Electron capture (K-electron capture, alsoK-capture, orL-electron capture,L-capture) is a process in which the proton-rich nucleus of an electrically neutralatom absorbs an inner atomicelectron, usually from the K or Lelectron shells. This process thereby changes a nuclearproton to a neutron and simultaneously causes the emission of anelectron neutrino.
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Since this single emitted neutrino carries the entiredecay energy, it has this single characteristic energy. Similarly, the momentum of the neutrino emission causes the daughter atom to recoil with a single characteristic momentum.
The resultingdaughter nuclide, if it is in anexcited state, then transitions to itsground state. Usually, agamma ray is emitted during this transition, but nuclear de-excitation may also take place byinternal conversion.
Following capture of an inner electron from the atom, an outer electron replaces the electron that was captured and one or morecharacteristic X-ray photons is emitted in this process. Electron capture sometimes also results in theAuger 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, theatomic number is reduced by one, the neutron number is increased by one, and there is no change inmass number. Simple electron capture by itself results in a neutral atom, since the loss of the electron in theelectron 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 ofweak interaction, one of the four fundamental forces.
Electron capture is the primarydecay mode forisotopes with a relative superabundance ofprotons in thenucleus, but with insufficient energy difference between the isotope and its prospective daughter (theisobar with one lesspositive charge) for the nuclide to decay by emitting apositron. Electron capture is always an alternative decay mode forradioactive isotopes thatdo have sufficient energy to decay bypositron emission. Electron capture is sometimes included as a type ofbeta decay,[1] because the basic nuclear process, mediated by the weak force, is the same. Innuclear physics, beta decay is a type ofradioactive decay in which abeta ray (fast energetic electron or positron) and a neutrino are emitted from an atomic nucleus. Electron capture is sometimes calledinverse beta decay, though this term usually refers to the interaction of anelectron antineutrino with a proton.[2]
If the energy difference between the parent atom and the daughter atom is less than 1.022 MeV, positron emission is forbidden as not enoughdecay 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 tokrypton-83 (36 protons, 47 neutrons) solely by electron capture (the energy difference, or decay energy, is about 0.9 MeV).
The theory of electron capture was first discussed byGian-Carlo Wick in a 1934 paper, and then developed byHideki Yukawa and others. K-electron capture was first observed byLuis Alvarez, invanadium,48
V, which he reported in 1937.[3][4][5] Alvarez went on to study electron capture ingallium (67
Ga) and other nuclides.[3][6][7]
The electron that is captured is one of the atom's own electrons, and not a new, incoming electron, as might be suggested by the way the reactions are written below. A few examples of electron capture are:
Radioactive isotopes that decay by pure electron capture can be inhibited from radioactive decay if they are fullyionized ("stripped" is sometimes used to describe such ions). It is hypothesized that such elements, if formed by ther-process in explodingsupernovae, 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[by whom?] to be partly a result of this effect on electron capture. Inverse decays can also be induced by full ionisation; for instance,163
Ho decays into163
Dy by electron capture; however, a fully ionised163
Dy decays into a bound state of163
Ho by the process ofbound-state β− decay.[8]
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, in7Be, a difference of 0.9% has been observed between half-lives in metallic and insulating environments.[9] This relatively large effect is because beryllium is a small atom that employs valence electrons that are close to the nucleus, and also in orbitals with no orbital angular momentum. Electrons ins orbitals (regardless of shell or primary quantum number), have a probability antinode at the nucleus, and are thus far more subject to electron capture thanp ord electrons, which have a probability node at the nucleus.
Around the elements in the middle of theperiodic 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 byelectron emission. Electron capture happens most often in the heavier neutron-deficient elements where the mass change is smallest and positron emission is not always possible. When the loss of mass in a nuclear reaction is greater than zero but less than2mec2 the process cannot occur by positron emission, but occurs spontaneously for electron capture.
Some common radionuclides that decay solely by electron capture include (a = annum or year):
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For a full list, see thetable of nuclides.
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