Positron (antielectron)
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.
CompositionElementary particle
StatisticsFermionic
GenerationFirst
InteractionsGravity, electromagnetic, weak
Symbol
e+
,
β+
AntiparticleElectron
TheorizedPaul Dirac (1928)
DiscoveredCarl D. Anderson (1932)
Massme
9.1093837015(28)×10−31 kg[1]
5.48579909065(16)×10−4 Da[2]
0.51099895000(15) MeV/c2[3]
Mean lifetimestable (same as electron)
Electric charge+1 e
+1.602176634×10−19 C[4]
Spin1/2 ħ (same as electron)
Weak isospinLH: 0, RH: 1/2

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.[5] 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.[6] 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.[5]

Dirac wrote a follow-up paper in December 1929[7] 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.[8] Weyl in 1931 showed that the negative-energy electron must have the same mass as that of the positive-energy electron.[9] 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.[10]

Richard Feynman, and earlier Ernst Stueckelberg, proposed an interpretation of the positron as an electron moving backward in time,[11] 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.[12] 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."[13] 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.

Experimental clues and discovery

Wilson cloud chambers 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.

Several sources have claimed that Dmitri Skobeltsyn first observed the positron long before 1930,[14] or even as early as 1923.[15] They state that while using a Wilson cloud chamber[16] 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[17] 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.[18] Later, Skobeltsyn rejected this claim even more strongly, calling it "nothing but sheer nonsense".[19]

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[20]), and by discovering charged particle cosmic rays,[21] for which he is credited in Carl David Anderson's Nobel lecture.[22] Skobeltzyn did observe likely positron tracks on images taken in 1931,[23] 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.[24] 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.[25]

Anderson discovered the positron on 2 August 1932,[26] for which he won the Nobel Prize for Physics in 1936.[27] 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.[28]

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.[24] 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.[28]

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.[29]

Natural production

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.[30] Antiprotons have also been found to exist in the Van Allen Belts around the Earth by the PAMELA module.[31][32]

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,[33] 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.[34]

Positron production from radioactive
β+
 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 70 kg (150 lb) mass, about 4,400 nuclei of 40K decay per second.[35] The activity of natural potassium is 31 Bq/g.[36] About 0.001% of these 40K decays produce about 4000 natural positrons per day in the human body.[37] 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.

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.[38][39][40]

Observation in cosmic rays

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.[41] 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.[42]

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.[43]

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.[44][45] 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.[46][47] These results on interpretation have been suggested to be due to positron production in annihilation events of massive dark matter particles.[48]

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 Space Shuttle Discovery 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.[49]

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.[50] 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.[51]

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.

The ALPHA experiment combines positrons with antiprotons to study properties of antihydrogen.[52]

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.[53]

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.[54]

