positron
{{Short description|Anti-particle to the electron}}
{{Other uses|Positron (disambiguation)}}
{{Use dmy dates|date=August 2018}}
{{Infobox Particle
| name = Positron (antielectron)
| image = 200px
| caption = Cloud chamber photograph by C. D. Anderson of the first positron ever identified. A 6 mm lead plate separates the chamber. The deflection and direction of the particle's ion trail indicate that the particle is a positron.
| num_types =
| composition = Elementary particle
| statistics = Fermionic
| generation = First
| interaction = Gravity, electromagnetic, weak
| antiparticle = Electron
| theorized = Paul Dirac (1928)
| discovered = Carl D. Anderson (1932)
| symbol = {{SubatomicParticle|Positron}}, {{SubatomicParticle|Beta+}}
| mass=me
{{physconst|me}}
{{physconst|me_Da}}
{{physconst|mec2_MeV|unit={{val|ul=MeV/c2}}}}
| mean_lifetime = stable (same as electron)
| decay_particle =
| electric_charge = +1 e
+{{physconst|e}}
| weak_isospin = {{nowrap|LH: 0, RH: {{sfrac|1|2}}}}
| color_charge =
| spin = {{sfrac|1|2}} ħ (same as electron)
| num_spin_states =
}}
The positron or antielectron is the particle with an electric charge of +1e, a spin of 1/2 (the same as the electron), and the same mass as an electron. It is the antiparticle (antimatter counterpart) of the electron. When a positron collides with an electron, annihilation occurs. If this collision occurs at low energies, it results in the production of two or more photons.
Positrons can be created by positron emission radioactive decay (through weak interactions), or by pair production from a sufficiently energetic photon which is interacting with an atom in a material.
History
= Theory =
In 1928, Paul Dirac published a paper proposing that electrons can have both a positive and negative charge.
{{cite journal
|last=Dirac |first=P. A. M.
|authorlink=Paul Dirac
|year=1928
|title=The quantum theory of the electron
|journal=Proceedings of the Royal Society A
|volume=117 |issue=778 |pages=610–624
|bibcode=1928RSPSA.117..610D
|doi=10.1098/rspa.1928.0023
|doi-access=free
}} This paper introduced the Dirac equation, a unification of quantum mechanics, special relativity, and the then-new concept of electron spin to explain the Zeeman effect. The paper did not explicitly predict a new particle but did allow for electrons having either positive or negative energy as solutions. Hermann Weyl then published a paper discussing the mathematical implications of the negative energy solution.
{{cite journal
|last=Weyl |first=H.
|year=1929
|title=Gravitation and the Electron
|journal=PNAS
|volume=15 |issue=4 |pages=323–334
|bibcode=1929PNAS...15..323W
|doi=10.1073/pnas.15.4.323
|pmid=16587474
|pmc=522457
|doi-access=free
}} The positive-energy solution explained experimental results, but Dirac was puzzled by the equally valid negative-energy solution that the mathematical model allowed. Quantum mechanics did not allow the negative energy solution to simply be ignored, as classical mechanics often did in such equations; the dual solution implied the possibility of an electron spontaneously jumping between positive and negative energy states. However, no such transition had yet been observed experimentally.
Dirac wrote a follow-up paper in December 1929
{{cite journal
|last=Dirac |first=P. A. M.
|authorlink=Paul Dirac
|year=1930
|title=A theory of electrons and protons
|journal=Proceedings of the Royal Society A
|volume=126 |issue=801 |pages=360–365
|bibcode=1930RSPSA.126..360D
|doi=10.1098/rspa.1930.0013
|doi-access=free
}} that attempted to explain the unavoidable negative-energy solution for the relativistic electron. He argued that "... an electron with negative energy moves in an external [electromagnetic] field as though it carries a positive charge." He further asserted that all of space could be regarded as a "sea" of negative energy states that were filled, so as to prevent electrons jumping between positive energy states (negative electric charge) and negative energy states (positive charge). The paper also explored the possibility of the proton being an island in this sea, and that it might actually be a negative-energy electron. Dirac acknowledged that the proton having a much greater mass than the electron was a problem, but expressed "hope" that a future theory would resolve the issue.
