(K-radiation). A type of radioactive decay in which an electron is captured by an atomic nucleus and immediately combines with a proton to form a neutron. The product of this radioactivity has the same mass number as the parent but the atomic number is one unit less. Thus 55iron with atomic number 26 decays by K-capture to form 55manganese, with atomic number 25. Terms synonymous with K-capture are K-electron capture and orbital electron capture.