See also

References

  1. ↑ "2018 CODATA Value: electron mass". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 20 May 2019.
  2. ↑ "2018 CODATA Value: electron mass in u". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 21 June 2020.
  3. ↑ "2018 CODATA Value: electron mass energy equivalent in MeV". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 11 July 2022.
  4. ↑ "2018 CODATA Value: elementary charge". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 20 May 2019.
  5. 1 2 Dirac, P. A. M. (1928). "The quantum theory of the electron". Proceedings of the Royal Society A. 117 (778): 610–624. Bibcode:1928RSPSA.117..610D. doi:10.1098/rspa.1928.0023.
  6. ↑ Weyl, H. (1929). "Gravitation and the Electron". PNAS. 15 (4): 323–334. Bibcode:1929PNAS...15..323W. doi:10.1073/pnas.15.4.323. PMC 522457. PMID 16587474.
  7. ↑ Dirac, P. A. M. (1930). "A theory of electrons and protons". Proceedings of the Royal Society A. 126 (801): 360–365. Bibcode:1930RSPSA.126..360D. doi:10.1098/rspa.1930.0013.
  8. ↑ Oppenheimer, J. R. (1930). "Note on the theory of the interaction of field and matter", Physical Review 35(5), 461.
  9. ↑ Weyl, H. (1931). Gruppentheorie und Quantenmechanik (Hirzel, Leipzig, 1928); H. Weyl. The Theory of Groups and Quantum Mechanics.
  10. ↑ Dirac, P. A. M. (1931). "Quantised Singularities in the Quantum Field". Proceedings of the Royal Society A. 133 (821): 60–72. Bibcode:1931RSPSA.133...60D. doi:10.1098/rspa.1931.0130.
  11. ↑ Feynman, R. (1949). "The theory of positrons". Physical Review. 76 (6): 749–759. Bibcode:1949PhRv...76..749F. doi:10.1103/PhysRev.76.749. S2CID 120117564. Archived from the original on 9 August 2022. Retrieved 28 December 2021.
  12. ↑ Feynman, R. (11 December 1965). The Development of the Space-Time View of Quantum Electrodynamics (Speech). Nobel Lecture. Retrieved 2 January 2007.
  13. ↑ Nambu, Y. (1950). "The Use of the Proper Time in Quantum Electrodynamics I". Progress of Theoretical Physics. 5 (1): 82–94. Bibcode:1950PThPh...5...82N. doi:10.1143/PTP/5.1.82.
  14. ↑ Wilson, David (1983). Rutherford, Simple Genius. Hodder and Stoughton. pp. 562–563. ISBN 0-340-23805-4.
  15. ↑ Close, F. (2009). Antimatter. Oxford University Press. pp. 50–52. ISBN 978-0-19-955016-6.
  16. ↑ Cowan, E. (1982). "The Picture That Was Not Reversed". Engineering & Science. 46 (2): 6–28.
  17. ↑ Hanson, Norwood Russel (1963). The Concept of the Positron. Cambridge University Press. pp. 136–139. ISBN 978-0-521-05198-9.
  18. ↑ Hanson, Norwood Russel (1963). The Concept of the Positron. Cambridge University Press. pp. 179–183. ISBN 978-0-521-05198-9.
  19. ↑ Brown, Laurie M.; Hoddeson, Lillian (1983). The Birth of Particle Physics. Cambridge University Press. pp. 118–119. ISBN 0-521-24005-0.
  20. ↑ Bazilevskaya, G.A. (2014). "Skobeltsyn and the early years of cosmic particle physics in the Soviet Union". Astroparticle Physics. 53: 61–66. Bibcode:2014APh....53...61B. doi:10.1016/j.astropartphys.2013.05.007.
  21. ↑ Skobeltsyn, D. (1929). "Uber eine neue Art sehr schneller beta-Strahlen". Z. Phys. 54 (9–10): 686–702. Bibcode:1929ZPhy...54..686S. doi:10.1007/BF01341600. S2CID 121748135.
  22. ↑ Anderson, Carl D. (1936). "The Production and Properties of Positrons". Retrieved 10 August 2020.
  23. ↑ Skobeltzyn, D. (1934). "Positive electron tracks". Nature. 133 (3349): 23–24. Bibcode:1934Natur.133...23S. doi:10.1038/133023a0. S2CID 4226799.
  24. 1 2 Merhra, J.; Rechenberg, H. (2000). The Historical Development of Quantum Theory, Volume 6: The Completion of Quantum Mechanics 1926–1941. Springer. p. 804. ISBN 978-0-387-95175-1.
  25. ↑ Cao, Cong (2004). "Chinese Science and the 'Nobel Prize Complex'" (PDF). Minerva. 42 (2): 154. doi:10.1023/b:mine.0000030020.28625.7e. ISSN 0026-4695. S2CID 144522961.
  26. ↑ Anderson, C. D. (1933). "The Positive Electron". Physical Review. 43 (6): 491–494. Bibcode:1933PhRv...43..491A. doi:10.1103/PhysRev.43.491.
  27. ↑ "The Nobel Prize in Physics 1936". Retrieved 21 January 2010.
  28. 1 2 Gilmer, P. J. (19 July 2011). "Irène Jolit-Curie, a Nobel laureate in artificial radioactivity" (PDF). p. 8. Archived from the original (PDF) on 19 May 2014. Retrieved 13 July 2013.
  29. ↑ "Atop the Physics Wave: Rutherford Back in Cambridge, 1919–1937". Rutherford's Nuclear World. American Institute of Physics. 2011–2014. Archived from the original on 21 October 2014. Retrieved 19 August 2014.
