Positron emission

<h2 id="discovery-of-positron-emission">Discovery of positron emission</h2>
In 1934 <a href="/facts/Fr%25C3%25A9d%25C3%25A9ric_Joliot-Curie/qTYsRfqb">Frédéric</a> and <a href="/facts/Ir%25C3%25A8ne_Joliot-Curie/rhBlqKxf">Irène Joliot-Curie</a> bombarded aluminium with <a href="/facts/Alpha_particle/dzNMs9WP">alpha particles</a> (emitted by <a href="/facts/Polonium/G4rzsyjP">polonium</a>) to effect the nuclear reaction 42He + 2713Al → 3015P + 10n, and observed that the product isotope 3015P emits a positron identical to those found in cosmic rays by <a href="/facts/Carl_David_Anderson/rIMqD9ax">Carl David Anderson</a> in 1932.<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a> This was the first example of β+ decay (positron emission). The Curies termed the phenomenon "artificial radioactivity", because 3015P 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.

<h2 id="positron-emitting-isotopes">Positron-emitting isotopes</h2>
<a href="/facts/Isotope/KD9B1y6o">Isotopes</a> which undergo this decay and thereby emit positrons include, but are not limited to: <a href="/facts/Carbon-11/K1LfqDNj">carbon-11</a>, <a href="/facts/Nitrogen-13/AMvEc8m7">nitrogen-13</a>, <a href="/facts/Isotopes_of_oxygen/ektDXb8B">oxygen-15</a>, <a href="/facts/Fluorine-18/4VpH1NYh">fluorine-18</a>, <a href="/facts/Copper-64/1KBmNStU">copper-64</a>, gallium-68, bromine-78, <a href="/facts/Rubidium-82/cnInn8hX">rubidium-82</a>, yttrium-86, zirconium-89,<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a> <a href="/facts/Isotopes_of_sodium/dGhdEPWX">sodium-22</a>, <a href="/facts/Aluminium-26/INwQYapg">aluminium-26</a>, <a href="/facts/Potassium-40/RMnkrhvc">potassium-40</a>, <a href="/facts/Strontium-83/ljDHPVaV">strontium-83</a>, and <a href="/facts/Isotopes_of_iodine/HxgH1yJD">iodine-124</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a><a class="footnote-ref" id="fnref:5" href="#fn:5">5</a> As an example, the following equation describes the beta plus decay of carbon-11 to <a href="/facts/Boron/Pmwgd7Mf">boron</a>-11, emitting a positron and a <a href="/facts/Neutrino/yGWVfSg2">neutrino</a>:

<table><tbody><tr><td><a href="/facts/Carbon-11/K1LfqDNj">116C</a> </td><td>→ </td><td><a href="/facts/Boron-11/Pmwgd7Mf">115B</a> </td><td>+ </td><td><a href="/facts/Positron/HgV4zBzA">e+</a> </td><td>+ </td><td><a href="/facts/Electron_neutrino/e5hQPWIQ">νe</a> </td><td>+ </td><td>0.96 <a href="/facts/Electronvolt/YMT1fqX9">MeV</a></td></tr></tbody></table>
<h2 id="emission-mechanism">Emission mechanism</h2>
Inside protons and neutrons, there are <a href="/facts/Fundamental_particle/CTFDM97U">fundamental particles</a> called <a href="/facts/Quark/hwuuVkV5">quarks</a>. The two most common types of quarks are <a href="/facts/Up_quark/Aw3Vp3vk">up quarks</a>, which have a charge of +2⁄3, and <a href="/facts/Down_quark/Nq10ayEE">down quarks</a>, with a −1⁄3 charge. Quarks arrange themselves in sets of three such that they make <a href="/facts/Proton/clCjepXP">protons</a> and <a href="/facts/Neutron/oRCM1geb">neutrons</a>. 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 <a href="/facts/Weak_interaction/f6upmKMs">weak interaction</a>, quarks can change <a href="/facts/Flavor_(particle_physics)/RR7rsuTD">flavor</a> from down to up, resulting in <a href="/facts/Beta_particle/x8b3ttLn">electron</a> emission. Positron emission happens when an up quark changes into a down quark, effectively converting a proton to a neutron.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a>
Nuclei which decay by positron emission may also decay by <a href="/facts/Electron_capture/EIccoV1u">electron capture</a>. For low-energy decays, electron capture is energetically favored by 2mec2 = 1.022 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 <a href="/facts/Branching_fraction/QrbbS9PI">branching fraction</a> of positron emission. However, if the energy difference is less than 2mec2, the positron emission cannot occur and electron capture is the sole decay mode. Certain otherwise electron-capturing isotopes (for instance, <a href="/facts/Beryllium-7/bNvHB5xg">7Be</a>) are stable in <a href="/facts/Galactic_cosmic_ray/uxCVdTKd">galactic cosmic rays</a>, because the electrons are stripped away and the decay energy is too small for positron emission.

