Helium-4

Helium-4 is the stable helium isotope whose nucleus contains two protons and two neutrons. It makes up nearly all naturally occurring helium and has unusually strong nuclear binding for such a light nucleus.

Helium: Helium is the chemical element with atomic number 2, whose atoms have two protons. Helium-4 is the element’s overwhelmingly common isotope.
Strong nuclear force: The strong nuclear force binds quarks into protons and neutrons and helps bind nucleons inside atomic nuclei. It overcomes proton repulsion and binds helium-4’s compact nucleus.
Big Bang nucleosynthesis: Big Bang nucleosynthesis is the formation of the lightest atomic nuclei during the first minutes of the universe. It produced most of the universe’s primordial helium-4.
Cryogenics: Cryogenics is the study and use of materials at very low temperatures. Liquid helium-4 cools systems to temperatures unreachable with ordinary refrigerants.
Helium-3: Helium-3 is a stable helium isotope with two protons and one neutron. Unlike helium-4, helium-3 is rare and is a fermion rather than a boson.
Isotope: An isotope is a form of an element with a particular number of neutrons in its atoms. Helium-4 differs from other helium isotopes by its neutron count.
Nuclear shell model: The nuclear shell model describes nucleons occupying quantized energy levels within an atomic nucleus. Its closed proton and neutron shells help account for helium-4’s stability.
Stellar nucleosynthesis: Stellar nucleosynthesis is the formation of atomic nuclei through nuclear reactions inside stars. Stars create helium-4 through hydrogen fusion and later nuclear burning.
Helium liquefaction: Helium liquefaction is the process of cooling gaseous helium until it becomes liquid. Liquefying helium-4 enables its use as a powerful cryogenic coolant.
Helium-4 superfluid transition: The helium-4 superfluid transition is the change from ordinary liquid helium-4 to its superfluid phase near 2.17 kelvin. It separates helium-4’s ordinary liquid state from its low-temperature quantum fluid.

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