Helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

(He) ( standard atomic weight: ) has nine known

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

s, but only

helium-3 Helium-3 (3He see also helion) is a light, stable isotope of helium with two protons and one neutron. (In contrast, the most common isotope, helium-4, has two protons and two neutrons.) Helium-3 and hydrogen-1 are the only stable nuclides with ...

(He) and

helium-4 Helium-4 () is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consi ...

(He) are

stable A stable is a building in which working animals are kept, especially horses or oxen. The building is usually divided into stalls, and may include storage for equipment and feed. Styles There are many different types of stables in use tod ...

. All

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...

s are short-lived; the longest-lived is He with

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

. The least stable is He, with half-life (), though He may have an even shorter half-life. In Earth's atmosphere, the ratio of He to He is . However, the isotopic abundance of helium varies greatly depending on its origin. In the Local Interstellar Cloud, the proportion of He to He is , 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 Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

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 He and He below separate into two

immiscible Miscibility () is the property of two chemical substance, substances to mix in all mixing ratio, proportions (that is, to fully dissolution (chemistry), dissolve in each other at any concentration), forming a homogeneity and heterogeneity, homoge ...

phases due to differences in quantum statistics: He atoms are

boson In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0, 1, 2, ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have half odd-intege ...

s while He atoms are

fermion In particle physics, a fermion is a subatomic particle that follows Fermi–Dirac statistics. Fermions have a half-integer spin (spin 1/2, spin , Spin (physics)#Higher spins, spin , etc.) and obey the Pauli exclusion principle. These particles i ...

s. Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few milli

kelvin The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature (absolute zero), taken to be 0 K. By de ...

. A mix of the two isotopes spontaneously separates into He-rich and He-rich regions. Phase separation also exists in ultracold gas systems. It has been shown experimentally in a two-component ultracold

Fermi gas A Fermi gas is an idealized model, an ensemble of many non-interacting fermions. Fermions are particles that obey Fermi–Dirac statistics, like electrons, protons, and neutrons, and, in general, particles with half-integer spin. These statis ...

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, HeIntermediate in the proton–proton chain , rowspan=2 style="text-align:right" , 2 , rowspan=2 style="text-align:right" , 0 , rowspan="2" , , rowspan=2 , ≪ , p (> ) , H , rowspan=2 , 0+# , rowspan=2 , , rowspan=2 , , - , β (< ) , H , - , HeProduced in

Big Bang nucleosynthesis In physical cosmology, Big Bang nucleosynthesis (also known as primordial nucleosynthesis, and abbreviated as BBN) is a model for the production of light nuclei, deuterium, 3He, 4He, 7Li, between 0.01s and 200s in the lifetime of the universe ...

This and H are the only stable nuclei with more protons than neutrons , style="text-align:right" , 2 , style="text-align:right" , 1 , , colspan=3 align=center, Stable , 1/2+ , , ref name="Isotopic Compositions of Elements 2013"> , - , HeProduced in

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an a ...

, style="text-align:right" , 2 , style="text-align:right" , 2 , , colspan=3 align=center, Stable , 0+ , , ref name="Isotopic Compositions of Elements 2013"/> , - , He , style="text-align:right" , 2 , style="text-align:right" , 3 , ,
[] , neutron emission, n , He , 3/2− , , , - , rowspan=2, HeHas 2 halo nucleus, halo neutrons , rowspan=2 style="text-align:right" , 2 , rowspan=2 style="text-align:right" , 4 , rowspan=2, , rowspan=2, , β (%) , Li , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , βdd: Deuteron emission (%) , He , -id=Helium-7 , He , style="text-align:right" , 2 , style="text-align:right" , 5 , ,
[] , n , He , (3/2)− , , , - , rowspan=3, HeHas 4 halo neutrons , rowspan=3 style="text-align:right" , 2 , rowspan=3 style="text-align:right" , 6 , rowspan=3, , rowspan=3, , β () , Li , rowspan=3, 0+ , rowspan=3, , rowspan=3, , - , βn () , Li , - , βtt: Triton emission () , He , -id=Helium-9 , He , style="text-align:right" , 2 , style="text-align:right" , 7 , , , n , , 1/2(+) , , , - , He , style="text-align:right" , 2 , style="text-align:right" , 8 , ,
[] , 2n , He , 0+ , ,

Helium-2 (diproton)

Helium-2, He, is extremely unstable. Its nucleus, a diproton, consists of two protons with no

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

s. According to theoretical calculations, it would be much more stable (but still β decay to

deuterium Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...

) if the strong force were 2% greater. Its instability is due to spin–spin interactions in the

nuclear force The nuclear force (or nucleon–nucleon interaction, residual strong force, or, historically, strong nuclear force) is a force that acts between hadrons, most commonly observed between protons and neutrons of atoms. Neutrons and protons, both ...

and the Pauli exclusion principle, which states that within a given

quantum system Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...

two or more identical particles with the same half-integer spins (that is, fermions) cannot simultaneously occupy the same quantum state; so He's two protons have opposite-aligned spins and the diproton itself has negative

binding energy In physics and chemistry, binding energy is the smallest amount of energy required to remove a particle from a system of particles or to disassemble a system of particles into individual parts. In the former meaning the term is predominantly use ...

. He 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 He. 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 In astrophysics, stellar nucleosynthesis is the creation of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. As a ...

