Electron capture is the primary decay mode for isotopes that have a relative superabundance of protons in the nucleus, but there is insufficient energy difference between the isotope and its prospective daughter with one less positive charge, for the nuclide to decay by simply emitting a positron. Electron capture also exists as a viable decay mode for radioactive isotopes that do have enough energy to decay by positron emission, and in that case, it competes with 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 because not enough decay energy is available to allow it, and thus electron capture is the sole decay mode. For example, rubidium-83 (37 protons, 46 neutrons) will decay to krypton-83 (36 protons, 47 neutrons) solely by electron capture (the energy difference, or decay energy, is about 0.9 MeV).
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. In the process of electron capture, 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.
Since the proton is changed to a neutron in electron capture, 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 now exists in an excited state with the inner shell missing an electron. While transiting to the ground state, the atom will emit an X-ray photon (a type of electromagnetic radiation) and/or Auger electrons, or both.
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.
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