Electron Capture(redirected from Epsilon decay)
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The process in which an atom or ion passing through a material medium either loses or gains one or more orbital electrons. In the passage of charged particles (defined here as nuclei having more or less than Z atomic electrons, where Z is the atomic number) through matter, the capture (and loss) of electrons is an important process in the slowing down of the particles and therefore has a strong influence on their range. Thus a neutral hydrogen atom loses only about half as much energy per centimeter as the positively charged proton in passing through matter consisting of light elements.
For the ordinary charged particles (alpha particles and protons) the capture process is important only at low energies, when the particle velocity is of the order of electron velocities in the stopping material, and thus is important at the end of the range. For fission fragments, however, which initially have a large excess of positive charges, electron capture occurs immediately and continues throughout the slowing-down process. This fact causes the energy-loss mechanisms at the latter part of the range to be different for fission fragments and protons or alpha particles. See Nuclear fission
The nuclear capture of electrons (K capture) occurs by a process quite different from atomic capture and is in fact a consequence of the general beta interaction. This general interaction includes ß - decay (the oldest known beta transformation and hence the name), ß + decay (or positron decay), and K capture, the latter so called because the electron captured by the nucleus is taken from the K shell (the shell nearest the nucleus) of atomic electrons. A second-order process, called L capture, can also occur, in which (to speak pictorially and thus somewhat imprecisely) an s electron (from the K shell) is captured with the simultaneous transition of a p electron (from the L shell) to the K shell with the emission of gamma radiation. See Radioactivity
a type of radioactive decay of nuclei in which a nucleus captures an electron from one of the inner shells of the atom, such as the K-, L-, or M-shell, and at the same time emits a neutrino (seeATOM and NEUTRINO). When this occurs, a nucleus with mass number A and atomic number Z is transformed into a nucleus with the same A and a Z that is smaller by 1: AZ + e– → AZ–1 + v. The vacancy formed in the atom’s electron shell is filled by electrons from other shells, and, as a result, a quantum of the characteristic X-radiation of the atom AZ–1 or a corresponding electron (an Auger electron) is emitted.
Electron capture is possible if the mass (in energy units) of the atom AZ greater than the mass of the atom AZ–1, by an amount greater than the binding energy of the captured electron. If this is greater than 2mc2 = 1.02 megaelectron volts (where m is the rest mass of the electron and c is the speed of light), β+ decay begins competing with electron capture.