View source for Electron capture

{{short description|Process in which a proton-rich nuclide absorbs an inner atomic electron}}
{{about|the radioactive decay mode|the fragmentation method used in mass spectrometry|Electron capture ionization|the detector used in gas chromatography|Electron-capture dissociation}}
{{Use British  English|date=December 2013}}
[[File:Atomic rearrangement following an electron capture.svg|thumb|Scheme of two types of electron capture. ''Top'': The nucleus absorbs an electron. ''Lower left'': An outer electron replaces the "missing" electron. An x-ray, equal in energy to the difference between the two electron shells, is emitted. ''Lower right'': In the Auger effect, the energy absorbed when the outer electron replaces the inner electron is transferred to an outer electron. The outer electron is ejected from the atom, leaving a positive ion.]]

'''Electron 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 shell]]s. This process thereby changes a nuclear [[charged current interaction|proton to a neutron]] and simultaneously causes the emission of an [[electron neutrino]].

: {{math| {{SubatomicParticle|link=yes|Proton}} &nbsp; + &nbsp; {{SubatomicParticle|link=yes|Electron}} &nbsp; &nbsp; → &nbsp;  &nbsp;{{SubatomicParticle|link=yes|Neutron}} &nbsp; + &nbsp;{{SubatomicParticle|link=yes|Electron Neutrino}} }}     
: or when written as a [[Nuclear reaction#Nuclear reaction equations|nuclear reaction equation]],    <chem>^{0}_{-1}e + ^{1}_{1}p -> ^{1}_{0}n  +  ^{0}_{0}   </chem>ν<math>_e</math>

Since this single emitted neutrino carries the entire [[decay 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 resulting [[daughter nuclide]], if it is in an [[excited state]], then transitions to its [[ground state]]. Usually, a [[gamma ray]] is emitted during this transition, but nuclear de-excitation may also take place by [[internal conversion]].

Following 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. 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.

Following electron capture, the [[atomic number]] is reduced by one, the neutron number is increased by one, and there is no change in [[mass number]]. 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. However, a positive atomic ion may result from further Auger electron emission.

Electron capture is an example of [[weak interaction]], one of the four fundamental forces.

Electron capture is the primary [[decay mode]] for [[isotope]]s with a relative superabundance of [[proton]]s in the [[atomic nucleus|nucleus]], but with insufficient energy difference between the isotope and its prospective daughter (the [[isobar (nuclide)|isobar]] with one less [[atomic number|positive charge]]) for the nuclide to decay by emitting a [[positron]]. Electron capture is always an alternative decay mode for [[radioactive]] isotopes that ''do'' have sufficient energy to decay by [[positron emission]]. Electron capture is sometimes included as a type of [[beta decay]],<ref>
{{cite book
 |last1=Cottingham |first1=W.N.
 |last2=Greenwood  |first2=D.A.
 |year=1986
 |title=An introduction to nuclear physics
 |page=[https://archive.org/details/introductiontonu0000cott/page/40 40]
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-31960-7
 |url=https://archive.org/details/introductiontonu0000cott/page/40
 }}</ref> because the basic nuclear process, mediated by the weak force, is the same. In [[nuclear physics]], beta decay is a type of [[radioactive decay]] in which a [[beta particle|beta ray]] (fast energetic electron or positron) and a neutrino are emitted from an atomic nucleus. Electron capture is sometimes called [[inverse beta decay]], though this term usually refers to the interaction of an [[Neutrino|electron antineutrino]] with a proton.<ref>
{{cite journal
 |year=1997
 |title=The Reines-Cowan experiments: Detecting the poltergeist
 |journal=[[Los Alamos National Laboratory]]
 |volume=25 |page=3
 |url=http://library.lanl.gov/cgi-bin/getfile?25-02.pdf
}}</ref>

If the energy difference between the parent atom and the daughter atom is less than 1.022&nbsp;[[MeV]], positron emission is forbidden as not enough [[decay energy]] is available to allow it, and thus electron capture is the sole decay mode. For example, [[rubidium-83]] (37&nbsp;protons, 46&nbsp;neutrons) will decay to [[krypton-83]] (36&nbsp;protons, 47&nbsp;neutrons) solely by electron capture (the energy difference, or decay energy, is about 0.9&nbsp;MeV).

