Isotopes of helium

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Although there are eight known isotopes of helium (He) (standard atomic mass: 4.002602(2) u), only helium-3 (3
He
) and helium-4 (4
He
) are stable. All radioisotopes are short-lived, the longest-lived being 6He with a half-life of 806.7 milliseconds. In the Earth's atmosphere, there is one 3
He
 atom for every million 4
He
 atoms.[1] However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of 3
He
 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 investigate the origin of rocks and the composition of the Earth's mantle.[3] The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.

Equal mixtures of liquid 3
He
 and 4
He
 below 0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: 4
He
 atoms are bosons while 3
He
 atoms are fermions).[4] Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.

Helium-2 (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.[citation needed] This atom would have two protons without any neutrons.

A diproton (or helium-2, symbol 2
He
) is a hypothetical type of helium nucleus consisting of two protons and no neutrons. Diprotons are not stable; this is due to spin-spin interactions in the nuclear force, and the Pauli exclusion principle, which forces the two protons to have anti-aligned spins and gives the diproton a negative binding energy.[5]

There may have been observations of unstable 2
He
. In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once - perhaps a 2
He
 nucleus.[6][7] The team led by Alfredo Galindo-Uribarri of the Oak Ridge National Laboratory announced that the discovery will help scientists understand the strong nuclear force and provide fresh insights into the creation of elements inside stars. Galindo-Uribarri and co-workers chose an isotope of neon with an energy structure that prevents it from emitting protons one at a time. This means that the two protons are ejected simultaneously. The team fired a beam of fluorine ions at a proton-rich target to produce 18
Ne
, which then decays into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. There are two ways in which the two-proton emission may proceed. The neon nucleus might eject a 'diproton' - a pair of protons bound together as a 2
He
 nucleus - which then decays into separate protons. Alternatively, the protons may be emitted separately but at the same time - so-called 'democratic decay'. The experiment was not sensitive enough to establish which of these two processes was taking place.

The best evidence of 2
He
 was found in 2008 at the Istituto Nazionale di Fisica Nucleare, in Italy. A beam of 20
Ne
 ions was collided into a foil of beryllium. In this collision some of the neon ended up as 18
Ne
 nuclei. These same nuclei then collided with a foil of lead. The second collision had the effect of exciting the 18
Ne
 nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the 18
Ne
 nucleus decayed into an 16
O
 nucleus, plus two protons detected exiting from the same direction. The new experiment showed that the two protons were initially ejected together before decaying into separate protons much less than a billionth of a second later.

Also, at RIKEN in Japan and JINR in Dubna, Russia, during productions of 5
He
 with collisions between a beam of 6
He
 nuclei and a cryogenic hydrogen target, it was discovered that the 6
He
 nucleus can donate all four of its neutrons to the hydrogen. This leaves two spare protons that may be simultaneously ejected from the target as a 2
He
 nucleus, which quickly decays into two protons. A similar reaction has also been observed from 8
He
 nuclei colliding with hydrogen.

Helium-3

Main article: Helium-3

There is only a trace amount of 3
He
 on Earth, primarily present since the formation of the Earth, although some falls to Earth trapped in cosmic dust.[3] Trace amounts are also produced by the beta decay of tritium.[8] In stars, however, 3
He
 is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of 3
He
 from being bombarded by solar winds.

Helium-4

Main article: Helium-4

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

Heavier helium isotopes

Although all heavier helium isotopes decay with a half-life of less than one second, researchers have eagerly created new 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 heavy helium isotope is helium-8. This isotope, as well as helium-6, are thought to consist of a normal helium-4 nucleus surrounded by a neutron "halo" (two for 6
He
 and four for 8
He
). 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.[9]

Table

nuclide
symbol 		Z(p) N(n) isotopic mass (u) half-life decay
mode(s)[10] 		daughter
isotope(s)[n 1] 		nuclear
spin 		representative
isotopic
composition
(mole fraction) 		range of natural
variation
(mole fraction)
3
He
[n 2] 		2 1 3.0160293191(26) Stable[n 3] 1/2+ 1.34(3)×10−6 4.6×10−10-4.1×10−5
4
He
[n 2] 		2 2 4.00260325415(6) Stable 0+ 0.99999866(3) 0.999959-1
5
He
 		2 3 5.01222(5) 700(30)×10−24 s
[0.60(2) MeV] 		n 4
He
 		3/2-
6
He
[n 4] 		2 4 6.0188891(8) 806.7(15) ms β- (99.99%) 6
Li
 		0+
β-, fission (2.8×10−4%) 4
He
, 2
H
7
He
 		2 5 7.028021(18) 2.9(5)×10−21 s
[159(28) keV] 		n 6
He
 		(3/2)-
8
He
[n 5] 		2 6 8.033922(7) 119.0(15) ms β- (83.1%) 8
Li
 		0+
β-,n (16.0%) 7
Li
β-, fission (.09%) 5
He
, 3
H
9
He
 		2 7 9.04395(3) 7(4)×10−21 s
[100(60) keV] 		n 8
He
 		1/2(-#)
10
He
 		2 8 10.05240(8) 2.7(18)×10−21 s
[0.17(11) MeV] 		2n 8
He
 		0+
  1. ^ Bold for stable isotopes
  2. ^ a b Produced during Big bang nucleosynthesis
  3. ^ This and 1H are the only stable nuclides with more protons than neutrons
  4. ^ Has 2 halo neutrons
  5. ^ Has 4 halo neutrons

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.

Template:Wikipedia-Books

References

  1. ^ J. Emsley (2001). Nature's Building Blocks: An A-Z Guide to the Elements. Oxford University Press. p. 178. ISBN 0-19-850340-7.
  2. ^ G.N. Zastenker; et al. (2002). "Isotopic Composition and Abundance of Interstellar Neutral Helium Based on Direct Measurements". Astrophysics. 45 (2): 131–142. Bibcode:2002Ap.....45..131Z. doi:10.1023/A:1016057812964. {{cite journal}}: Explicit use of et al. in: |author= (help)
  3. ^ a b "Helium Fundamentals".
  4. ^ The Encyclopedia of the Chemical Elements. p. 264.
  5. ^ “Nuclear Physics in a Nutshell”, C.A. Bertulani, Princeton University Press, Princeton, NJ, 2007, Chapter 1, ISBN 978-0-691-12505-3.
  6. ^ Physicists discover new kind of radioactivity, in physicsworld.com Oct 24, 2000
  7. ^ Decay of a Resonance in 18Ne by the Simultaneous Emission of Two Protons, Physical Review Online Archive, by del Campo, Galindo-Uribarri et al.
  8. ^ K. L. Barbalace. "Periodic Table of Elements: Li - Lithium". EnvironmentalChemistry.com. Retrieved 2010-09-13.
  9. ^ The Encyclopedia of the Chemical Elements. p. 260.
  10. ^ http://www.nucleonica.net/unc.aspx

External links

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