Electron capture is sometimes called inverse beta decay, though this term usually refers to the interaction of an electron antineutrino with a proton. In nuclear physics, beta decay is a type of radioactive decay in which a beta ray (fast energetic electron or positron) and a neutrino are emitted from an atomic nucleus. Electron capture is sometimes included as a type of beta decay, because the basic nuclear process, mediated by the weak force, is the same. Electron capture is always an alternative decay mode for radioactive isotopes that do have sufficient energy to decay by positron emission. However, a positive atomic ion may result from further Auger electron emission.Įlectron capture is an example of weak interaction, one of the four fundamental forces.Įlectron 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. Simple electron capture by itself results in a neutral atom, since the loss of the electron in the electron shell is balanced by a loss of positive nuclear charge. 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.įollowing electron capture, the atomic number is reduced by one, the neutron number is increased by one, and there is no change in mass number. Usually, a gamma ray is emitted during this transition, but nuclear de-excitation may also take place by internal conversion.įollowing 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. The resulting daughter nuclide, if it is in an excited state, then transitions to its ground state. Similarly, the momentum of the neutrino emission causes the daughter atom to recoil with a single characteristic momentum. Since this single emitted neutrino carries the entire decay energy, it has this single characteristic energy. This process thereby changes a nuclear proton to a neutron and simultaneously causes the emission of an electron neutrino.Į or when written as a nuclear reaction equation, e − 1 0 + p 1 1 ⟶ n 0 1 + 0 0 The outer electron is ejected from the atom, leaving a positive ion.Įlectron 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 shells. Lower right: In the Auger effect, the energy absorbed when the outer electron replaces the inner electron is transferred to an outer electron. An x-ray, equal in energy to the difference between the two electron shells, is emitted. Lower left: An outer electron replaces the "missing" electron. For the detector used in gas chromatography, see Electron-capture dissociation. For the fragmentation method used in mass spectrometry, see Electron capture ionization. So the answer is, both of these diagrams have to be included in calculating the crossection, so sometimes it is a W- one way and/or a W+ the other, due to the uncertainty principle in time ordering.This article is about the radioactive decay mode. This is why some books show W+ going one way and others show a W- going the other. The existence of one diagram implies the other. The uncertainty principle means that you don't know exactly when the particles react. When one writes the quark diagrams for electron capture, the reverse can also happen, within the Uncertainty principle of quantum mechanics, depending on the time evolution, there are two diagrams:īoth of these reactions occur superposed. Looking at the quark content, proton uud and neutron udd, an up quark (charge +2/3) has to turn into a down quark (charge -1/3), which happens with the an up -> d +W*+, the star because the W is in a virtual state, off mass shell. We are in the quantum mechanical regime,for a proton to turn into a neutron a quark interaction has to happen, again from conservation of charge. BUT the Feynman diagram rules can read it as W- from right to left. Usually one plots the arrow from left to right. In electron capture, ignoring the quark level, this is the diagram:Ĭonservation of charge needs a positron, which means the intermediate virtual boson exchanged going from left to right has to carry a + charge.
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