Isotopes of helium

Although there are nine known isotopes of helium (2He) (standard atomic weight: $4.003$), only helium-3 (3Helium) and helium-4 (4Helium) are stable. All radioisotopes are short-lived, the longest-lived being 6Helium with a half-life of $806.92 milliseconds$. The least stable is 10Helium, with a half-life of $260 yoctoseconds$ ($0 s$), although it is possible that 2Helium may have an even shorter half-life.

In the Earth's atmosphere, the ratio of 3Helium to 4Helium is $0$. However, the isotopic abundance of helium varies greatly depending on its origin. In the Local Interstellar Cloud, the proportion of 3Helium to 4Helium is $0$, which is $121$ 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 3Helium and 4Helium below $0.8 K$ separate into two immiscible phases due to differences in quantum statistics: 4Helium atoms are bosons while 3Helium atoms are fermions. Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.

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

List of isotopes

 * rowspan=2|[[Helium|2Helium]]
 * rowspan=2 style="text-align:right" | 2
 * rowspan=2 style="text-align:right" | 0
 * rowspan="2" | $2.016$
 * rowspan=2 | ≪ $s$
 * p (> $99.99 %$)
 * [[Hydrogen|1Hydrogen]]
 * rowspan=2 | 0+#
 * rowspan=2 |
 * rowspan=2 |
 * β+ (< $0.01 %$)
 * [[Hydrogen|2Hydrogen]]
 * [[Helium|3Helium]]
 * style="text-align:right" | 2
 * style="text-align:right" | 1
 * colspan=3 align=center|Stable
 * 1/2+
 * [$3.016$, $0$]
 * [[Helium|4Helium]]
 * style="text-align:right" | 2
 * style="text-align:right" | 2
 * colspan=3 align=center|Stable
 * 0+
 * [$4.6$, $0$]
 * 5Helium
 * style="text-align:right" | 2
 * style="text-align:right" | 3
 * $4.003$ [$1$]
 * n
 * [[Helium|4Helium]]
 * 3/2−
 * rowspan=2|6Helium
 * rowspan=2 style="text-align:right" | 2
 * rowspan=2 style="text-align:right" | 4
 * rowspan=2|$1$
 * rowspan=2|$1$
 * β− ($5.012$%)
 * 6Lithium
 * rowspan=2|0+
 * rowspan=2|
 * rowspan=2|
 * β−d ($6.02 s$%)
 * 4Helium
 * 7Helium
 * style="text-align:right" | 2
 * style="text-align:right" | 5
 * $758 keV$ [$6.019$]
 * n
 * 6Helium
 * (3/2)−
 * rowspan=3|8Helium
 * rowspan=3 style="text-align:right" | 2
 * rowspan=3 style="text-align:right" | 6
 * rowspan=3|$806.92 ms$
 * rowspan=3|$100$
 * β− ($0$)
 * 8Lithium
 * rowspan=3|0+
 * rowspan=3|
 * rowspan=3|
 * β−n ($7.028$)
 * 7Lithium
 * β−t ($2.51 s$)
 * 5Helium
 * 9Helium
 * style="text-align:right" | 2
 * style="text-align:right" | 7
 * n
 * 8Helium
 * 1/2(+)
 * 10Helium
 * style="text-align:right" | 2
 * style="text-align:right" | 8
 * $182 keV$ [$8.034$]
 * 2n
 * 8Helium
 * 0+
 * rowspan=3|
 * rowspan=3|
 * β−n ($119.5 ms$)
 * 7Lithium
 * β−t ($83.1 %$)
 * 5Helium
 * 9Helium
 * style="text-align:right" | 2
 * style="text-align:right" | 7
 * n
 * 8Helium
 * 1/2(+)
 * 10Helium
 * style="text-align:right" | 2
 * style="text-align:right" | 8
 * $16 %$ [$0.9 %$]
 * 2n
 * 8Helium
 * 0+
 * 10Helium
 * style="text-align:right" | 2
 * style="text-align:right" | 8
 * $9.044$ [$2.5 s$]
 * 2n
 * 8Helium
 * 0+
 * 8Helium
 * 0+

Helium-2 (diproton)
Helium-2, or 2Helium, 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 state—all 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 2Helium. In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once—perhaps a 2Helium 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 18Neon, 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 2Helium 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 2Helium was found in 2008 at the Istituto Nazionale di Fisica Nucleare, in Italy. A beam of 20Neon ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into 18Neon nuclei. These nuclei then collided with a foil of lead. The second collision excited the 18Neon nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the 18Neon nucleus decayed into an 16Oxygen 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 the Joint Institute for Nuclear Research in Dubna, Russia, where beams of 6Helium nuclei were directed at a cryogenic hydrogen target to produce 5Hydrogen. It was discovered that the 6Helium nucleus can donate all four of its neutrons to the hydrogen. The two remaining protons could be simultaneously ejected from the target as a 2Helium nucleus, which quickly decayed into two protons. A similar reaction has also been observed from 8Helium 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 the electron cooling of the beam.

2Helium 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 + + $10.053$ → ,

followed by the immediate beta-plus decay of the diproton to deuterium:
 * → Deuterium + $+$ + $2.6 s$,

with the overall formula

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 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
3Helium is stable and is the only stable isotope other than 1Hydrogen with more protons than neutrons. (There are many such unstable isotopes, the lightest being 7Beryllium and 8Boron.) There is only a trace amount ($1.76 MeV$) of 3Helium 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, 3Helium is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, has trace amounts of 3Helium from solar wind bombardment.

For helium-3 to form a superfluid, it must be cooled to a temperature of $1.25 MeV$, or almost a thousand times lower than helium-4 ($1.67 MeV$). 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
The most common isotope, 4Helium, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized 4Helium nuclei. 4Helium 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 ($0.42 MeV$) of this isotope. Helium-4's boiling point of $0$ is the second lowest of all known substances, second only to helium-3. When cooled further to $0.003 K$, it transforms to a unique superfluid state of zero viscosity. It solidifies only at pressures above 25 atmospheres, where its melting point is $2.17 K$.

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 $1$. Helium-6 decays by emitting a beta particle and has a half-life of $4.2 K$. 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 6Helium and four neutrons in 8Helium). Halo nuclei have become an area of intense research. Isotopes up to helium-10, with two protons and eight neutrons, have been confirmed. 10Helium, despite being a doubly magic isotope, has a very short half-life; it is not particle-bound and near-instantaneously drips out two neutrons.