Robert Oppenheimer argued strongly against the proton being the negative-energy electron solution to Dirac's equation. He asserted that if it were, the hydrogen atom would rapidly self-destruct.{{Cite journal
|last=Oppenheimer |first=J. R.
|author-link=J. Robert Oppenheimer
|date=March 1930
|title=Note on the Theory of the Interaction of Field and Matter
|journal=Physical Review
|volume=35 |issue=5 |pages=461–477
|doi=10.1103/PhysRev.35.461 |bibcode=1930PhRv...35..461O
|issn=0031-899X
}} Weyl in 1931 showed that the negative-energy electron must have the same mass as that of the positive-energy electron.{{Cite journal
|last=Weyl |first=H.
|author-link=Hermann Weyl
|date=November 1927
|title=Quantenmechanik und Gruppentheorie
|url=https://www.thphys.uni-heidelberg.de/~wolschin/qms1920_7s.pdf
|journal=Zeitschrift für Physik
|language=de |volume=46 |issue=1–2 |pages=1–46
|doi=10.1007/BF02055756 |bibcode=1927ZPhy...46....1W
|issn=1434-6001
}} Persuaded by Oppenheimer's and Weyl's argument, Dirac published a paper in 1931 that predicted the existence of an as-yet-unobserved particle that he called an "anti-electron" that would have the same mass and the opposite charge as an electron and that would mutually annihilate upon contact with an electron.
{{cite journal
|last=Dirac |first=P. A. M.
|authorlink=Paul Dirac
|year=1931
|title=Quantised Singularities in the Quantum Field
|journal=Proceedings of the Royal Society A
|volume=133 |issue=821 |pages=60–72
|bibcode=1931RSPSA.133...60D
|doi=10.1098/rspa.1931.0130
|doi-access=free
}}
Ernst Stueckelberg, and later Richard Feynman, proposed an interpretation of the positron as an electron moving backward in time,{{cite journal
|last=Feynman
|first=R.
|year=1949
|title=The theory of positrons
|journal=Physical Review
|volume=76
|issue=6
|pages=749–759
|bibcode=1949PhRv...76..749F
|doi=10.1103/PhysRev.76.749
|s2cid=120117564
|url=https://authors.library.caltech.edu/3520/
|access-date=28 December 2021
|archive-date=9 August 2022
|archive-url=https://web.archive.org/web/20220809030941/https://authors.library.caltech.edu/3520/
|url-status=dead
}} reinterpreting the negative-energy solutions of the Dirac equation. Electrons moving backward in time would have a positive electric charge. John Archibald Wheeler invoked this concept to explain the identical properties shared by all electrons, suggesting that "they are all the same electron" with a complex, self-intersecting worldline.{{cite speech |title=The Development of the Space-Time View of Quantum Electrodynamics |last=Feynman |first=R. |date=11 December 1965 |location=Nobel Lecture |url=http://nobelprize.org/nobel_prizes/physics/laureates/1965/feynman-lecture.html |access-date=2 January 2007}} Yoichiro Nambu later applied it to all production and annihilation of particle-antiparticle pairs, stating that "the eventual creation and annihilation of pairs that may occur now and then is no creation or annihilation, but only a change of direction of moving particles, from the past to the future, or from the future to the past."
{{cite journal
|last=Nambu |first=Y.
|year=1950
|title=The Use of the Proper Time in Quantum Electrodynamics I
|journal=Progress of Theoretical Physics
|volume=5 |issue=1 |pages=82–94
|bibcode=1950PThPh...5...82N
|doi=10.1143/PTP/5.1.82
|doi-access=free
}} The backwards in time point of view is nowadays accepted as completely equivalent to other pictures, but it does not have anything to do with the macroscopic terms "cause" and "effect", which do not appear in a microscopic physical description.{{citation needed|date=July 2020}}
= Experimental clues and discovery =
File:Cloud chambers played an important role of particle detectors.jpgs used to be very important particle detectors in the early days of particle physics. They were used in the discovery of the positron, muon, and kaon.]]
