beta decay

A nucleus with the wrong balance of neutrons and protons cannot fix it by emitting nucleons, but it can convert one kind into the other. Beta decay is that conversion. In beta-minus decay a neutron turns into a proton, and in beta-plus decay a proton turns into a neutron, while a fast electron or positron (the historical 'beta particle') is shot out to conserve charge, along with a nearly invisible neutrino to balance the books.

The two channels are: beta-minus, n -> p + e^- + antineutrino_e (raising Z by one), and beta-plus, p -> n + e^+ + neutrino_e (lowering Z by one, possible only inside a nucleus where energy allows). Unlike the monoenergetic alpha, the emitted electron shows a continuous energy spectrum from zero up to a maximum, because the released energy is shared among three bodies (daughter, electron, neutrino). That continuous spectrum was so alarming, it seemed to violate energy conservation, that Pauli in 1930 postulated the neutrino precisely to carry off the missing energy and momentum. Beta decay is mediated by the weak interaction, at a deeper level a quark changing flavor (a down quark becoming an up quark, or vice versa) by emitting a virtual W boson that then materializes as the electron-antineutrino pair.

Beta decay is the great re-sorter of the nuclear chart, driving unstable isobars along the mass parabola toward the valley of stability. It is central to reactor physics (fission fragments are neutron-rich and beta-decay copiously), to nucleosynthesis (converting the neutrons captured in the s- and r-processes into protons), and to fundamental physics, where its parity-violating nature first revealed that the weak interaction distinguishes left from right.