitted to the atmosphere. Mid-ocean ridges emit another 3 kg per year. Around subduction zones, various sources produce helium-3 in natural gas deposits which possibly contain a thousand tonnes of helium-3. Wittenberg estimated that United States crustal natural gas sources may have only half a tonne total. Wittenberg cited Anderson's estimate of another 1200 tonnes in interplanetary dust particles on the ocean floors. In the 1994 study, extracting helium-3 from these sources consumes more energy than fusion would release.