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Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shell. This process thereby changes a nuclear proton to a neutron and simultaneously causes the emission of an electron neutrino. : + → + The daughter nuclide, if it is in an excited state, then transitions to its ground state. Usually, a gamma ray is emitted during this transition, but nuclear de-excitation may also take place by internal conversion. Following capture of an inner electron from the atom, an outer electron replaces the electron that was captured and one or more characteristic X-ray photons is emitted in this process. Electron capture sometimes also results in the Auger effect, where an electron is ejected from the atom's electron shell due to interactions between the atom's electrons in the process of seeking a lower energy electron state. Following electron capture, the atomic number is reduced by one, the neutron number is increased by one, and there is no change in atomic mass. Simple electron capture results in a neutral atom, since the loss of the electron in the electron shell is balanced by a loss of positive nuclear charge. However, a positive atomic ion may result from further Auger electron emission. Electron capture is an example of weak interaction, one of the four fundamental forces. Electron capture is the primary decay mode for isotopes with a relative superabundance of protons in the nucleus, but with insufficient energy difference between the isotope and its prospective daughter (the isobar with one less positive charge) for the nuclide to decay by emitting a positron. Electron capture is always an alternate decay mode for radioactive isotopes that do not have sufficient energy to decay by positron emission. It is sometimes called inverse beta decay, though this term can also refer to the interaction of an electron antineutrino with a proton.〔 〕 If the energy difference between the parent atom and the daughter atom is less than 1.022 MeV, positron emission is forbidden as 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). 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. ==History== 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 ''Physical Review''.〔Luis W. Alvarez, W. Peter Trower (1987). ("Chapter 3: K-Electron Capture by Nuclei (with the commentary of Emilio Segré)" ) In ''Discovering Alvarez: selected works of Luis W. Alvarez, with commentary by his students and colleagues''. University of Chicago Press, pp. 11–12, ISBN 978-0-226-81304-2.〕〔("Luis Alvarez, The Nobel Prize in Physics 1968" ), biography, nobelprize.org. Accessed October 7, 2009.〕 Alvarez went on to study electron capture in gallium-67 and other nuclides.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Electron capture」の詳細全文を読む スポンサード リンク
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