==History==
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 Walter Alvarez|Luis Alvarez]], in [[vanadium]], {{SimpleNuclide|link=y|Vanadium|48}}, which he reported in 1937.<ref name=k>{{cite book
 |first1=Luis W.  |last1=Alvarez
 |first2=W. Peter |last2=Trower
 |others=Segré, Emilio (commentary) 
 |year=1987
 |chapter=Chapter&nbsp;3: K-electron capture by nuclei
 |title=Discovering Alvarez: Selected works of Luis W. Alvarez, with commentary by his students and colleagues
 |pages=11–12 
 |publisher=University of Chicago Press
 |isbn=978-0-226-81304-2
 |via=archive.org
 |chapter-url=https://archive.org/details/discoveringalvar0000alva/page/11
}}</ref><ref>{{cite web
 |title=Luis Alvarez, biography
 |series=The Nobel Prize in Physics 1968
 |website=Nobel Prize
 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1968/alvarez-bio.html
 |access-date=7 October 2009
}}</ref><ref>{{cite journal
 |last1= Alvarez |first1= Luis W.
 |year= 1937
 |title= Nuclear K Electron Capture
 |journal= Physical Review
 |volume= 52 |issue= 2
 |pages= 134–135
 |doi= 10.1103/PhysRev.52.134 |bibcode= 1937PhRv...52..134A
}}</ref> Alvarez went on to study electron capture in [[gallium]] ({{SimpleNuclide|link=y|Gallium|67}}) and other nuclides.<ref name=k/><ref>{{cite journal
 |last1= Alvarez |first1= Luis W.
 |year= 1937
 |title= Electron Capture and Internal Conversion in Gallium 67
 |journal= Physical Review
 |volume= 53 |issue= 7
 |page= 606
 |doi= 10.1103/PhysRev.53.606 |bibcode= 1938PhRv...53..606A
}}</ref><ref>{{cite journal
 |last1= Alvarez |first1= Luis W.
 |year= 1938
 |title= The capture of orbital electrons by nuclei
 |journal= Physical Review
 |volume= 54 |issue= 7
 |pages= 486–497
 |doi= 10.1103/PhysRev.54.486 |bibcode= 1938PhRv...54..486A
}}</ref>

==Reaction details==
[[File:Electron-capture.svg|alt=Leading-order EC Feynman diagrams|thumb|309x309px|The leading-order [[Feynman diagram]]s for electron capture decay. An [[electron]] interacts with an [[up quark]] in the nucleus via a [[W and Z bosons|W boson]] to create a [[down quark]] and [[electron neutrino]]. Two diagrams comprise the leading (second) order, though as a [[virtual particle]], the type (and charge) of the W-boson is indistinguishable.]]
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:
:{|
|-
| {{math| {{nuclide|link=yes|aluminium|26}} &nbsp; }}
| {{math| + &nbsp; {{SubatomicParticle|link=yes|Electron}} &nbsp; &nbsp; → &nbsp; &thinsp; &nbsp; }}
| {{math| {{nuclide|link=yes|magnesium|26}} &nbsp; }}
| {{math| + &nbsp; {{SubatomicParticle|link=yes|Electron Neutrino}} }}
|-
| {{math| {{nuclide|link=yes|nickel|59}} &nbsp; }}
| {{math| + &nbsp; {{SubatomicParticle|link=yes|Electron}} &nbsp; &nbsp; → &nbsp; &thinsp; &nbsp; }}
| {{math| {{nuclide|link=yes|cobalt|59}} &nbsp; }}
| {{math| + &nbsp; {{SubatomicParticle|link=yes|Electron Neutrino}} }}
|-
| {{math| {{nuclide|link=yes|potassium|40}} &nbsp; }}
| {{math| + &nbsp; {{SubatomicParticle|link=yes|Electron}} &nbsp; &nbsp; → &nbsp;  &thinsp; &nbsp; }}
| {{math| {{nuclide|link=yes|argon|40}} &nbsp; }}
| {{math| + &nbsp; {{SubatomicParticle|link=yes|Electron Neutrino}} }}
|}

