Editing Beta particle (section)

=== β<sup>−</sup> decay (electron emission) ===
{{main|Beta_decay#.CE.B2.E2.88.92_decay|l1=β<sup>−</sup> decay}}
[[File:Beta-minus Decay.svg|thumb|Beta decay. A beta particle (in this case a negative electron) is shown being emitted by a [[Atomic nucleus|nucleus]]. An antineutrino (not shown) is always emitted along with an electron. Insert: in the decay of a free neutron, a proton, an electron (negative beta ray), and an [[electron antineutrino]] are produced.]]

An unstable atomic nucleus with an excess of [[neutron]]s may undergo β<sup>−</sup> decay, where a neutron is converted into a [[proton]], an electron, and an [[electron antineutrino]] (the [[antiparticle]] of the [[neutrino]]):

:{{SubatomicParticle|neutron}} → {{SubatomicParticle|proton}} + {{SubatomicParticle|electron}} + {{SubatomicParticle|electron antineutrino}}

This process is mediated by the [[weak interaction]]. The neutron turns into a proton through the emission of a [[virtual particle|virtual]] [[weak interaction|W<sup>−</sup> boson]]. At the [[quark]] level, W<sup>−</sup> emission turns a down quark into an up quark, turning a neutron (one up quark and two down quarks) into a proton (two up quarks and one down quark).
The virtual W<sup>−</sup> boson then decays into an electron and an antineutrino.

β− decay commonly occurs among the neutron-rich [[Nuclear fission product|fission byproducts]] produced in [[nuclear reactor]]s. Free neutrons also decay via this process. Both of these processes contribute to the copious quantities of beta rays and electron antineutrinos produced by fission-reactor fuel rods.