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Isotopes of helium

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Isotopes of helium (2He)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
3He 0.0002% stable
4He 99.9998% stable
Standard atomic weight Ar°(He)

Helium (2He) (standard atomic weight: 4.002602(2)) has nine known isotopes, but only helium-3 (3He) and helium-4 (4He) are stable.[4] All radioisotopes are short-lived; the longest-lived is 6He with half-life 806.92(24) milliseconds. The least stable is 10He, with half-life 260(40) yoctoseconds (2.6(4)×10−22 s), though 2He may have an even shorter half-life.

In Earth's atmosphere, the ratio of 3He to 4He is 1.343(13)×10−6.[5] However, the isotopic abundance of helium varies greatly depending on its origin. In the Local Interstellar Cloud, the proportion of 3He to 4He is 1.62(29)×10−4,[6] which is ~121 times higher than in Earth's atmosphere. Rocks from 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.[7] The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.

Equal mixtures of liquid 3He and 4He below 0.8 K separate into two immiscible phases due to differences in quantum statistics: 4He atoms are bosons while 3He atoms are fermions.[8] Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvin.

A mix of the two isotopes spontaneously separates into 3He-rich and 4He-rich regions.[9] Phase separation also exists in ultracold gas systems.[10] It has been shown experimentally in a two-component ultracold Fermi gas case.[11][12] The phase separation can compete with other phenomena as vortex lattice formation or an exotic Fulde–Ferrell–Larkin–Ovchinnikov phase.[13]

List of isotopes

[edit]
Nuclide
 Z N Isotopic mass (Da)[14]
[n 1] 		Half-life[1]

[resonance width] 		Decay
mode[1]
[n 2] 		Daughter
isotope
[n 3] 		Spin and
parity[1]
[n 4][n 5] 		Natural abundance (mole fraction)
Normal proportion[1] Range of variation
2He[n 6] 2 0 2.015894(2) ≪ 10−9 s[15] p (> 99.99%) 1H 0+#
β+ (< 0.01%) 2H
3He[n 7][n 8] 2 1 3.016029321967(60) Stable 1/2+ 0.000002(2)[16] [4.6×10−10, 0.000041][17]
4He[n 7] 2 2 4.002603254130(158) Stable 0+ 0.999998(2)[16] [0.999959, 1.000000][17]
5He 2 3 5.012057(21) 6.02(22)×10−22 s
[758(28) keV] 		n 4He 3/2−
6He[n 9] 2 4 6.018885889(57) 806.92(24) ms β− (99.999722(18)%) 6He 0+
β−d[n 10] (0.000278(18)%) 4He
7He 2 5 7.027991(8) 2.51(7)×10−21 s
[182(5) keV] 		n 6He (3/2)−
8He[n 11] 2 6 8.033934388(95) 119.5(1.5) ms β− (83.1(1.0)%) 8Li 0+
β−n (16(1)%) 7Li
β−t[n 12] (0.9(1)%) 5He
9He 2 7 9.043946(50) 2.5(2.3)×10−21 s n 8
He
 		1/2(+)
10He 2 8 10.05281531(10) 2.60(40)×10−22 s
[1.76(27) MeV] 		2n 8He 0+
This table header & footer:
  1. ^ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  2. ^ Modes of decay:
    n: Neutron emission
    p: Proton emission
  3. ^ Bold symbol as daughter – Daughter product is stable.
  4. ^ ( ) spin value – Indicates spin with weak assignment arguments.
  5. ^ # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  6. ^ Intermediate in the proton–proton chain
  7. ^ a b Produced in Big Bang nucleosynthesis
  8. ^ This and 1H are the only stable nuclei with more protons than neutrons
  9. ^ Has 2 halo neutrons
  10. ^ d: Deuteron emission
  11. ^ Has 4 halo neutrons
  12. ^ t: Triton emission

Helium-2 (diproton)

[edit]

Helium-2, 2He, is extremely unstable. Its nucleus, a diproton, consists of two protons with no neutrons. According to theoretical calculations, it would be much more stable (but still β+ decay to deuterium) if the strong force were 2% greater.[18] Its instability is due to spin–spin interactions in the nuclear force and the Pauli exclusion principle, which states that within a given quantum system two or more identical particles with the same half-integer spins (that is, fermions) cannot simultaneously occupy the same quantum state; so 2He's two protons have opposite-aligned spins and the diproton itself has negative binding energy.[19]

2He may have been observed. In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once—perhaps 2He.[20][21] The team led by Alfredo Galindo-Uribarri of Oak Ridge National Laboratory announced that the discovery will help understand the strong nuclear force and provide fresh insights into stellar nucleosynthesis. 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 the two protons are ejected simultaneously. The team fired a beam of fluorine ions at a proton-rich target to produce 18Ne, which then decayed into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. The two-proton emission may proceed in two ways. The neon might eject a diproton, which then decays into separate protons. Or the protons may be emitted separately but simultaneously: "democratic decay". The experiment was not sensitive enough to establish which of these two processes was taking place.

