beta decay

Beta decay is radioactive decay in which a nucleus emits a beta particle (i.e., electron or positron), leaving the nucleus with the same mass number (virtually the same mass) but with an atomic number one more (if the beta particle is an electron) or less (if a positron). Nucleosynthesis often involves a neutron capture or alpha capture leading to an unstable isotope followed by beta decay one or more times until the nucleus is again stable, i.e., until it reaches the valley of beta stability.

Beta decay happens spontaneously (at a random time) in neutron-rich or neutron-poor isotopes, emitting two particles (a beta particle and a neutrino), changing a nucleon between proton and neutron. A inverse of either reaction can happen: a nucleus is not likely to receive the two necessary particles simultaneously, but it can receive one, and emit the antiparticle of the other, which is equivalent to receiving the two at once. Four types of incoming particles can trigger this, i.e., either type of beta particle, or an electron neutrino or an anti-electron neutrino:

• electron capture - absorption of an electron, changing a proton to a neutron and emitting an electron neutrino. The term electron capture is generally used for this when the electron is taken from an inner orbit of the atom, but the same reaction can occur given arrival of a free electron with sufficient kinetic energy.
• inverse beta decay aka reverse beta decay - absorption of an electron neutrino or antineutrino, changing a nucleon and emitting a beta particle. Both these are used in neutrino observatories for detecting electron neutrinos or electron antineutrinos.
• (free) positron capture is considered a possibility in a pair-instability supernova (PISN).

The term beta rays refers to moving beta particles generated by beta decay.