{{Antimatter}}
Several sources have claimed that Dmitri Skobeltsyn first observed the positron long before 1930,
{{cite book
|last=Wilson |first=David
|year=1983
|title=Rutherford, Simple Genius
|pages=562–563
|publisher=Hodder and Stoughton
|isbn=0-340-23805-4
{{cite book
|last=Close |first=F. |author-link=Frank Close
|year=2009
|title=Antimatter
|pages=50–52
|publisher=Oxford University Press
|isbn=978-0-19-955016-6
}} They state that while using a Wilson cloud chamber
{{cite journal
|last=Cowan |first=E.
|date=1982
|title=The Picture That Was Not Reversed
|journal=Engineering & Science
|volume=46 |issue=2 |pages=6–28
|url=http://calteches.library.caltech.edu/3360/
}} in order to study the Compton effect, Skobeltsyn detected particles that acted like electrons but curved in the opposite direction in an applied magnetic field, and that he presented photographs with this phenomenon in a conference in the University of Cambridge, on 23–27 July 1928. In his book
{{cite book
|last=Hanson |first=Norwood Russel
|year=1963
|title=The Concept of the Positron
|pages=136–139
|publisher=Cambridge University Press
|isbn=978-0-521-05198-9
}} on the history of the positron discovery from 1963, Norwood Russell Hanson has given a detailed account of the reasons for this assertion, and this may have been the origin of the myth. But he also presented Skobeltsyn's objection to it in an appendix.
{{cite book
|last=Hanson |first=Norwood Russel
|year=1963
|title=The Concept of the Positron
|pages=179–183
|publisher=Cambridge University Press
|isbn=978-0-521-05198-9
}} Later, Skobeltsyn rejected this claim even more strongly, calling it "nothing but sheer nonsense".
{{cite book
|last1=Brown |first1=Laurie M.
|last2=Hoddeson |first2=Lillian
|year=1983
|title=The Birth of Particle Physics
|pages=118–119
|publisher=Cambridge University Press
|isbn=0-521-24005-0
}}
Skobeltsyn did pave the way for the eventual discovery of the positron by two important contributions: adding a magnetic field to his cloud chamber (in 1925
{{cite journal
|last=Bazilevskaya |first=G.A.
|year=2014
|title=Skobeltsyn and the early years of cosmic particle physics in the Soviet Union
|pages=61–66
|journal=Astroparticle Physics
|volume=53
|doi=10.1016/j.astropartphys.2013.05.007
|bibcode=2014APh....53...61B
}}), and by discovering charged particle cosmic rays,
{{cite journal
|last=Skobeltsyn |first=D.
|year=1929
|title=Uber eine neue Art sehr schneller beta-Strahlen
|pages=686–702
|journal=Z. Phys.
|volume=54
|issue=9–10
|doi=10.1007/BF01341600
|bibcode=1929ZPhy...54..686S
|s2cid=121748135
}} for which he is credited in Carl David Anderson's Nobel lecture.
{{cite web
|last=Anderson |first=Carl D.
|year=1936
|title=The Production and Properties of Positrons
|url=https://www.nobelprize.org/prizes/physics/1936/anderson/lecture/
|access-date=10 August 2020
}} Skobeltsyn did observe likely positron tracks on images taken in 1931,
{{cite journal
|last=Skobeltzyn |first=D.
|year=1934
|title=Positive electron tracks
|pages=23–24
|journal=Nature
|volume=133
|issue=3349
|doi=10.1038/133023a0
|bibcode=1934Natur.133...23S
|s2cid=4226799
}} but did not identify them as such at the time.
Likewise, in 1929 Chung-Yao Chao, a Chinese graduate student at Caltech, noticed some anomalous results that indicated particles behaving like electrons, but with a positive charge, though the results were inconclusive and the phenomenon was not pursued.
