electron capture

A proton-rich nucleus needs to turn a proton into a neutron, but sometimes it lacks the energy to shoot out a positron. It has another option: instead of emitting a particle, it reaches out and swallows one of its own atomic electrons. Electron capture is a nucleus pulling in an inner-shell electron to convert a proton into a neutron. It is, in effect, beta-plus decay run with an absorbed electron rather than an emitted positron.

The elementary process is p + e^- -> n + neutrino_e: a bound electron (usually from the innermost K shell, hence 'K-capture') combines with a nuclear proton, producing a neutron and emitting a single neutrino. The daughter has Z reduced by one and A unchanged, exactly like beta-plus decay. Electron capture competes with beta-plus emission whenever both are allowed, but it wins, or is the only option, when the decay energy is below 2 m_e c^2 = 1.022 MeV, the threshold needed to create a positron. Because only two bodies emerge and the recoiling nucleus takes little energy, the neutrino is essentially monoenergetic, in contrast to the continuous spectrum of ordinary beta decay.

Capturing an inner electron leaves a vacancy in the atomic shell, which is promptly filled by an outer electron with the emission of a characteristic X-ray or an Auger electron, the observable tell-tale of an EC event. Electron capture matters in the late stages of stellar evolution, where it removes electrons, softens the electron degeneracy pressure that supports a stellar core, and helps trigger core collapse in massive stars.