View source for Double electron capture

{{short description|Mode of radioactive decay}}
{{Nuclear physics|cTopic=Capturing processes}}
'''Double electron capture''' is a [[decay mode]] of an [[atomic nucleus]].<ref>{{cite journal |journal=Zeitschrift für Physik A |year=1994 |volume=347 |issue=3 |pages=151–160 |author1=Hirsch, M. |display-authors=etal |title=Nuclear structure calculation of β<sup>+</sup>β<sup>+</sup>, β<sup>+</sup>/EC and EC/EC decay matrix elements |doi=10.1007/BF01292371|bibcode=1994ZPhyA.347..151H |s2cid=120191487 }}</ref> For a nuclide (''A'', ''Z'') with a number of [[nucleon]]s ''A'' and [[atomic number]] ''Z'', double electron capture is only possible if the mass of the nuclide (''A'', ''Z''−2) is lower.

In this mode of decay, two of the orbital [[electron]]s are captured via the [[weak interaction]] by two [[proton]]s in the nucleus, forming two [[neutron]]s (Two [[neutrino]]s are emitted in the process). Since the protons are changed to neutrons, the number of neutrons increases by two, while the number of protons ''Z'' decreases by two, and the atomic [[mass number]] ''A'' remains unchanged. As a result, by reducing the atomic number by two, double electron capture transforms the [[nuclide]] into a different [[chemical element|element]].<ref>{{Cite journal|last1=Abe|first1=K.|last2=Hiraide|first2=K.|last3=Ichimura|first3=K.|last4=Kishimoto|first4=Y.|last5=Kobayashi|first5=K.|last6=Kobayashi|first6=M.|last7=Moriyama|first7=S.|last8=Nakahata|first8=M.|last9=Norita|first9=T.|last10=Ogawa|first10=H.|last11=Sato|first11=K.|date=2018-05-01|title=Improved search for two-neutrino double electron capture on 124Xe and 126Xe using particle identification in XMASS-I|url=https://academic.oup.com/ptep/article/2018/5/053D03/5021518|journal=Progress of Theoretical and Experimental Physics|language=en|volume=2018|issue=5|doi=10.1093/ptep/pty053|doi-access=free|arxiv=1801.03251}}</ref>

Example:
:{| border="0"
|- style="height:2em;"
|{{nuclide|link=yes|barium|130}}&nbsp;||+&nbsp;||2&nbsp;{{SubatomicParticle|link=yes|Electron}}&nbsp;||→&nbsp;||{{nuclide|link=yes|xenon|130}}&nbsp;||+&nbsp;||2&nbsp;{{math|{{SubatomicParticle|link=yes|Electron Neutrino}}}}
|}

== Rarity ==
In most cases this decay mode is masked by other, more probable modes involving fewer particles, such as single [[electron capture]]. When all other modes are “forbidden” (strongly suppressed) double electron capture becomes the main mode of decay. There exist 34&nbsp;naturally occurring nuclei that are believed to undergo double electron capture, but the process has been confirmed by observation in the decay of only three nuclides: {{chem|78|36|Kr}}, {{chem|124|54|Xe}}, and {{chem|130|56|Ba}}.{{NUBASE2016|ref}}

One reason is that the probability of double electron capture is stupendously small; the [[half-life|half-lives]] for this mode lie well above 10{{sup|20}}&nbsp;years. A second reason is that the only detectable particles created in this process are [[X-ray]]s and [[Auger electron]]s that are emitted by the excited atomic shell. In the range of their energies (~1–10&nbsp;[[keV]]), the background is usually high. Thus, the experimental detection of double electron capture is more difficult than that for [[double beta decay]].

Double electron capture can be accompanied by the excitation of the daughter nucleus. Its de-excitation, in turn, is accompanied by an emission of photons with energies of hundreds of keV.{{citation needed|date=July 2019}}

== Modes with positron emission ==
If the mass difference between the mother and daughter atoms is more than two masses of an electron (1.022&nbsp;[[MeV]]), the energy released in the process is enough to allow another mode of decay, called '''electron capture with positron emission'''. It occurs along with double electron capture, their [[branching ratio]] depending on nuclear properties.

