Helium

Isotopes

Although there are eight known isotopes of helium, only helium-3 and helium-4 are stable. In the Earth's atmosphere, there is one He-3 atom for every million He-4. However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of He-3 is around a hundred times higher. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to study the origin of such rocks.

Related Topics:
Isotope - Helium-3 - Helium-4 - Stable - Interstellar medium - Geology

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The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis, and its abundance serves as a test of cosmological models.

Related Topics:
Alpha decay - Alpha particle - Nucleon - Complete shells - Big Bang nucleosynthesis

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Equal mixtures of liquid He-3 and He-4 below 0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: He-4 atoms are bosons while He-3 atoms are fermions). There is only a trace amount of helium-3 on Earth, primarily present since the formation of the Earth, although some falls to Earth trapped in cosmic dust. Trace amounts are also produced by the beta decay of tritium. In stars, however, helium-3 is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds.

Related Topics:
Quantum statistics - Boson - Fermion - Beta decay - Tritium - Star - Nuclear fusion - Lunar - Asteroid - Regolith - Solar wind

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The different formation processes of the two stable isotopes of helium produce the differing isotope abundances. These differing isotope abundances can be used to investigate the origin of rocks and the composition of the Earth's mantle.

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It is possible to produce exotic helium isotopes, which rapidly decay into other substances. The shortest-lived isotope is helium-5 with a half-life of 7.6×10−22 second. Helium-6 decays by emitting a beta particle and has a half life of 0.8 second. Helium-7 also emits a beta particle as well as a gamma ray. Helium-7 and helium-8 are hyperfragments that are created in certain nuclear reactions.

Related Topics:
Exotic helium isotopes - Half-life - Beta particle - Gamma ray - Nuclear reaction

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