Editing Alpha decay (section)

{{Short description|Type of radioactive decay}}
[[File:Alpha Decay.svg|thumb|240px|right|Visual representation of alpha decay]]
{{Nuclear physics}}
'''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 [[atomic number]] that is reduced by two. An alpha particle is identical to the nucleus of a [[helium-4]] atom, which consists of two [[proton]]s and two [[neutron]]s. It has a charge of {{val|+2|ul=e}} and a mass of {{val|4|ul=Da}}. For example, [[uranium-238]] decays to form [[thorium-234]].

While alpha particles have a [[electric charge|charge]] {{val|+2|u=e}}, this is not usually shown because a nuclear equation describes a nuclear reaction without considering the electrons – a convention that does not imply that the nuclei necessarily occur in neutral atoms.

Alpha decay typically occurs in the heaviest [[nuclide]]s. Theoretically, it can occur only in nuclei somewhat heavier than [[nickel]] (element 28), where the overall [[binding energy]] per [[nucleon]] is no longer a maximum and the nuclides are therefore unstable toward spontaneous fission-type processes. In practice, this mode of decay has only been observed in nuclides considerably heavier than nickel, with the lightest known alpha emitter being the second lightest [[isotope]] of [[antimony]], [[Isotopes of antimony|<sup>104</sup>Sb]].<ref>F.G. Kondev et al 2021 Chinese Phys. C 45 030001</ref> Exceptionally, however, [[beryllium-8]] decays to two alpha particles.

Alpha decay is by far the most common form of [[cluster decay]], where the parent [[atom]] ejects a defined [[decay product|daughter]] collection of nucleons, leaving another defined product behind. It is the most common form because of the combined extremely high [[nuclear binding energy]] and relatively small mass of the alpha particle. Like other cluster decays, alpha decay is fundamentally a [[quantum tunneling]] process. Unlike [[beta decay]], it is governed by the interplay between both the [[nuclear force|strong nuclear force]] and the [[electromagnetic force]].

[[Alpha particle]]s have a typical kinetic energy of 5&nbsp;MeV (or ≈&nbsp;0.13% of their total energy, 110&nbsp;TJ/kg) and have a speed of about 15,000,000&nbsp;m/s, or 5% of the [[speed of light]]. There is surprisingly small variation around this energy, due to [[Geiger–Nuttall law|the strong dependence]] of the half-life of this process on the energy produced. Because of their relatively large mass, the electric charge of {{val|+2|u=e}} and relatively low velocity, alpha particles are very likely to interact with other atoms and lose their energy, and their forward motion can be stopped by a few centimeters of [[air]].

Approximately 99% of the [[helium]] produced on [[Earth]] is the result of the alpha decay of underground deposits of [[mineral]]s containing [[uranium]] or [[thorium]]. The helium is brought to the surface as a by-product of [[natural gas]] production.