More evidence of 2He was found in 2008 at Istituto Nazionale di Fisica Nucleare, in Italy.[15][22] A beam of 20Ne ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into 18Ne nuclei. These nuclei then collided with a foil of lead. The second collision excited the 18Ne nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the 18Ne nucleus decayed into an 16O nucleus, plus two protons detected exiting from the same direction. The new experiment showed that the two protons were initially ejected together, correlated in a quasibound 1S configuration, before decaying into separate protons much less than a nanosecond later.

Further evidence comes from Riken in Japan and Joint Institute for Nuclear Research in Dubna, Russia, where beams of 6He nuclei were directed at a cryogenic hydrogen target to produce 5H. It was discovered that the 6He can donate all four of its neutrons to the hydrogen.[citation needed] The two remaining protons could be simultaneously ejected from the target as a diproton, which quickly decayed into two protons. A similar reaction has also been observed from 8He nuclei colliding with hydrogen.[23]

Under the influence of electromagnetic interactions, the Jaffe-Low primitives [24] may leave the unitary cut, creating narrow two-nucleon resonances, like a diproton resonance with a mass of 2000 MeV and a width of a few hundred keV. [25] To search for this resonance, a beam of protons with kinetic energy 250 MeV, and an energy spread below 100 keV, is required, which is feasible considering the electron cooling of the beam.

2He is an intermediate in the first step of the proton–proton chain. The first step of the proton-proton chain is a two-stage process: first, two protons fuse to form a diproton:

1H + 1H + 1.25 MeV → 2He;

then the diproton immediately beta-plus decays into deuterium:

2He → 2H + e+ + νe + 1.67 MeV;

with the overall formula

1H + 1H → 2H + e+ + νe 0.42 MeV.

The hypothetical effect of a bound diproton on Big Bang and stellar nucleosynthesis, has been investigated.[18] Some models suggest that variations in the strong force allowing a bound diproton would enable the conversion of all primordial hydrogen to helium in the Big Bang, which would be catastrophic for the development of stars and life. This notion is an example of the anthropic principle. However, a 2009 study suggests that such a conclusion can't be drawn, as the formed diproton would still decay to deuterium, whose binding energy would also increase. In some scenarios, it is postulated that hydrogen (in the form of 2H) could still survive in large amounts, rebutting arguments that the strong force is tuned within a precise anthropic limit.[26]

Helium-3

[edit]
Main article: Helium-3

3He is the only stable isotope other than 1H with more protons than neutrons. (There are many such unstable isotopes; the lightest are 7Be and 8B.) There is only a trace (~2ppm)[16] of 3He on Earth, mainly present since the formation of the Earth, although some falls to Earth trapped in cosmic dust.[7] Trace amounts are also produced by the beta decay of tritium.[27] In stars, however, 3He is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, has traces of 3He from solar wind bombardment.

To become superfluid, 3He must be cooled to 2.5 millikelvin, ~900 times lower than 4He (2.17 K). This difference is explained by quantum statistics: 3He atoms are fermions, while 4He atoms are bosons, which condense to a superfluid more easily.

Helium-4

[edit]
Main article: Helium-4

The most common isotope, 4He, is produced on Earth by alpha decay of heavier elements; the alpha particles that emerge are fully ionized 4He nuclei. 4He is an unusually stable nucleus because it is doubly magic. It was formed in enormous quantities in Big Bang nucleosynthesis.

Terrestrial helium consists almost exclusively (all but ~2ppm)[16] of 4He. 4He's boiling point of 4.2 K is the lowest of all known substances except 3He. When cooled further to 2.17 K, it becomes a unique superfluid with zero viscosity. It solidifies only at pressures above 25 atmospheres, where it melts at 0.95 K.

Heavier helium isotopes

[edit]

Though all heavier helium isotopes decay with a half-life of <1 second, particle accelerator collisions have been used, to create unusual nuclei of elements such as helium, lithium and nitrogen. The unusual nuclear structures of such isotopes may offer insights into the isolated properties of neutrons and physics beyond the Standard Model.[28][29]

The shortest-lived isotope is 10He with half-life ~260 yoctoseconds. 6He beta decays with half-life 807 milliseconds. The most widely studied heavy helium isotope is 8He. 8He and 6He are thought to consist of a normal 4He nucleus surrounded by a neutron "halo" (of two neutrons in 6He and four neutrons in 8He). Halo nuclei have become an area of intense research. Isotopes up to 10He, with two protons and eight neutrons, have been confirmed. 10He, despite being a doubly magic isotope, is not particle-bound and near-instantly drips out two neutrons.[30]

References

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