Everything about Electron Capture totally explained
Electron capture (sometimes called
inverse beta decay) is a
decay mode for
isotopes that will occur when there are too many
protons in the
nucleus of an
atom and insufficient energy to emit a
positron; however, it continues to be a viable decay mode for
radioactive isotopes that can decay by
positron emission. If the energy difference between the parent atom and the daughter atom is less than 1.022
MeV, positron emission is forbidden and electron capture is the sole decay mode. For example,
Rubidium-83 will decay to
Krypton-83 solely by electron capture (the energy difference is about 0.9 MeV).
In this case, 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, 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 moves into an
excited state with the inner shell missing an electron. When transiting to the ground state, the atom will emit an
X-ray photon (a type of
electromagnetic radiation) and/or
Auger electrons.
Reaction Details
(Please note that it's one of the initial atom's own electrons that's captured, not a new, incoming electron as might be suggested by the way the above reactions are written.) Radioactive isotopes which decay by pure electron capture can, in theory, be inhibited from radioactive decay if they're 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 don't undergo radioactive decay as long as they don't 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 (generally less than 1%) depending on the proximity of electrons to the nucleus.
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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. A good example of this is
silver.
Common Examples
Some common radioisotopes that decay by electron capture include:
| Radioisotope |
Half-life |
| Be-7 | 53.28 d
|
| Ar-37 | 35.0 d
|
| Ca-41 | 1.03E5 a
|
| Ti-44 | 52 a
|
| V-49 | 337 d
|
| Cr-51 | 27.7 d
|
| Mn-53 | 3.7E6 a
|
| Co-57 | 271.8 d
|
| Ni-56 | 6.10 d
|
| Ga-67 | 3.260 d
|
| Ge-68 | 270.8 d
|
| Se-72 | 8.5 d
|
For a full list, see the
table of nuclides.
Further Information
Get more info on 'Electron Capture'.
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