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Beta decay is when an unstable atomic nucleus decays (radioactively) by emitting a beta particle; when the beta particle is an electron, it is β- decay, and when a positron, β+ decay.
Beta rays, as a distinct component of the rays given off in radioactivity, were discovered by Rutherford, in 1899, just a few years after radioactivity itself was discovered (in 1896). However, this is beta minus decay … the discovery of beta plus decay (by Irène and Frédéric Joliot-Curie, in 1934) came after the discovery of the positron (in cosmic rays, in 1932) and the (then) controversial ‘invention’ of the neutrino (by Pauli, in 1931) to account for the continuous energy spectrum of electrons in beta decay. It was also in 1934 that Fermi published – in Italian and German (Nature considered the idea too speculative!!) – his theory of beta decay (for more details on this, check out this Hyperphysics page).
In beta minus decay, a neutron changes into a proton, antineutrino, and electron; this conversion is due to the weak interaction (or weak force) … a down quark (in the neutron) becomes an up quark and emits a W- boson (one of three bosons which mediate the weak interaction), which then decays into an electron and an antineutrino.
Beta plus decay – which is also known as inverse beta decay – involves the conversion of a proton to a neutron, positron, and neutrino.
So why do isolated neutrons decay (but those in stable nuclei, and those in neutron stars, don’t)? And why are isolated protons stable, but those in certain radioactive nuclei not? It’s all down to energy … if one state (an isolated neutron, say) has a higher energy than another (proton plus electron plus antineutrino), then the first will decay into the second (the baryon number of the two states must be the same, ditto lepton number, and so on).
There is also a rare double beta decay, in which two beta particles are emitted; it has been observed, in some unstable isotopes, as predicted. There is one kind of double beta decay – called neutrino-less double beta decay (the image above is from the COBRA Project, one study of this) – which is being studied intensely (though no such decay has yet been observed), because it may be one of the very few easily opened windows into physics beyond the Standard Model (see this WIPP page for more details).
Berkeley Lab has a neat Guide to the Nuclear Wallchart (subtitled “You don’t need to be a Nuclear Physicist to understand Nuclear Science“!) on beta decay, and this Ohio University page – Alpha and beta decay – puts more technical meat on the bare overview bones.
Pushing the Polite Boundaries of Science About Dark Matter is a Universe Today story which has a tangential reference to beta decay (it’s in the comments!).