Beta and gamma decays

1 Simple Fermi theory of beta decay 2 Beta decay is one of the most easily found kinds of radioactivity. As we have seen, this result of the weak interaction leads to conversion of a neutron into a proton or vice versa, with the necessary charge change being made possible by the emission of a positive or negative beta particle (positron or electron). To ensure conservation of lepton number, each such event is accompanied by emission of an electron neutrino. Alternatively, an orbital electron may be absorbed (electron capture or K-capture), changing a proton into a neutron (this process competes with ̄ emission), with emission of an electron neutrino. 2 The spectra of electrons or positrons emitted in beta decay is a continuum of energies, up to a maximum value, with most emitted betas having intermediate energies. The emitted, unobserved, neutrinos also have a continuous energy distribution. In K-capture, there is no electron emitted; the neutrino is mono-energetic. (Mono-energetic electrons also emerge from de-excitation of an excited nucleus by internal conversion [see below]; this process leads to electron emission but is quite a di®erent physical mechanism than beta decay.)