View source for Positron emission

{{Short description|Type of radioactive decay}}
{{For|the medical imaging method|Positron emission tomography}}
{{Nuclear physics}}
'''Positron emission''', '''beta plus decay''', or '''β<sup>+</sup> decay''' is a subtype of [[radioactive decay]] called [[beta decay]], in which a [[proton]] inside a [[radionuclide]] nucleus is converted into a [[neutron]] while releasing a [[positron]] and an [[electron neutrino]] ({{math|''ν''<sub>e</sub>}}).<ref>{{cite web| publisher = The University of North Carolina at Chapel Hill|title=Nuclear Chemistry|url=http://www.shodor.org/unchem/advanced/nuc/|access-date=2012-06-14}}</ref> Positron emission is mediated by the [[weak force]]. The positron is a type of [[beta particle]] (β<sup>+</sup>), the other beta particle being the electron (β<sup>−</sup>) emitted from the β<sup>−</sup> decay of a nucleus.

An example of positron emission (β<sup>+</sup> decay) is shown with [[magnesium-23]] decaying into [[sodium-23]]:

:{{nuclide|Magnesium|23}} → {{nuclide|Sodium|23}} + {{math|{{subatomic particle|positron}}}} + {{math|{{subatomic particle|electron neutrino}}}}

Because positron emission decreases proton number relative to neutron number, positron decay happens typically in large "proton-rich" radionuclides. Positron decay results in [[nuclear transmutation]], changing an atom of one chemical element into an atom of an element with an [[atomic number]] that is less by one unit.

Positron emission occurs extremely rarely in nature on Earth. Known instances include [[cosmic ray]] interactions and the decay of certain [[isotopes]], such as [[potassium-40]]. This rare form of potassium makes up only 0.012% of the element on Earth and has a 1 in 100,000 chance of decaying via positron emission{{Citation needed|reason=Missing context/innacurate|date=June 2025}}.

Positron emission should not be confused with [[beta decay#β− decay|electron emission]] or beta minus decay (β<sup>−</sup> decay), which occurs when a neutron turns into a proton and the nucleus emits an electron and an antineutrino.

Positron emission is different from [[proton decay]], the hypothetical decay of protons, not necessarily those bound with neutrons, not necessarily through the emission of a positron, and not as part of nuclear physics, but rather of [[particle physics]].

==Discovery of positron emission==
In 1934 [[Frédéric Joliot-Curie|Frédéric]] and [[Irène Joliot-Curie]] bombarded aluminium with [[alpha particle]]s (emitted by [[polonium]]) to effect the nuclear reaction {{nuclide|Helium|4}}&nbsp;+&nbsp;{{nuclide|Aluminium|27}}&nbsp;→ {{nuclide|Phosphorus|30}}&nbsp;+&nbsp;{{nuclide|neutronium|1}}, and observed that the product isotope {{nuclide|Phosphorus|30}} emits a positron identical to those found in cosmic rays by [[Carl David Anderson]] in 1932.<ref>{{cite journal |vauthors= Joliot MF, Curie I |journal= J. Phys. |date= 1934 |volume= 5 |issue= 153 |pages= 254 |title= Un nouveau type de radioactivité |trans-title= A new type of radioactivity |language= French |url= https://gallica.bnf.fr/ark:/12148/bpt6k31506/f254.image}}</ref> This was the first example of {{subatomic particle|Beta+}}&nbsp;decay (positron emission). The Curies termed the phenomenon "artificial radioactivity", because {{nuclide|Phosphorus|30}} is a short-lived nuclide which does not exist in nature. The discovery of artificial radioactivity would be cited when the husband-and-wife team won the Nobel Prize.

==Positron-emitting isotopes==
[[Isotope]]s which undergo this decay and thereby emit positrons include, but are not limited to: [[carbon-11]], [[nitrogen-13]], [[isotopes of oxygen|oxygen-15]], [[fluorine-18]], [[copper-64]], gallium-68, bromine-78, [[rubidium-82]], yttrium-86, zirconium-89,<ref name="Conti 2016">{{cite journal | vauthors = Conti M, Eriksson L | title = Physics of pure and non-pure positron emitters for PET: a review and a discussion | journal = EJNMMI Physics | volume = 3 | issue = 1 | pages = 8 | date = December 2016 | pmid = 27271304 | pmc = 4894854 | doi = 10.1186/s40658-016-0144-5 | doi-access = free }}</ref> [[isotopes of sodium|sodium-22]], [[aluminium-26]], [[potassium-40]], [[strontium-83]], and [[isotopes of iodine|iodine-124]].<ref name="Conti 2016"/>{{NUBASE2020|ref}} As an example, the following equation describes the beta plus decay of carbon-11 to [[boron]]-11, emitting a positron and a [[neutrino]]:

<!-- Autogenerated using Phykiformulae 0.12 [[User:SkyLined#Phykiformulae]]
	C-11 -> B-11 + e+ + ve + 0.96 MeV
-->:{| border="0"
|- style="height:2em;"
|{{nuclide|link=yes|carbon|11}}&nbsp;||→&nbsp;||{{nuclide|link=yes|boron|11}}&nbsp;||+&nbsp;||{{math|{{subatomic particle|link=yes|Positron}}}}&nbsp;||+&nbsp;||{{math|{{subatomic particle|link=yes|Electron Neutrino}}}}&nbsp;||+&nbsp;||{{val|0.96|ul=MeV}}
|}

