Isotopes of helium

Helium (He)
Standard atomic mass: 4.002602(2) u

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Natural helium isotopes

For more details on this topic, see Helium-3.

For more details on this topic, see Helium-4.

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 atoms.^[1] 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.^[2] 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.

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.

Equal mixtures of liquid helium-3 and helium-4 below 0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: helium-4 atoms are bosons while helium-3 atoms are fermions).^[3] Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins. 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.^[4] Trace amounts are also produced by the beta decay of tritium.^[5] 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.

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.^[4]

Exotic helium isotopes

A subset of exotic light nuclei, the exotic helium isotopes have larger atomic masses than helium's natural isotopes. Although all exotic helium isotopes decay with a half-life of less than one second, researchers have eagerly created exotic light isotopes through particle accelerator collisions to create unusual atomic nuclei for elements such as helium, lithium, and nitrogen. The bizarre nuclear structures of such isotopes may offer insight into the isolated properties of neutrons.

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. The most widely-studied exotic helium isotope is helium-8. This isotope is thought to consist of a normal helium-4 nucleus surrounded by four neutrons dubbed a "halo" (⁶He also has a halo of neutrons). Halo nuclei have become an area of intense research. Isotopes up to helium-10, with two protons and eight neutrons, have been confirmed. Helium-7 and helium-8 are hyperfragments that are created in certain nuclear reactions.^[6]

Helium-2 (diproton)

For more details on this topic, see Diproton.

Helium-2 is a hypothetical isotope of Helium which according to theoretical calculations would have existed if the strong force had been 2% greater.

Table

Notes

• The isotopic composition refers to that in air.
• The precision of the isotope abundances and atomic mass is limited through variations. The given ranges should be applicable to any normal terrestrial material.
• Geologically exceptional samples are known in which the isotopic composition lies outside the reported range. The uncertainty in the atomic mass may exceed the stated value for such specimens.
• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.

References

• Isotope masses from Ame2003 Atomic Mass Evaluation by G. Audi, A.H. Wapstra, C. Thibault, J. Blachot and O. Bersillon in Nuclear Physics A729 (2003).
• Isotopic compositions and standard atomic masses from Atomic weights of the elements. Review 2000 (IUPAC Technical Report). Pure Appl. Chem. Vol. 75, No. 6, pp. 683-800, (2003) and Atomic Weights Revised (2005).
• Half-life, spin, and isomer data selected from these sources. Editing notes on this article's talk page.
  • Audi, Bersillon, Blachot, Wapstra. The Nubase2003 evaluation of nuclear and decay properties, Nuc. Phys. A 729, pp. 3-128 (2003).
  • National Nuclear Data Center, Brookhaven National Laboratory. Information extracted from the NuDat 2.1 database (retrieved Sept. 2005).
  • David R. Lide (ed.), Norman E. Holden in CRC Handbook of Chemistry and Physics, 85th Edition, online version. CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes.