Английская Википедия:Isotopes of helium

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Шаблон:Short description Шаблон:Infobox helium isotopes Although there are nine known isotopes of helium (₂He) (standard atomic weight: Шаблон:Val), only helium-3 (Шаблон:SimpleNuclide) and helium-4 (Шаблон:SimpleNuclide) are stable. All radioisotopes are short-lived, the longest-lived being Шаблон:SimpleNuclide with a half-life of Шаблон:Val. The least stable is Шаблон:SimpleNuclide, with a half-life of Шаблон:Val (Шаблон:Val), although it is possible that Шаблон:SimpleNuclide may have an even shorter half-life.

In the Earth's atmosphere, the ratio of Шаблон:SimpleNuclide to Шаблон:SimpleNuclide is Шаблон:Val. However, the isotopic abundance of helium varies greatly depending on its origin. In the Local Interstellar Cloud, the proportion of Шаблон:SimpleNuclide to Шаблон:SimpleNuclide is Шаблон:Val, which is Шаблон:Val times higher than that of atmospheric helium. 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. The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.

Equal mixtures of liquid Шаблон:SimpleNuclide and Шаблон:SimpleNuclide below Шаблон:Val separate into two immiscible phases due to differences in quantum statistics: Шаблон:SimpleNuclide atoms are bosons while Шаблон:SimpleNuclide atoms are fermions. Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.

List of isotopes

Шаблон:Isotopes table |- | rowspan=2|[[#Helium-2_(diproton)|Шаблон:SimpleNuclide]] | rowspan=2 style="text-align:right" | 2 | rowspan=2 style="text-align:right" | 0 | rowspan="2" | Шаблон:Val | rowspan=2 | ≪ Шаблон:Val | p (> Шаблон:Val) | [[Hydrogen atom|Шаблон:SimpleNuclide]] | rowspan=2 | 0+# | rowspan=2 | | rowspan=2 | |- | β⁺ (< Шаблон:Val) | [[Deuterium|Шаблон:SimpleNuclide]] |- | [[Helium-3|Шаблон:SimpleNuclide]] | style="text-align:right" | 2 | style="text-align:right" | 1 | Шаблон:Val | colspan=3 align=center|Stable | 1/2+ | Шаблон:Val | [[[:Шаблон:Val]], Шаблон:Val] |- | [[Helium-4|Шаблон:SimpleNuclide]] | style="text-align:right" | 2 | style="text-align:right" | 2 | Шаблон:Val | colspan=3 align=center|Stable | 0+ | Шаблон:Val | [[[:Шаблон:Val]], Шаблон:Val] |- | Шаблон:SimpleNuclide | style="text-align:right" | 2 | style="text-align:right" | 3 | Шаблон:Val | Шаблон:Val
[[[:Шаблон:Val]]] | n | [[Helium-4|Шаблон:SimpleNuclide]] | 3/2− | | |- | rowspan=2|Шаблон:SimpleNuclide | rowspan=2 style="text-align:right" | 2 | rowspan=2 style="text-align:right" | 4 | rowspan=2|Шаблон:Val | rowspan=2|Шаблон:Val | β⁻ (Шаблон:Val%) | Шаблон:SimpleNuclide | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β⁻d (Шаблон:Val%) | Шаблон:SimpleNuclide |- | Шаблон:SimpleNuclide | style="text-align:right" | 2 | style="text-align:right" | 5 | Шаблон:Val | Шаблон:Val
[[[:Шаблон:Val]]] | n | Шаблон:SimpleNuclide | (3/2)− | | |- | rowspan=3|Шаблон:SimpleNuclide | rowspan=3 style="text-align:right" | 2 | rowspan=3 style="text-align:right" | 6 | rowspan=3|Шаблон:Val | rowspan=3|Шаблон:Val | β⁻ (Шаблон:Val) | Шаблон:SimpleNuclide | rowspan=3|0+ | rowspan=3| | rowspan=3| |- | β⁻n (Шаблон:Val) | Шаблон:SimpleNuclide |- | β⁻t (Шаблон:Val) | Шаблон:SimpleNuclide |- | Шаблон:SimpleNuclide | style="text-align:right" | 2 | style="text-align:right" | 7 | Шаблон:Val | Шаблон:Val | n | Шаблон:SimpleNuclide | 1/2(+) | | |- | Шаблон:SimpleNuclide | style="text-align:right" | 2 | style="text-align:right" | 8 | Шаблон:Val | Шаблон:Val
[[[:Шаблон:Val]]] | 2n | Шаблон:SimpleNuclide | 0+ | | Шаблон:Isotopes table/footer

Helium-2 (diproton)

Helium-2, or Шаблон:SimpleNuclide, is an extremely unstable isotope of helium. Its nucleus, a diproton, consists of two protons with no neutrons. According to theoretical calculations, it would have been much more stable (although still undergoing β⁺ decay to deuterium) if the strong interaction had been 2% greater. Its instability is due to spin–spin interactions in the nuclear force and to the quantum mechanics described by 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 stateШаблон:Mdashall which presents for helium-2 that its two protons (of the diproton) have opposite-aligned spins and the diproton itself has a negative binding energy.

There may have been observations of Шаблон:SimpleNuclide. In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once—perhaps a Шаблон:SimpleNuclide nucleus. 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 Шаблон:SimpleNuclide, which then decayed 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 bundled together as a Шаблон:SimpleNuclide nucleus—which then decays into separate protons. Alternatively, the protons may be emitted separately but simultaneously—so-called "democratic decay". The experiment was not sensitive enough to establish which of these two processes was taking place.

More evidence of Шаблон:SimpleNuclide was found in 2008 at the Istituto Nazionale di Fisica Nucleare, in Italy. A beam of Шаблон:SimpleNuclide ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into Шаблон:SimpleNuclide nuclei. These nuclei then collided with a foil of lead. The second collision had the effect of exciting the Шаблон:SimpleNuclide nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the Шаблон:SimpleNuclide nucleus decayed into an Шаблон:SimpleNuclide 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 JapanШаблон:Citation needed and the Joint Institute for Nuclear Research in Dubna, Russia,Шаблон:Citation needed where beams of Шаблон:SimpleNuclide nuclei were directed at a cryogenic hydrogen target to produce Шаблон:SimpleNuclide. It was discovered that the Шаблон:SimpleNuclide nucleus can donate all four of its neutrons to the hydrogen.Шаблон:Citation needed The two remaining protons could be simultaneously ejected from the target as a Шаблон:SimpleNuclide nucleus, which quickly decayed into two protons. A similar reaction has also been observed from Шаблон:SimpleNuclide nuclei colliding with hydrogen.

Under the influence of electromagnetic interactions, the Jaffe-Low primitives 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. To search for this resonance, a beam of protons with kinetic energy T = 250 MeV and an energy spread below 100 keV is required, which is feasible considering electron cooling of the beam.

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

[[Hydrogen atom|Шаблон:Nuclide]] + Шаблон:Nuclide + Шаблон:ValШаблон:Citation needed → Шаблон:Nuclide,

followed by the immediate beta-plus decay of the diproton to deuterium:

Шаблон:Nuclide → [[Deuterium|Шаблон:Nuclide]] + Шаблон:Math + Шаблон:ValШаблон:Citation needed,

with the overall formula

Шаблон:Nuclide + Шаблон:Nuclide → Шаблон:Nuclide + Шаблон:Math + Шаблон:ValШаблон:Citation needed.

The hypothetical effect of the binding of the diproton on Big Bang and stellar nucleosynthesis has been investigated. Some models suggest that variations in the strong force allowing the existence of a bound diproton would enable the conversion of all primordial hydrogen to helium in the Big Bang, with catastrophic consequences on the development of stars and life. This proposition is used as an example of the anthropic principle. However, a 2009 study suggests that such a conclusion cannot be drawn, as the formed diprotons would still decay to deuterium, whose binding energy would also increase. In some scenarios, it is postulated that hydrogen (in the form of deuterium) could still survive in relatively large quantities, rebutting arguments that the strong force is tuned within a precise anthropic limit.

Helium-3

Шаблон:Main Шаблон:SimpleNuclide is stable and is the only stable isotope other than Шаблон:SimpleNuclide with more protons than neutrons. (There are many such unstable isotopes, the lightest being Шаблон:SimpleNuclide and Шаблон:SimpleNuclide.) There is only a trace amount (Шаблон:Val) of Шаблон:SimpleNuclide 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, Шаблон:SimpleNuclide is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, has trace amounts of Шаблон:SimpleNuclide from solar wind bombardment.

For helium-3 to form a superfluid, it must be cooled to a temperature of Шаблон:Val, or almost a thousand times lower than helium-4 (Шаблон:Val). This difference is explained by quantum statistics, since helium-3 atoms are fermions, while helium-4 atoms are bosons, which condense to a superfluid more easily.

Helium-4

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

Terrestrial helium consists almost exclusively (Шаблон:Val) of this isotope. Helium-4's boiling point of Шаблон:Val is the second lowest of all known substances, second only to helium-3. When cooled further to Шаблон:Val, it transforms to a unique superfluid state of zero viscosity. It solidifies only at pressures above 25 atmospheres, where its melting point is Шаблон:Val.

Heavier helium isotopes

Although all heavier helium isotopes decay with a half-life of less than one second, researchers have used particle accelerator collisions to create unusual atomic nuclei for 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.

The shortest-lived isotope is helium-10 with a half-life of Шаблон:Val. Helium-6 decays by emitting a beta particle and has a half-life of Шаблон:Val. The most widely studied heavy helium isotope is helium-8. This isotope, as well as helium-6, is thought to consist of a normal helium-4 nucleus surrounded by a neutron "halo" (containing two neutrons in Шаблон:SimpleNuclide and four neutrons in Шаблон:SimpleNuclide). Halo nuclei have become an area of intense research. Isotopes up to helium-10, with two protons and eight neutrons, have been confirmed. Шаблон:SimpleNuclide, despite being a doubly magic isotope, has a very short half-life; it is not particle-bound and near-instantaneously drips out two neutrons.

External links

• General Tables — abstracts for helium and other exotic light nuclei