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electron capture A mechanism by which a nucleus can decrease its proton number and increase its neutron number by capturing one of its extranuclear electrons. The process can be envisaged as the reaction between an extranuclear electron and a proton in the nucleus to form a neutron and a neutrino, the neutrino then being emitted from the nucleus. The event may leave the product nucleus in an excited state, and this is then followed by the emission of a gamma ray. Removal of an extranuclear electron from the k-shell, or from higher-energy shells, leaves a vacancy that is subsequently filled by other electrons that fall into the vacant position. In the process these electrons emit a series of X-rays that can be detected.