When the mass difference is more than four electron masses (2.044&nbsp;MeV), the third mode, called '''[[double positron decay]]''', is allowed. Only six naturally occurring nuclides (<sup>78</sup>Kr, <sup>96</sup>Ru, <sup>106</sup>Cd, <sup>124</sup>Xe, <sup>130</sup>Ba, and <sup>136</sup>Ce) plus the non-primordial <sup>148</sup>Gd and <sup>154</sup>Dy are energetically allowed to decay via these three modes simultaneously.<ref>{{cite journal |last1=Campani |first1=Alice |last2=Dompè |first2=Valentina |last3=Fantini |first3=Guido |date=2021 |title=Status and Perspectives on Rare Decay Searches in Tellurium Isotopes |journal=Universe |volume=7 |issue=7 |pages=212 |doi=10.3390/universe7070212 |doi-access=free |bibcode=2021Univ....7..212C |hdl=11573/1557551 |hdl-access=free }}</ref><ref>{{cite journal |last1=Ren |first1=Yuejiao |last2=Ren |first2=Zhongzhou |date=15 November 2014 |title=Half-lives of double β<sup>+</sup>-decay with two neutrinos |journal=Nuclear Physics |volume=58 |issue=2015 |pages=1–4 |doi=10.1007/s11433-014-5625-8 |arxiv=1501.01374 }}</ref><ref>{{cite conference|last=Ren|first=Zhongzhou|date=10 January 2015|title=Law of double-β decay half-lives with two neutrinos|url=http://tpcsf.ihep.ac.cn/FHNP2015/2015.1.10/ren150110.pdf|conference=Frontiers in Hadron and Nuclear Physics (FHNP'15)}}</ref>

== Neutrinoless double electron capture ==
The above-described process with the capture of two electrons and emission of two neutrinos (two-neutrino double electron capture) is allowed by the [[Standard Model]] of [[particle physics]]: No conservation laws (including [[lepton number]] conservation) are violated. However, if the lepton number is not conserved, or equivalently the [[neutrino]] is [[Majorana fermion|its own antiparticle]], another kind of process can occur: the so-called neutrinoless double electron capture. In this case, two electrons are captured by nucleus, but neutrinos are not emitted.<ref>{{cite journal |journal=Nuclear Physics B |volume=223 |issue=1 |date=1985-08-15 |df=dmy-all |pages=15–28 |title=Neutrinoless double electron capture as a tool to measure the electron neutrino mass |author1=Bernabeu, J. |author2=de&nbsp;Rujula, A. |author3=Jarlskog, C. |doi=10.1016/0550-3213(83)90089-5 |bibcode=1983NuPhB.223...15B|url=https://cds.cern.ch/record/143368/files/198303194.pdf }}</ref> The energy released in this process is carried away by an internal [[bremsstrahlung]] [[gamma rays|gamma quantum]].

Example:
:{| border="0"
|- style="height:2em;"
|{{nuclide|link=yes|barium|130}}&nbsp;||+&nbsp;||2&nbsp;{{SubatomicParticle|Electron}}&nbsp;||→&nbsp;||{{nuclide|link=yes|xenon|130}}
|}

This mode of decay has never been observed experimentally, and would contradict the [[Standard Model]] if it were observed.

==See also==
*[[Double beta decay]]
*[[Neutrinoless double beta decay]]
*[[Beta decay]]
*[[Neutrino]]
*[[Particle radiation]]
*[[Radioactive isotope]]

==References==
{{reflist|30em}}

==External links==
* {{cite journal |url=https://arstechnica.com/science/2019/04/dark-matter-detector-identifies-extraordinarily-rare-radioactive-decay/ |title=Radioactivity detected from a half-life of once every trillion universes |date=April 2019 |journal=[[Ars Technica]] |department=Science |doi=10.1038/s41586-019-1124-4|arxiv=1904.11002 |last1=Aprile |first1=E. |last2=Aalbers |first2=J. |last3=Agostini |first3=F. |last4=Alfonsi |first4=M. |last5=Althueser |first5=L. |last6=Amaro |first6=F. D. |last7=Anthony |first7=M. |last8=Antochi |first8=V. C. |last9=Arneodo |first9=F. |last10=Baudis |first10=L. |last11=Bauermeister |first11=B. |last12=Benabderrahmane |first12=M. L. |last13=Berger |first13=T. |last14=Breur |first14=P. A. |last15=Brown |first15=A. |last16=Brown |first16=A. |last17=Brown |first17=E. |last18=Bruenner |first18=S. |last19=Bruno |first19=G. |last20=Budnik |first20=R. |last21=Capelli |first21=C. |last22=Cardoso |first22=J. M. R. |last23=Cichon |first23=D. |last24=Coderre |first24=D. |last25=Colijn |first25=A. P. |last26=Conrad |first26=J. |last27=Cussonneau |first27=J. P. |last28=Decowski |first28=M. P. |last29=de Perio |first29=P. |last30=Di Gangi |first30=P. |volume=568 |issue=7753 |pages=532–535 |pmid=31019319 |s2cid=186243086 |display-authors=1 }}

{{Nuclear_processes}}

[[Category:Nuclear physics]]
[[Category:Radioactivity]]