==Emission mechanism==
Inside protons and neutrons, there are [[fundamental particle]]s called [[quark]]s. The two most common types of quarks are ''[[up quark]]s'', which have a charge of +{{2/3}}, and ''[[down quark]]s'', with a −{{1/3}} charge. Quarks arrange themselves in sets of three such that they make [[proton]]s and [[neutron]]s. In a proton, whose charge is +1, there are two ''up'' quarks and one ''down'' quark ({{2/3}} + {{2/3}} − {{1/3}} = 1). Neutrons, with no charge, have one ''up'' quark and two ''down'' quarks ({{2/3}} − {{1/3}} − {{1/3}} = 0). Via the [[weak interaction]], quarks can change [[flavor (particle physics)|flavor]] from ''down'' to ''up'', resulting in [[beta particle|electron]] emission. Positron emission happens when an ''up'' quark changes into a ''down'' quark, effectively converting a proton to a neutron.<ref>[http://courses.washington.edu/bioen508/Lecture5-B-PET.pdf How it works:Positron emission]</ref>

Nuclei which decay by positron emission may also decay by [[electron capture]]. For low-energy decays, electron capture is energetically favored by 2''m''<sub>e</sub>''c''<sup>2</sup> = {{val|1.022|u=MeV}}, since the final state has an electron removed rather than a positron added. As the energy of the decay goes up, so does the [[branching fraction]] of positron emission. However, if the energy difference is less than 2''m''<sub>e</sub>''c''<sup>2</sup>, the positron emission cannot occur and electron capture is the sole decay mode. Certain otherwise electron-capturing isotopes (for instance, {{SimpleNuclide|link=yes|Beryllium|7}}) are stable in [[galactic cosmic ray]]s, because the electrons are stripped away and the decay energy is too small for positron emission.

==Energy conservation==
A positron is ejected from the parent nucleus, but the daughter (Z−1) atom still has Z atomic electrons from the parent, i.e. the daughter is a negative ion (at least immediately after the positron emission). Since tables of masses are for ''atomic'' masses, 
<math>
_Z^A \textrm {X} \rightarrow _{Z-1}^A \textrm {Y} +_{+1}^0 \textrm e^{+}+_{-1}^0 \textrm e^{-}
</math>
, and, since the mass of the positron is identical to that of the electron, the overall result is that the mass-energy of ''two'' electrons is required, and the β<sup>+</sup> decay is energetically possible [[if and only if]] the mass of the parent atom exceeds the mass of the daughter atom by at least two electron masses (2''m''<sub>e</sub> c<sup>2</sup> = 1.022&nbsp;MeV).<ref name="L'Annunziata_180">{{cite book |first=Michael F. |last=L'Annunziata |title=Radioactivity: Introduction and History, From the Quantum to Quarks |year=2016 |publisher=Elsevier |isbn=9780444634962 |page=180}}</ref>

Isotopes which increase in mass under the conversion of a proton to a neutron, or which decrease in mass by less than 2''m''<sub>e</sub>, cannot spontaneously decay by positron emission.<ref name="L'Annunziata_180"/>

==Application==
These isotopes are used in [[positron emission tomography]], a technique used for medical imaging. The energy emitted depends on the isotope that is decaying; the figure of {{val|0.96|u=MeV}} applies only to the decay of [[carbon-11]].

The short-lived positron emitting isotopes <sup>11</sup>C (T<sub>{{1/2}}</sub> = {{val|20.4|u=min}}), <sup>13</sup>N (T<sub>{{1/2}}</sub> = {{val|9.9|u=min}}), <sup>15</sup>O (T<sub>{{1/2}}</sub> = {{val|2.0|u=min}}), and <sup>18</sup>F (T<sub>{{1/2}}</sub> = {{val|109.8|u=min}}) used for positron emission tomography are typically produced by proton or deuteron irradiation of natural or enriched targets.<ref>{{cite web |url= http://www.pet.ubc.ca/radioisotopes |title= Positron Emission Tomography Imaging at the University of British Columbia |access-date= 11 May 2012 |work= Positron Emission Tomography Imaging |publisher= University of British Columbia |url-status=dead |archive-url= https://web.archive.org/web/20180122030757/http://www.pet.ubc.ca/radioisotopes |archive-date= 22 January 2018}}</ref><ref>{{cite journal | vauthors = Ledingham KW, McKenna P, McCanny T, Shimizu S, Yang JM, Robson L, Zweit J, Gillies JM, Bailey J, Chimon GN, Clarke RJ | doi = 10.1088/0022-3727/37/16/019 | title = High power laser production of short-lived isotopes for positron emission tomography | year = 2004 | journal = Journal of Physics D: Applied Physics | volume = 37 | issue = 16 | pages = 2341–2345 |bibcode = 2004JPhD...37.2341L| s2cid = 250744282 }}</ref>

==References==
{{Reflist}}

==External links==
*[[File:Ndslivechart.png]] '''[https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html Live Chart of Nuclides: nuclear structure and decay data]''' (main decay modes) - IAEA 
{{Nuclear processes}}

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[[Category:Radioactivity]]
[[Category:Electron]]
[[Category:Antimatter]]