  30. ↑ Palmer, J. (11 January 2011). "Antimatter caught streaming from thunderstorms on Earth". BBC News. Archived from the original on 12 January 2011. Retrieved 11 January 2011.
  31. ↑ Adriani, O.; et al. (2011). "The Discovery of Geomagnetically Trapped Cosmic-Ray Antiprotons". The Astrophysical Journal Letters. 737 (2): L29. arXiv:1107.4882. Bibcode:2011ApJ...737L..29A. doi:10.1088/2041-8205/737/2/L29.
  32. ↑ Than, K. (10 August 2011). "Antimatter Found Orbiting Earth—A First". National Geographic Society. Archived from the original on 10 October 2011. Retrieved 12 August 2011.
  33. ↑ "What's the Matter with Antimatter?". NASA. 29 May 2000. Archived from the original on 4 June 2008. Retrieved 24 May 2008.
  34. ↑ "Riddle of matter remains unsolved: Proton and antiproton share fundamental properties". Johannes Gutenberg University Mainz. 19 October 2017.
  35. ↑ "Radiation and Radioactive Decay. Radioactive Human Body". Harvard Natural Sciences Lecture Demonstrations. Retrieved 18 May 2011.
  36. ↑ Wintergham, F. P. W. (1989). Radioactive Fallout in Soils, Crops and Food. Food and Agriculture Organization. p. 32. ISBN 978-92-5-102877-3.
  37. ↑ Engelkemeir, D. W.; Flynn, K. F.; Glendenin, L. E. (1962). "Positron Emission in the Decay of K40". Physical Review. 126 (5): 1818. Bibcode:1962PhRv..126.1818E. doi:10.1103/PhysRev.126.1818.
  38. ↑ "Electron-positron Jets Associated with Quasar 3C 279" (PDF).
  39. ↑ "Vast Cloud of Antimatter Traced to Binary Stars". NASA.
  40. ↑ https://www.youtube.com/watch?v=Sw-og52UUVg start FOUR minutes into video: Sagittarius produces 15 billion tons/sec of electron-positron matter
  41. ↑ Golden (February 1996). "Measurement of the Positron to Electron Ratio in Cosmic Rays above 5 GeV". Astrophysical Journal Letters. 457 (2). Bibcode:1996ApJ...457L.103G. doi:10.1086/309896. hdl:11576/2514376. S2CID 122660096. Retrieved 19 October 2021.
  42. ↑ Boudaud (19 December 2014). "A new look at the cosmic ray positron fraction". Astronomy & Astrophysics. 575. Retrieved 19 October 2021.
  43. ↑ "Towards Understanding the Origin of Cosmic-Ray Positrons". The Alpha Magnetic Spectrometer on the International Space Station. Retrieved 19 October 2021.
  44. ↑ Accardo, L.; et al. (AMS Collaboration) (2014). "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" (PDF). Physical Review Letters. 113 (12): 121101. Bibcode:2014PhRvL.113l1101A. doi:10.1103/PhysRevLett.113.121101. PMID 25279616.
  45. ↑ Schirber, M. (2014). "Synopsis: More Dark Matter Hints from Cosmic Rays?". Physical Review Letters. 113 (12): 121102. arXiv:1701.07305. Bibcode:2014PhRvL.113l1102A. doi:10.1103/PhysRevLett.113.121102. hdl:1721.1/90426. PMID 25279617. S2CID 2585508.
  46. ↑ "New results from the Alpha Magnetic Spectrometer on the International Space Station" (PDF). AMS-02 at NASA. Retrieved 21 September 2014.
  47. ↑ "Positron fraction". Archived from the original on 22 July 2018. Retrieved 22 July 2018.
  48. ↑ Aguilar, M.; et al. (2013). "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" (PDF). Physical Review Letters. 110 (14): 141102. Bibcode:2013PhRvL.110n1102A. doi:10.1103/PhysRevLett.110.141102. PMID 25166975.
  49. ↑ Aguilar, M.; et al. (AMS Collaboration) (2002). "The Alpha Magnetic Spectrometer (AMS) on the International Space Station: Part I – results from the test flight on the space shuttle". Physics Reports. 366 (6): 331–405. Bibcode:2002PhR...366..331A. doi:10.1016/S0370-1573(02)00013-3. hdl:2078.1/72661. S2CID 122726107.
  50. ↑ Bland, E. (1 December 2008). "Laser technique produces bevy of antimatter". NBC News. Retrieved 6 April 2016. The LLNL scientists created the positrons by shooting the lab's high-powered Titan laser onto a one-millimeter-thick piece of gold.
  51. ↑ https://lasers.llnl.gov/workshops/user_group_2012/docs/7.3_chen.pdf Lab production of 5MeV positron-electron beams
  52. ↑ Charman, A. E. (30 April 2013). "Description and first application of a new technique to measure the gravitational mass of antihydrogen". Nature Communications. 4 (1): 1785–. Bibcode:2013NatCo...4.1785A. doi:10.1038/ncomms2787. ISSN 2041-1723. PMC 3644108. PMID 23653197.
  53. ↑ Phelps, M. E. (2006). PET: physics, instrumentation, and scanners. Springer. pp. 2–3. ISBN 978-0-387-32302-2.
  54. ↑ "Introduction to Positron Research". St. Olaf College. Archived from the original on 5 August 2010.
Wikimedia Commons has media related to Positrons.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.