<h2 id="energy-conservation">Energy conservation</h2>
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,

Z
          
          
            A
          
        
        
          
            X
          
        
        
          →
          
            Z
            −
            1
          
          
            A
          
        
        
          
            Y
          
        
        
          +
          
            +
            1
          
          
            0
          
        
        
          
            
              e
            
          
          
            +
          
        
        
          +
          
            −
            1
          
          
            0
          
        
        
          
            
              e
            
          
          
            −
          
        
      
    
    {\displaystyle _{Z}^{A}{\textrm {X}}\rightarrow _{Z-1}^{A}{\textrm {Y}}+_{+1}^{0}{\textrm {e}}^{+}+_{-1}^{0}{\textrm {e}}^{-}}

, 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+ decay is energetically possible <a href="/facts/If_and_only_if/bYSxGJ66">if and only if</a> the mass of the parent atom exceeds the mass of the daughter atom by at least two electron masses (2me c2 = 1.022 MeV).<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>
Isotopes which increase in mass under the conversion of a proton to a neutron, or which decrease in mass by less than 2me, cannot spontaneously decay by positron emission.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a>

<h2 id="application">Application</h2>
These isotopes are used in <a href="/facts/Positron_emission_tomography/5SmcW6qz">positron emission tomography</a>, a technique used for medical imaging. The energy emitted depends on the isotope that is decaying; the figure of 0.96 MeV applies only to the decay of <a href="/facts/Carbon-11/K1LfqDNj">carbon-11</a>.
The short-lived positron emitting isotopes 11C (T1⁄2 = 20.4 min), 13N (T1⁄2 = 9.9 min), 15O (T1⁄2 = 2.0 min), and 18F (T1⁄2 = 109.8 min) used for positron emission tomography are typically produced by proton or deuteron irradiation of natural or enriched targets.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a><a class="footnote-ref" id="fnref:10" href="#fn:10">10</a>

<h2 id="external-links">External links</h2>
<ul><li> <a href="https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html">Live Chart of Nuclides: nuclear structure and decay data</a> (main decay modes) - IAEA</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">"Nuclear Chemistry". The University of North Carolina at Chapel Hill. Retrieved 2012-06-14. <a href="http://www.shodor.org/unchem/advanced/nuc/" target="_blank">http://www.shodor.org/unchem/advanced/nuc/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Joliot MF, Curie I (1934). "Un nouveau type de radioactivité" [A new type of radioactivity]. J. Phys. (in French). 5 (153): 254. <a href="https://gallica.bnf.fr/ark:/12148/bpt6k31506/f254.image" target="_blank">https://gallica.bnf.fr/ark:/12148/bpt6k31506/f254.image</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Conti M, Eriksson L (December 2016). "Physics of pure and non-pure positron emitters for PET: a review and a discussion". EJNMMI Physics. 3 (1): 8. doi:10.1186/s40658-016-0144-5. PMC 4894854. PMID 27271304. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4894854" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4894854</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Conti M, Eriksson L (December 2016). "Physics of pure and non-pure positron emitters for PET: a review and a discussion". EJNMMI Physics. 3 (1): 8. doi:10.1186/s40658-016-0144-5. PMC 4894854. PMID 27271304. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4894854" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4894854</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
 <a href="https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf" target="_blank">https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">How it works:Positron emission <a href="http://courses.washington.edu/bioen508/Lecture5-B-PET.pdf" target="_blank">http://courses.washington.edu/bioen508/Lecture5-B-PET.pdf</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">L'Annunziata, Michael F. (2016). Radioactivity: Introduction and History, From the Quantum to Quarks. Elsevier. p. 180. ISBN 9780444634962. <a href="9780444634962" target="_blank">9780444634962</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">L'Annunziata, Michael F. (2016). Radioactivity: Introduction and History, From the Quantum to Quarks. Elsevier. p. 180. ISBN 9780444634962. <a href="9780444634962" target="_blank">9780444634962</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">"Positron Emission Tomography Imaging at the University of British Columbia". Positron Emission Tomography Imaging. University of British Columbia. Archived from the original on 22 January 2018. Retrieved 11 May 2012. <a href="https://web.archive.org/web/20180122030757/http://www.pet.ubc.ca/radioisotopes" target="_blank">https://web.archive.org/web/20180122030757/http://www.pet.ubc.ca/radioisotopes</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Ledingham KW, McKenna P, McCanny T, Shimizu S, Yang JM, Robson L, Zweit J, Gillies JM, Bailey J, Chimon GN, Clarke RJ (2004). "High power laser production of short-lived isotopes for positron emission tomography". Journal of Physics D: Applied Physics. 37 (16): 2341–2345. Bibcode:2004JPhD...37.2341L. doi:10.1088/0022-3727/37/16/019. S2CID 250744282. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
</ol>

Positron emission open-in-new

Positron emission