Radioactive isotopes that decay by pure electron capture can be inhibited from radioactive decay if they are fully [[ion]]ized ("stripped" is sometimes used to describe such ions). It is hypothesized that such elements, if formed by the [[r-process]] in exploding [[supernova]]e, 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|date=September 2012}} to be partly a result of this effect on electron capture. Inverse decays can also be induced by full ionisation; for instance, {{SimpleNuclide|link=yes|Holmium|163}} decays into {{SimpleNuclide|link=yes|Dysprosium|163}} by electron capture; however, a fully ionised {{SimpleNuclide|Dysprosium|163}} decays into a bound state of {{SimpleNuclide|Holmium|163}} by the process of [[bound-state β− decay|bound-state β<sup>−</sup> decay]].<ref>{{cite journal
 |author-first=Fritz |author-last=Bosch
 |year=1995
 |title=Manipulation of Nuclear Lifetimes in Storage Rings
 |journal=Physica Scripta
 |volume=T59  |pages=221–229
 |doi=10.1088/0031-8949/1995/t59/030 |bibcode=1995PhST...59..221B
 |s2cid=250860726
 |url=http://www.ca.infn.it/~oldeman/resneu/physscr5_T59_030.pdf
 |url-status=dead
 |archive-url=https://web.archive.org/web/20131226210737/http://www.ca.infn.it/~oldeman/resneu/physscr5_T59_030.pdf
 |archive-date=2013-12-26
}}</ref>

[[Chemical bond]]s 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 <sup>7</sup>Be, a difference of 0.9% has been observed between half-lives in metallic and insulating environments.<ref>{{cite journal
 |author1-first=B. |author1-last=Wang
 |display-authors=etal
 |year=2006
 |title=Change of the <sup>7</sup>Be electron capture half-life in metallic environments
 |journal=The European Physical Journal A
 |volume=28 |issue=3
 |pages=375–377
 |bibcode = 2006EPJA...28..375W
 |doi = 10.1140/epja/i2006-10068-x  <!-- prior author/editor noted "subscription required" but no URL -->
|s2cid=121883028
 }}</ref> This relatively large effect is due to the fact that 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 in '''s''' orbitals (regardless of shell or primary quantum number), have a probability antinode at the nucleus, and are thus far more subject to electron capture than '''p''' or '''d''' electrons, which have a probability node at 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]]. 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 than {{math|2''m''<sub>e</sub>''c''<sup>2</sup>}}  the process cannot occur by positron emission, but occurs spontaneously for electron capture.

==Common examples==
Some common radionuclides that decay solely by electron capture include:

{{col-begin |width=90%}}
{{col-break |width=10%}}
{{col-break |width=30%}}
{| class="wikitable"
|- align="right"
! Nuclide
! Half-life
|- align="right"
| {{Nuclide|link=yes|Beryllium|7}}
| 53.28&nbsp;d
|- align="right"
| {{Nuclide|link=yes|Argon|37}}
| 35.0&nbsp;d 
|- align="right"
| {{Nuclide|link=yes|Calcium|41}}
| 1.03{{e|5}}&nbsp;a
|- align="right"
| {{Nuclide|link=yes|Titanium|44}}
| 60&nbsp;a
|- align="right"
| {{Nuclide|link=yes|Vanadium|49}}
| 337&nbsp;d
|}
{{col-break |width=30%}}
{| class="wikitable"
|- align="right"
! Nuclide
! Half-life
|- align="right"
| {{Nuclide|link=yes|Chromium|51}}
| 27.7&nbsp;d
|- align="right"
| {{Nuclide|link=yes|Manganese|53}}
| 3.7{{e|6}}&nbsp;a
|- align="right"
| {{Nuclide|link=yes|Iron|55}}
| 2.6&nbsp;a
|- align="right"
| {{Nuclide|link=yes|Cobalt|57}}
| 271.8&nbsp;d
|- align="right"
| {{Nuclide|link=yes|Nickel|59}}
| 7.5{{e|4}}&nbsp;a
|}
{{col-break |width=30%}}
{| class="wikitable"
|- align="right"
! Nuclide
! Half-life
|- align="right"
| {{Nuclide|link=yes|Gallium|67}}
| 3.260&nbsp;d
|- align="right"
| {{Nuclide|link=yes|Germanium|68}}
| 270.8&nbsp;d
|- align="right"
| {{Nuclide|link=yes|Selenium|72}}
| 8.5&nbsp;d
|- align="right"
| {{su|p=&nbsp;|b=&nbsp;}}
| &nbsp;
|- align="right"
| {{su|p=&nbsp;|b=&nbsp;}}
| &nbsp;
|}
{{col-end}}

For a full list, see the [[table of nuclides]].

== See also ==
* [[Chandrasekhar limit]]

== References ==
{{reflist|25em}}

== External links ==
* {{cite web
 |title=The LIVEChart of Nuclides 
 |publisher=[[International Atomic Energy Agency]]
 |department=IAEA Nuclear Data Section
 |place=Vienna, Austria
 |url=http://www-nds.iaea.org/livechart
 |access-date=2020-08-16 |df=dmy-all
}} with filter on electron capture

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