{{cite book
|last1=Merhra |first1=J. |author-link1=Jagdish Mehra
|last2=Rechenberg |first2=H. |author-link2=Helmut Rechenberg
|year=2000
|title=The Historical Development of Quantum Theory, Volume 6: The Completion of Quantum Mechanics 1926–1941
|url=https://books.google.com/books?id=9l61Dy9FBfYC&q=Chung-Yao%20Chao%20positron&pg=PA804
|page=804
|publisher=Springer
|isbn=978-0-387-95175-1
}} Fifty years later, Anderson acknowledged that his discovery was inspired by the work of his Caltech classmate Chung-Yao Chao, whose research formed the foundation from which much of Anderson's work developed but was not credited at the time.{{Cite journal|last=Cao|first=Cong|date=2004|title=Chinese Science and the 'Nobel Prize Complex'|url=http://china-us.uoregon.edu/pdf/Minerva-2004.pdf|journal=Minerva|language=en|volume=42|issue=2|page=154|doi=10.1023/b:mine.0000030020.28625.7e|s2cid=144522961|issn=0026-4695}}
Anderson discovered the positron on 2 August 1932,
{{cite journal
|last=Anderson |first=C. D.
|date=1933
|title=The Positive Electron
|journal=Physical Review
|volume=43 |issue=6 |pages=491–494
|bibcode=1933PhRv...43..491A
|doi=10.1103/PhysRev.43.491
|doi-access=free
}} for which he won the Nobel Prize for Physics in 1936.
{{cite web
|title=The Nobel Prize in Physics 1936
|url=http://nobelprize.org/nobel_prizes/physics/laureates/1936/index.html
|access-date=21 January 2010
}} Anderson did not coin the term positron, but allowed it at the suggestion of the Physical Review journal editor to whom he submitted his discovery paper in late 1932. The positron was the first evidence of antimatter and was discovered when Anderson allowed cosmic rays to pass through a cloud chamber and a lead plate. A magnet surrounded this apparatus, causing particles to bend in different directions based on their electric charge. The ion trail left by each positron appeared on the photographic plate with a curvature matching the mass-to-charge ratio of an electron, but in a direction that showed its charge was positive.
{{cite web
|last=Gilmer
|first=P. J.
|date=19 July 2011
|title=Irène Jolit-Curie, a Nobel laureate in artificial radioactivity
|url=http://www.chem.fsu.edu/~gilmer/PDFs/Ch%202_Irene_Curie_Penny_Gilmer_6-19-11_pg_mh.pdf
|page=8
|access-date=13 July 2013
|archive-url=https://web.archive.org/web/20140519131211/http://www.chem.fsu.edu/~gilmer/PDFs/Ch%202_Irene_Curie_Penny_Gilmer_6-19-11_pg_mh.pdf
|archive-date=19 May 2014
|url-status=dead
}}
Anderson wrote in retrospect that the positron could have been discovered earlier based on Chung-Yao Chao's work, if only it had been followed up on. Frédéric and Irène Joliot-Curie in Paris had evidence of positrons in old photographs when Anderson's results came out, but they had dismissed them as protons.
The positron had also been contemporaneously discovered by Patrick Blackett and Giuseppe Occhialini at the Cavendish Laboratory in 1932. Blackett and Occhialini had delayed publication to obtain more solid evidence, so Anderson was able to publish the discovery first.
{{cite web
|date=2011–2014
|title=Atop the Physics Wave: Rutherford Back in Cambridge, 1919–1937
|url=http://www.aip.org/history/exhibits/rutherford/sections/atop-physics-wave.html
|website=Rutherford's Nuclear World
|publisher=American Institute of Physics
|access-date=19 August 2014
|archive-date=21 October 2014
|archive-url=https://web.archive.org/web/20141021094704/http://www.aip.org/history/exhibits/rutherford/sections/atop-physics-wave.html
|url-status=dead
}}
Natural production
{{main|Positron emission}}
Positrons are produced, together with neutrinos naturally in β+ decays of naturally occurring radioactive isotopes (for example, potassium-40) and in interactions of gamma quanta (emitted by radioactive nuclei) with matter. Antineutrinos are another kind of antiparticle produced by natural radioactivity (β− decay). Many different kinds of antiparticles are also produced by (and contained in) cosmic rays. In research published in 2011 by the American Astronomical Society, positrons were discovered originating above thunderstorm clouds; positrons are produced in gamma-ray flashes created by electrons accelerated by strong electric fields in the clouds.
{{Cite news
|last=Palmer |first=J.
|date=11 January 2011
|title=Antimatter caught streaming from thunderstorms on Earth
|journal=BBC News
|url=https://www.bbc.co.uk/news/science-environment-12158718
|access-date=11 January 2011
|archive-url=https://web.archive.org/web/20110112080623/http://www.bbc.co.uk/news/science-environment-12158718
|archive-date=12 January 2011
|url-status=live
}} Antiprotons have also been found to exist in the Van Allen Belts around the Earth by the PAMELA module.
