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Concept
Helium-4
Summary
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 consists of two protons and two neutrons.
Alpha decay of heavy elements in the Earth's crust is the source of most naturally occurring helium-4 on Earth, produced after the planet cooled and solidified. While it is also produced by nuclear fusion in stars, most helium-4 in the Sun and in the universe is thought to have been produced by the Big Bang, and is referred to as "primordial helium". However, primordial helium-4 is largely absent from the Earth, having escaped during the high-temperature phase of Earth's formation.
Helium-4 makes up about one quarter of the ordinary matter in the universe by mass, with almost all of the rest being hydrogen.
When liquid helium-4 is cooled to below , it becomes a superfluid, with properties that are very unlike those of an ordinary liquid. For example, if superfluid helium-4 is kept in an open vessel, a thin film will climb up the sides of the vessel and overflow. In this state and situation, it is called a "Rollin film". This strange behavior is a result of the Clausius–Clapeyron relation and cannot be explained by the current model of classical mechanics, nor by nuclear or electrical modelsit can only be understood as a quantum-mechanical phenomenon. The total spin of the helium-4 nucleus is an integer (zero), and therefore it is a boson (as are neutral atoms of helium-4). The superfluid behavior is now understood to be a manifestation of Bose–Einstein condensation, which occurs only with collections of bosons.
It is theorized that at 0.2 K and 50 atm, solid helium-4 may be a superglass (an amorphous solid exhibiting superfluidity).
Helium-4 also exists on the Moon andas on Earthit is the most abundant helium isotope.
Helium atom
The helium atom is the second simplest atom (hydrogen is the simplest), but the extra electron introduces a third "body", so the solution to its wave equation becomes a "three-body problem", which has no analytic solution.
Official source
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Related concepts (19)
Isotope
Isotopes are distinct nuclear species (or nuclides, as technical term) of the same element. They have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), but differ in nucleon numbers (mass numbers) due to different numbers of neutrons in their nuclei. While all isotopes of a given element have almost the same chemical properties, they have different atomic masses and physical properties.
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 produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α. The symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are also sometimes written as He2+ or 42He2+ indicating a helium ion with a +2 charge (missing its two electrons).
Neutron generator
Neutron generators are neutron source devices which contain compact linear particle accelerators and that produce neutrons by fusing isotopes of hydrogen together. The fusion reactions take place in these devices by accelerating either deuterium, tritium, or a mixture of these two isotopes into a metal hydride target which also contains deuterium, tritium or a mixture of these isotopes. Fusion of deuterium atoms (D + D) results in the formation of a helium-3 ion and a neutron with a kinetic energy of approximately 2.