K-capture

    The K-capture is a form of decay in which the closest electron in the electron cloud is captured by the nucleus. It will occur when there are too many protons in the nucleus of an atom and insufficient energy to emit a positron; however, it continues to be a viable decay mode for radioactive isotopes that can decay by positron emission. It is sometimes called inverse beta decay, though this term can also refer to the capture of a neutrino through a similar process. If the energy difference between the parent atom and the daughter atom is less than 1.022 MeV, positron emission is forbidden and electron capture is the sole decay mode. In this case, one of the orbital electrons, usually from the K or L electron shell (K-electron capture, also K-capture, or L-electron capture, L-capture), is captured by a proton in the nucleus, forming a neutron and a neutrino. 

p     +     e−     →     n     +     νe

    Note that a free proton cannot normally be changed to a free neutron by this process. The proton and neutron must be part of a larger nucleus. Since the proton is changed to a neutron, the number of neutrons increases by 1, the number of protons decreases by 1, and the atomic mass number remains unchanged. By changing the number of protons, electron capture transforms the nuclide into a new element. The atom moves into an excited state with the inner shell missing an electron. 

    The theory of electron capture was first discussed by Gian-Carlo Wick in a 1934 paper, and then developed by Hideki Yukawa and others. K-electron capture was first observed by Luis Alvarez, in vanadium-48. He reported it in a 1937 paper in the Physical Review. Alvarez went on to study electron capture in gallium-67 and other nuclides.