{{cite journal
|last1=Adriani |first1=O.
|display-authors=etal
|date=2011
|title=The Discovery of Geomagnetically Trapped Cosmic-Ray Antiprotons
|journal=The Astrophysical Journal Letters
|volume=737 |issue=2 |pages=L29
|arxiv=1107.4882
|bibcode=2011ApJ...737L..29A |bibcode-access=free
|doi=10.1088/2041-8205/737/2/L29 |doi-access=free
{{cite news
|last=Than |first=K.
|date=10 August 2011
|title=Antimatter Found Orbiting Earth—A First
|url=http://news.nationalgeographic.com/news/2011/08/110810-antimatter-belt-earth-trapped-pamela-space-science/
|archive-url=https://web.archive.org/web/20111010014111/http://news.nationalgeographic.com/news/2011/08/110810-antimatter-belt-earth-trapped-pamela-space-science
|url-status=dead
|archive-date=10 October 2011
|publisher=National Geographic Society
|access-date=12 August 2011
}}
Antiparticles, of which the most common are antineutrinos and positrons due to their low mass, are also produced in any environment with a sufficiently high temperature (mean particle energy greater than the pair production threshold). During the period of baryogenesis, when the universe was extremely hot and dense, matter and antimatter were continually produced and annihilated. The presence of remaining matter, and absence of detectable remaining antimatter,
{{cite web
|date=29 May 2000
|title=What's the Matter with Antimatter?
|url=https://science.nasa.gov/headlines/y2000/ast29may_1m.htm
|publisher=NASA
|access-date=24 May 2008
|archive-url=https://web.archive.org/web/20080604155823/https://science.nasa.gov/headlines/y2000/ast29may_1m.htm
|archive-date=4 June 2008
|url-status=dead
}} also called baryon asymmetry, is attributed to CP-violation: a violation of the CP-symmetry relating matter to antimatter. The exact mechanism of this violation during baryogenesis remains a mystery.
{{cite web
|date=19 October 2017
|title=Riddle of matter remains unsolved: Proton and antiproton share fundamental properties
|url=http://www.uni-mainz.de/presse/aktuell/3027_ENG_HTML.php
|publisher=Johannes Gutenberg University Mainz
}}
Positron production from radioactive {{Subatomic particle|beta+}} decay can be considered both artificial and natural production, as the generation of the radioisotope can be natural or artificial. Perhaps the best known naturally-occurring radioisotope which produces positrons is potassium-40, a long-lived isotope of potassium which occurs as a primordial isotope of potassium. Even though it is a small percentage of potassium (0.0117%), it is the single most abundant radioisotope in the human body. In a human body of {{Convert|70|kg|lb|abbr=on}} mass, about 4,400 nuclei of 40K decay per second.
{{cite web
|title=Radiation and Radioactive Decay. Radioactive Human Body
|url=http://www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/RadioactiveHumanBody/RadioactiveHumanBody.html
|access-date=18 May 2011
|publisher=Harvard Natural Sciences Lecture Demonstrations
}} The activity of natural potassium is 31 Bq/g.
{{cite book
|last1=Wintergham |first1=F. P. W.
|date=1989
|title=Radioactive Fallout in Soils, Crops and Food
|url=https://books.google.com/books?id=KRVXMiQWi0cC&pg=PA32
|page=32
|publisher=Food and Agriculture Organization
|isbn=978-92-5-102877-3
}} About 0.001% of these 40K decays produce about 4000 natural positrons per day in the human body.
