Editing Beta decay (section)

==Description==
The two types of beta decay are known as ''beta minus'' and ''beta plus''. In beta minus (β<sup>−</sup>) decay, a neutron is converted to a proton, and the process creates an electron and an [[electron antineutrino]]; while in beta plus (β<sup>+</sup>) decay, a proton is converted to a neutron and the process creates a positron and an electron neutrino. β<sup>+</sup> decay is also known as [[positron emission]].<ref>{{Cite book
 |last1=Basdevant |first1=J.-L.
 |last2=Rich |first2=J.
 |last3=Spiro |first3=M.
 |year=2005
 |title=Fundamentals in Nuclear Physics: From Nuclear Structure to Cosmology
 |publisher=[[Springer (publisher)|Springer]]
 |isbn=978-0-387-01672-6 }}</ref>

Beta decay conserves a quantum number known as the [[lepton number]], or the number of electrons and their associated neutrinos (other leptons are the [[muon]] and [[Tau (particle)|tau]] particles). These particles have lepton number +1, while their antiparticles have lepton number −1. Since a proton or neutron has lepton number zero, β<sup>+</sup> decay (a positron, or antielectron) must be accompanied with an electron neutrino, while β<sup>−</sup> decay (an electron) must be accompanied by an electron antineutrino.

An example of electron emission (β<sup>−</sup> decay) is the decay of [[carbon-14]] into [[nitrogen-14]] with a [[half-life]] of about 5,700 years:
:{{nuclide|carbon|14}} → {{nuclide|nitrogen|14}} + {{subatomic particle|electron}} + {{math|{{subatomic particle|electron antineutrino}}}}

In this form of decay, the original element becomes a new chemical element in a process known as [[nuclear transmutation]]. This new element has an unchanged [[mass number]] {{mvar|A}}, but an [[atomic number]] {{mvar|Z}} that is increased by one. As in all nuclear decays, the decaying element (in this case {{nuclide|carbon|14}}) is known as the ''parent nuclide'' while the resulting element (in this case {{nuclide|nitrogen|14}}) is known as the ''daughter nuclide''.

Another example is the decay of hydrogen-3 ([[tritium]]) into [[helium-3]] with a half-life of about 12.3 years:
:{{nuclide|hydrogen|3}} → {{nuclide|helium|3}} + {{subatomic particle|electron}} + {{math|{{subatomic particle|electron antineutrino}}}}

An example of positron emission (β<sup>+</sup> decay) is the decay of [[magnesium-23]] into [[sodium-23]] with a half-life of about 11.3 s:

:{{nuclide|Magnesium|23}} → {{nuclide|Sodium|23}} + {{subatomic particle|positron}} + {{math|{{subatomic particle|electron neutrino}}}}
β<sup>+</sup> decay also results in nuclear transmutation, with the daughter element having an atomic number that is decreased by one.

[[File:RaE1.jpg|thumb|A beta spectrum, showing a typical division of energy between electron and antineutrino]]
The beta spectrum, or distribution of energy values for the beta particles, is continuous. The total energy of the decay process is divided between the electron, the antineutrino, and the recoiling nuclide. In the figure to the right, an example of an electron with 0.40 MeV energy from the beta decay of <sup>210</sup>Bi is shown. In this example, the total decay energy is 1.16 MeV, so the antineutrino has the remaining energy: {{nowrap|1=1.16 MeV − 0.40 MeV = 0.76 MeV}}. An electron at the far right of the curve would have the maximum possible kinetic energy, leaving the energy of the neutrino to be only its small rest mass.