Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron (changing a nuclear proton to a neutron) and simultaneously emits a neutrino. Various photon emissions follow, in order to allow the energy of the atom to fall to the ground state of the new nuclide.
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 (with one less positive charge) for the nuclide to decay by emitting a positron. Electron capture also exists as a viable decay mode for radioactive isotopes with sufficient energy to decay by positron emission, where 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, although still neutral in charge, now exists in an energetically 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. Often the nucleus exists in an excited state as well, and emits a gamma ray in order to reach the ground state energy of the new nuclide just formed.
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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.[1][2][3] Alvarez went on to study electron capture in gallium-67 and other nuclides.[1][4][5]
Note that it is one of the initial atom's own electrons that is captured, not a new, incoming electron, as might be suggested by the way the above reactions are written. Radioactive isotopes that decay by pure electron capture can, in theory, be inhibited from radioactive decay if they are fully ionized ("stripped" is sometimes used to describe such ions). It is hypothesized that such elements, if formed by the r-process in exploding supernovae, are ejected fully ionized and so do not undergo radioactive decay as long as they do not encounter electrons in outer space. Anomalies in elemental distributions are thought to be partly a result of this effect on electron capture.
Chemical bonds can also affect the rate of electron capture to a small degree (in general, less than 1%) depending on the proximity of electrons to the nucleus. For example in 7Be, a difference of 0.9% has been observed between half-lives in metallic and insulating environments.[6] This relatively large effect is due to the fact that beryllium is a small atom whose valence electrons are close to the nucleus.
Around the elements in the middle of the periodic table, isotopes that are lighter than stable isotopes of the same element tend to decay through electron capture, while isotopes heavier than the stable ones decay by electron emission.
Some common radioisotopes that decay by electron capture include:
| Radioisotope | Half-life |
|---|---|
| 7 Be |
53.28 d |
| 37 Ar |
35.0 d |
| 41 Ca |
1.03E5 a |
| 44 Ti |
52 a |
| 49 V |
337 d |
| 51 Cr |
27.7 d |
| 53 Mn |
3.7E6 a |
| 55 Fe |
2.6 a |
| 57 Co |
271.8 d |
| 56 Ni |
6.10 d |
| 67 Ga |
3.260 d |
| 68 Ge |
270.8 d |
| 72 Se |
8.5 d |
For a full list, see the table of nuclides.
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