Positron

History

Theory

In 1928, Paul Dirac published a paper proposing that electrons can have both a positive and negative charge. |doi-access=free

Dirac wrote a follow-up paper in December 1929

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 |doi-access=free

Ernst Stueckelberg, and later Richard Feynman, proposed an interpretation of the positron as an electron moving backward in time,{{cite journal |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-access=subscription

Experimental clues and discovery

Beginning in 1923, while using a Wilson cloud chamber to study the Compton effect, Dmitri Skobeltsyn observed tracks that acted like electrons but curved in the opposite direction in an applied magnetic field. Skobeltsyn presented photographs with this phenomenon in a conference in the University of Cambridge, on 23–27 July 1928. Similar photographic evidence had been seen by Irene and Frederic Joliot-Curie and others but no one at the time had an explanation for these anomalous tracks. |access-date=10 August 2020

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.

Anderson discovered the positron on 2 August 1932, |access-date=21 January 2010 |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

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. |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

Natural production

Main article: 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. |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 |display-authors=etal |archive-url=https://web.archive.org/web/20111010014111/http://news.nationalgeographic.com/news/2011/08/110810-antimatter-belt-earth-trapped-pamela-space-science |archive-date=10 October 2011 |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, |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 |access-date=21 September 2019 |archive-date=21 September 2019 |archive-url=https://web.archive.org/web/20190921212147/http://www.uni-mainz.de/presse/aktuell/3027_ENG_HTML.php

Positron production from radioactive 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 mass, about 4,400 nuclei of 40K decay per second. |access-date=18 May 2011

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.

Observation in cosmic rays

Main article: 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. 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.

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.

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. |article-number=121101 |article-number=121102 |access-date=21 September 2014 |display-authors=etal

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 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. |display-authors=etal |hdl-access=free

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. |access-date=6 April 2016

In 2023, a collaboration between CERN and University of Oxford performed an experiment at the HiRadMat facility 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. 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.

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

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.

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. |archive-url = https://web.archive.org/web/20100805002736/http://www.stolaf.edu/academics/positron/intro.htm |archive-date = 5 August 2010

References

References

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2. Skobelzyn, D.. (September 1929). "Über eine neue Art sehr schneller ?-Strahlen". Zeitschrift für Physik.
3. Chao, C. Y.. (1930-11-15). "Scattering of Hard γ -Rays". Physical Review.
4. Cao, Cong. (2004). "Chinese Science and the 'Nobel Prize Complex'". Minerva.
5. (1932). "Émission de protons de grande vitesse par les substances hydrogénées sous l'influence des rayons y très pénétrants". Comptes rendus hebdomadaires des séances de l'Académie des sciences.
6. "Electron-positron Jets Associated with Quasar 3C 279".
7. (1 September 2008). "Science With Integral".
8. (February 1996). "Measurement of the Positron to Electron Ratio in Cosmic Rays above 5 GeV". Astrophysical Journal Letters.
9. (19 December 2014). "A new look at the cosmic ray positron fraction". Astronomy & Astrophysics.
10. "Towards Understanding the Origin of Cosmic-Ray Positrons".
11. "Positron fraction".
12. Chen, Hui. (February 2012). "Relativistic Electron-positron Plasma Jets Using High-intensity Lasers".
13. (2024-06-12). "Laboratory realization of relativistic pair-plasma beams". Nature Communications.
14. Charman, A. E.. (2013-04-30). "Description and first application of a new technique to measure the gravitational mass of antihydrogen". Nature Communications.