{{cite journal
|last1=Engelkemeir |first1=D. W.
|last2=Flynn |first2=K. F.
|last3=Glendenin |first3=L. E.
|date=1962
|title=Positron Emission in the Decay of K40
|journal=Physical Review
|volume=126 |issue=5 |page=1818
|bibcode=1962PhRv..126.1818E
|doi=10.1103/PhysRev.126.1818
}} These positrons soon find an electron, undergo annihilation, and produce pairs of 511 keV photons, in a process similar (but much lower intensity) to that which happens during a PET scan nuclear medicine procedure.{{citation needed|date=April 2016}}
Recent observations indicate black holes and neutron stars produce vast amounts of positron-electron plasma in astrophysical jets. Large clouds of positron-electron plasma have also been associated with neutron stars.{{Cite web|url=http://pc.astro.brandeis.edu/pdfs/elec-pos.pdf|title=Electron-positron Jets Associated with Quasar 3C 279}}
{{cite web
|url=http://www.nasa.gov/centers/goddard/news/topstory/2007/antimatter_binary.html
|title=Vast Cloud of Antimatter Traced to Binary Stars
|publisher=NASA
= Observation in cosmic rays =
{{main|Cosmic ray}}
Satellite experiments have found evidence of positrons (as well as a few antiprotons) in primary cosmic rays, amounting to less than 1% of the particles in primary cosmic rays.{{cite journal |last1=Golden |title=Measurement of the Positron to Electron Ratio in Cosmic Rays above 5 GeV |journal=Astrophysical Journal Letters |date=February 1996 |volume=457 |issue=2 |doi=10.1086/309896 |bibcode=1996ApJ...457L.103G |hdl=11576/2514376 |s2cid=122660096 |url=https://ui.adsabs.harvard.edu/abs/1996ApJ...457L.103G/abstract |access-date=19 October 2021|hdl-access=free }} However, the fraction of positrons in cosmic rays has been measured more recently with improved accuracy, especially at much higher energy levels, and the fraction of positrons has been seen to be greater in these higher energy cosmic rays.{{cite journal |last1=Boudaud |title=A new look at the cosmic ray positron fraction |journal=Astronomy & Astrophysics |date=19 December 2014 |volume=575 |pages=A67 |url=https://www.aanda.org/articles/aa/full_html/2015/03/aa25197-14/aa25197-14.html |access-date=19 October 2021 |doi=10.1051/0004-6361/201425197 |doi-access=free|arxiv=1410.3799 }}
These do not appear to be the products of large amounts of antimatter from the Big Bang, or indeed complex antimatter in the universe (evidence for which is lacking, see below). Rather, the antimatter in cosmic rays appear to consist of only these two elementary particles. Recent theories suggest the source of such positrons may come from annihilation of dark matter particles, acceleration of positrons to high energies in astrophysical objects, and production of high energy positrons in the interactions of cosmic ray nuclei with interstellar gas.{{cite web |title=Towards Understanding the Origin of Cosmic-Ray Positrons |url=https://ams02.space/physics/towards-understanding-origin-cosmic-ray-positrons |website=The Alpha Magnetic Spectrometer on the International Space Station |access-date=19 October 2021}}
Preliminary results from the presently operating Alpha Magnetic Spectrometer (AMS-02) on board the International Space Station show that positrons in the cosmic rays arrive with no directionality, and with energies that range from 0.5 GeV to 500 GeV.
{{cite journal
|last1=Accardo |first1=L.
|collaboration=AMS Collaboration
|date=2014
|title=High Statistics Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–500 GeV with the Alpha Magnetic Spectrometer on the International Space Station
|url=http://ams.nasa.gov/Documents/AMS_Publications/PhysRevLett.113.121101.pdf
|journal=Physical Review Letters
|volume=113 |issue=12
|page=121101
|bibcode=2014PhRvL.113l1101A
|doi=10.1103/PhysRevLett.113.121101
|pmid=25279616
|doi-access=free
{{Cite journal|last1=Schirber |first1=M.
|title=Synopsis: More Dark Matter Hints from Cosmic Rays?
|journal=Physical Review Letters
|volume=113
|issue=12
|pages=121102
|doi=10.1103/PhysRevLett.113.121102
|pmid=25279617
|year=2014
|arxiv=1701.07305
|bibcode=2014PhRvL.113l1102A
|url=https://cds.cern.ch/record/1756487
|hdl=1721.1/90426
|s2cid=2585508
}} Positron fraction peaks at a maximum of about 16% of total electron+positron events, around an energy of 275 ± 32 GeV. At higher energies, up to 500 GeV, the ratio of positrons to electrons begins to fall again. The absolute flux of positrons also begins to fall before 500 GeV, but peaks at energies far higher than electron energies, which peak about 10 GeV.
