The ratio of helium-3 to helium-4 in natural Earth-bound sources varies greatly. Crustal sources are dominated by the 4He produced by radioactive decay. Ratios of 3He/ 4He in excess of atmospheric are indicative of a contribution of 3He from the mantle. Over Earth's history alpha-particle decay of uranium, thorium and other radioactive isotopes has generated significant amounts of 4He, such that only around 7% of the helium now in the mantle is primordial helium, lowering the total 3He/ 4He ratio to around 20 ppm. ppm), leading to the assumption that the original ratio of these primordial gases in the mantle was around 200-300 ppm when Earth was formed. 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.ģHe has a cosmological ratio of 300 atoms per million atoms of 4He (at. Natural abundance Terrestrial abundance ģHe is a primordial substance in the Earth's mantle, considered to have become entrapped within the Earth during planetary formation. Its latent heat of vaporization is also considerably lower at 0.026 kJ/mol compared with the 0.0829 kJ/mol of helium-4. 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. Helium-3 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. 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. This implies that helium-3 can overcome dipole–dipole interactions with less thermal energy than helium-4 can.

Also, the microscopic properties of helium-3 cause it to have a higher zero-point energy than helium-4. Because of the weak, induced dipole–dipole interaction between the helium atoms, their microscopic physical properties are mainly determined by their zero-point energy. Physical properties īecause of its low atomic mass of 3.016 u, helium-3 has some physical properties different from those of helium-4, with a mass of 4.0026 u. 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. Isolation of helium-3 was first accomplished by Luis Alvarez and Robert Cornog in 1939. Oliphant had performed experiments in which fast deuterons collided with deuteron targets (incidentally, the first demonstration of nuclear fusion). 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. 5.4 Radio energy absorber for tokamak plasma experiments.3.3 Solar nebula (primordial) abundance.The abundance of helium-3 is thought to be greater on the Moon than on Earth, having been embedded 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. However, the temperatures required to achieve helium-3 fusion reactions are much higher than in traditional fusion reactions, and the process may unavoidably create other reactions that themselves would cause the surrounding material to become radioactive. Unlike most nuclear fusion reactions, the fusion of helium-3 atoms releases large amounts of energy without causing the surrounding material to become radioactive. Much speculation has been made over the possibility of helium-3 as a future energy source. Some of the helium-3 found in the terrestrial atmosphere is also an artifact of atmospheric and underwater nuclear weapons testing.

Helium-3 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. Helium-3 occurs as a primordial nuclide, escaping from Earth's crust into its atmosphere and into outer space over millions of years. Other than protium (ordinary hydrogen), helium-3 is the only stable isotope of any element with more protons than neutrons.

Helium-3 ( 3He see also helion) is a light, stable isotope of helium with two protons and one neutron (the most common isotope, helium-4, having two protons and two neutrons in contrast). For the record label, see Helium 3 (record label).

Mass of helium atom