. 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 Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

ions at a proton-rich target to produce Ne, 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 in a "democratic decay". The experiment was not sensitive enough to establish which of these two processes was taking place. More evidence of He was found in 2008 at Istituto Nazionale di Fisica Nucleare, in Italy. A beam of Ne ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into Ne nuclei. These nuclei then collided with a foil of lead. The second collision excited the Ne nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the Ne nucleus decayed into an O nucleus, plus two

protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' ( elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an electron (the pro ...

detected exiting from the same direction. The new experiment showed that the two protons were initially ejected together, correlated in a quasibound S configuration, before decaying into separate protons much less than a nanosecond later. Further evidence comes from Riken in

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

and Joint Institute for Nuclear Research in

Dubna Dubna ( rus, Дубна́, p=dʊbˈna) is a town in Moscow Oblast, Russia. It has a status of '' naukograd'' (i.e. town of science), being home to the Joint Institute for Nuclear Research, an international nuclear physics research center and o ...

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

, where beams of He nuclei were directed at a cryogenic

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

target to produce H. It was discovered that the He can donate all four of its neutrons to the hydrogen. 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 He nuclei colliding with hydrogen. Under the influence of

electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

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 250 MeV and an energy spread below 100 keV is required, which is feasible considering the electron cooling of the beam. He 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: :H + H + → He; in a low-probability branch, the diproton beta-plus decays into

: :He → H + e + ν + ; with the overall formula :H + H → H + e + ν + . More than 99.99% of the time the diproton fissions back to two protons. The hypothetical effect of a bound diproton on

Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...

and

, has been investigated. 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 In cosmology, the anthropic principle, also known as the observation selection effect, is the proposition that the range of possible observations that could be made about the universe is limited by the fact that observations are only possible in ...

. 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 H) could still survive in large amounts, rebutting arguments that the strong force is tuned within a precise anthropic limit.

Helium-3

He is the only stable isotope other than H with more protons than neutrons. (There are many such unstable isotopes; the lightest are Be and B.) There is only a trace (~2ppm) of He on Earth, mainly 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 In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron ...

tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...

. In

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

s, however, He is more abundant, a product of

nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...

. Extraplanetary material, such as lunar and

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

regolith, has traces of He from

solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...

bombardment. To become

superfluid Superfluidity is the characteristic property of a fluid with zero viscosity which therefore flows without any loss of kinetic energy. When stirred, a superfluid forms vortex, vortices that continue to rotate indefinitely. Superfluidity occurs ...

, He must be cooled to 2.5 milli

, ~900 times lower than He (). This difference is explained by quantum statistics: He atoms are

s, while He atoms are

s, which condense to a superfluid more easily.

Helium-4

The most common isotope, He, is produced on Earth by

of heavier elements; the

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay but may also be produce ...

s that emerge are fully ionized He nuclei. He is an unusually stable nucleus because it is doubly magic. It was formed in enormous quantities in

. Terrestrial helium consists almost exclusively (all but ~2ppm) of He. He's boiling point of is the lowest of all known substances except He. When cooled further to , it becomes a unique

with zero

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

. It solidifies only at pressures above 25 atmospheres, where it melts at .

Helium-5

Helium-5 is extremely unstable, decaying to helium-4 with a half-life of 602 yoctoseconds. It is briefly produced in the favorable fusion reaction:

^2\mathrm + ^3\mathrm \longrightarrow ^5\mathrm^ \longrightarrow ^4\mathrm + n + 17.6\ \mathrm

The reaction is greatly enhanced by the existence of a resonance. Helium-5, which has a natural spin state of -3/2 at the 0 MeV ground state, has a +3/2 excited spin state at 16.84 MeV. Because the reaction creates helium-5 nuclei with an energy level close to this state, it happens more frequently. This was discovered by Egon Bretscher, who was investigating weaponization of fusion reactions for the

Manhattan Project The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. From 1942 to 1946, the ...

. The DT reaction specifically is 100 times more likely than the DD reaction at relevant energies, but would be similar without the resonance. The H-He reaction benefits from a similar resonance in lithium-5, but is

Coulomb The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). It is defined to be equal to the electric charge delivered by a 1 ampere current in 1 second, with the elementary charge ''e'' as a defining c ...

-suppressed i.e. the +2 helium nucleus charge increases the electrostatic repulsion for fusing nuclei.

Helium-6 and helium-8

Helium-6 is the longest-lived radioactive isotope of helium; it

s with a half-life of 806.92 milliseconds. The most widely studied heavy helium isotope is He, which beta decays with a half-life of 119.5 milliseconds. He and He are thought to consist of a normal He nucleus surrounded by a neutron "halo" (of two neutrons in He and four neutrons in He). The unusual structures of halo nuclei may offer insights into the isolated properties of

s and

physics beyond the Standard Model Physics beyond the Standard Model (BSM) refers to the theoretical developments needed to explain the deficiencies of the Standard Model, such as the inability to explain the fundamental parameters of the standard model, the strong CP problem, neut ...

Helium-10

The shortest-lived and heaviest known helium isotope is He. Despite being a doubly magic isotope, He is not particle-bound and near-instantly drips out two neutrons (half-life ~260 yoctoseconds).

References

External links

General Tables
— abstracts for helium and other exotic light nuclei {{Navbox element isotopes Helium