{{cite web
|title=New results from the Alpha Magnetic Spectrometer on the International Space Station
|url=http://ams.nasa.gov/Documents/AMS_Publications/ams_new_results_-_18.09.2014.pdf
|website=AMS-02 at NASA
|access-date=21 September 2014
}}{{Cite web|url=http://www1b.physik.rwth-aachen.de/~pebs/?PEBS_physics:Positron_fraction|title=Positron fraction|access-date=22 July 2018|archive-date=22 July 2018|archive-url=https://web.archive.org/web/20180722184917/http://www1b.physik.rwth-aachen.de/~pebs/?PEBS_physics:Positron_fraction|url-status=dead}} These results on interpretation have been suggested to be due to positron production in annihilation events of massive dark matter particles.
{{Cite journal
|last1=Aguilar |first1=M.
|display-authors=etal
|year=2013
|title=First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350 GeV
|journal=Physical Review Letters
|volume=110 |issue=14 |pages=141102
|bibcode=2013PhRvL.110n1102A
|doi=10.1103/PhysRevLett.110.141102
|pmid=25166975
|url=https://boa.unimib.it/bitstream/10281/44680/1/2013_PhysRevLett.110.141102_positron_fraction.pdf|doi-access=free
}}
Positrons, like anti-protons, do not appear to originate from any hypothetical "antimatter" regions of the universe. On the contrary, there is no evidence of complex antimatter atomic nuclei, such as antihelium nuclei (i.e., anti-alpha particles), in cosmic rays. These are actively being searched for. A prototype of the AMS-02 designated AMS-01, was flown into space aboard the {{OV|103}} on STS-91 in June 1998. By not detecting any antihelium at all, the AMS-01 established an upper limit of 1.1×10−6 for the antihelium to helium flux ratio.
{{cite journal
|last1=Aguilar |first1=M.
|display-authors=etal
|collaboration=AMS Collaboration
|date=2002
|title=The Alpha Magnetic Spectrometer (AMS) on the International Space Station: Part I – results from the test flight on the space shuttle
|journal=Physics Reports
|volume=366 |issue=6 |pages=331–405
|bibcode=2002PhR...366..331A
|doi=10.1016/S0370-1573(02)00013-3
|hdl=2078.1/72661
|s2cid=122726107
}}
Artificial production
Physicists at the Lawrence Livermore National Laboratory in California have used a short, ultra-intense laser to irradiate a millimeter-thick gold target and produce more than 100 billion positrons.
{{cite news
|last=Bland |first=E.
|date=1 December 2008
|title=Laser technique produces bevy of antimatter
|url=https://www.nbcnews.com/id/wbna27998860
|quote=The LLNL scientists created the positrons by shooting the lab's high-powered Titan laser onto a one-millimeter-thick piece of gold.
|publisher=NBC News
|access-date=6 April 2016
}} Presently significant lab production of 5 MeV positron-electron beams allows investigation of multiple characteristics such as how different elements react to 5 MeV positron interactions or impacts, how energy is transferred to particles, and the shock effect of gamma-ray bursts.{{Cite web |last=Chen |first=Hui |date=February 2012 |title=Relativistic Electron-positron Plasma Jets Using High-intensity Lasers |url=https://lasers.llnl.gov/workshops/user_group_2012/docs/7.3_chen.pdf |url-status=dead |archive-url=https://web.archive.org/web/20120512122456/https://lasers.llnl.gov/workshops/user_group_2012/docs/7.3_chen.pdf |archive-date=12 May 2012 |website=Lawrence Livermore National Laboratory}} Lab production of 5MeV positron-electron beams.
In 2023, a collaboration between CERN and University of Oxford performed an experiment at the HiRadMat facility{{cite web |url=https://hiradmat.web.cern.ch/ |title=The HiRadMat Facility at SPS |date= 8 Dec 2023}}
in which nano-second duration beams of electron-positron pairs were produced containing more than 10 trillion electron-positron pairs, so creating the first 'pair plasma' in the laboratory with sufficient density to support collective plasma behavior.{{Cite journal |last1=Arrowsmith |first1=C. D. |last2=Simon |first2=P. |last3=Bilbao |first3=P. J. |last4=Bott |first4=A. F. A. |last5=Burger |first5=S. |last6=Chen |first6=H. |last7=Cruz |first7=F. D. |last8=Davenne |first8=T. |last9=Efthymiopoulos |first9=I. |last10=Froula |first10=D. H. |last11=Goillot |first11=A. |last12=Gudmundsson |first12=J. T. |last13=Haberberger |first13=D. |last14=Halliday |first14=J. W. D. |last15=Hodge |first15=T. |date=2024-06-12 |title=Laboratory realization of relativistic pair-plasma beams |journal=Nature Communications |language=en |volume=15 |issue=1 |pages=5029 |doi=10.1038/s41467-024-49346-2 |pmid=38866733 |pmc=11169600 |issn=2041-1723|arxiv=2312.05244 |bibcode=2024NatCo..15.5029A }} Future experiments offer the possibility to study physics relevant to extreme astrophysical environments where copious electron-positron pairs are generated, such as gamma-ray bursts, fast radio bursts and blazar jets.
Applications
Certain kinds of particle accelerator experiments involve colliding positrons and electrons at relativistic speeds. The high impact energy and the mutual annihilation of these matter/antimatter opposites create a fountain of diverse subatomic particles. Physicists study the results of these collisions to test theoretical predictions and to search for new kinds of particles.{{citation needed|date=July 2020}}
The ALPHA experiment combines positrons with antiprotons to study properties of antihydrogen.{{Cite journal|last=Charman|first=A. E.|date=2013-04-30|title=Description and first application of a new technique to measure the gravitational mass of antihydrogen|journal=Nature Communications|language=en|volume=4|issue=1|pages=1785–|doi=10.1038/ncomms2787|pmid=23653197|issn=2041-1723|pmc=3644108|bibcode=2013NatCo...4.1785A}}
Gamma rays, emitted indirectly by a positron-emitting radionuclide (tracer), are detected in positron emission tomography (PET) scanners used in hospitals. PET scanners create detailed three-dimensional images of metabolic activity within the human body.
{{cite book
|first=M. E. |last=Phelps
|date=2006
|title=PET: physics, instrumentation, and scanners
|pages=2–3
|publisher=Springer
|isbn=978-0-387-32302-2
}}
An experimental tool called positron annihilation spectroscopy (PAS) is used in materials research to detect variations in density, defects, displacements, or even voids, within a solid material.
{{cite web
|title = Introduction to Positron Research
|url = http://www.stolaf.edu/academics/positron/intro.htm
|publisher = St. Olaf College
|url-status = dead
|archive-url = https://web.archive.org/web/20100805002736/http://www.stolaf.edu/academics/positron/intro.htm
|archive-date = 5 August 2010
}}
See also
References
{{Reflist}}
External links
{{Commons category|Positrons}}
- [https://web.archive.org/web/20051028075601/http://www.positron.edu.au/faq.html What is a Positron?] (from the Frequently Asked Questions :: Center for Antimatter-Matter Studies)
- [https://archive.today/20121127065037/http://discover.positron.edu.au/ Website about positrons and antimatter ]
- [https://web.archive.org/web/20071012192435/http://www-search.slac.stanford.edu:8765/query.html?qt=positron Positron information search at SLAC]
- [http://positronannihilation.net Positron Annihilation as a method of experimental physics used in materials research.] {{Webarchive|url=https://web.archive.org/web/20220303032726/http://www.positronannihilation.net/ |date=3 March 2022 }}
- [https://web.archive.org/web/20090825002231/https://publicaffairs.llnl.gov/news/news_releases/2008/NR-08-11-03.html New production method to produce large quantities of positrons]
- [http://www.como.polimi.it/positron Website about antimatter (positrons, positronium and antihydrogen). Positron Laboratory, Como, Italy]
- [https://web.archive.org/web/20110811045124/http://aegis.web.cern.ch/aegis/home.html Website of the AEgIS: Antimatter Experiment: Gravity, Interferometry, Spectroscopy, CERN]
- [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.110.255002 Synopsis: Tabletop Particle Accelerator] ... new tabletop method for generating electron–positron streams.
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