/poz"i tron'/, n. Physics.an elementary particle having the same mass and spin as an electron but having a positive charge equal in magnitude to that of the electron's negative charge; the antiparticle of the electron.[1930-35; POSI(TIVE) + (ELEC)TRON]
* * *Subatomic particle having the same mass as an electron but with an electric charge of +1 (an electron has a charge of -1).It constitutes the antiparticle (see antimatter) of an electron. The existence of the positron was a consequence of the electron theory of P. A. M. Dirac (1928), and the particle was discovered in cosmic rays by Carl D. Anderson (1905–1991) in 1932. Though they are stable in a vacuum, positrons react quickly with the electrons of ordinary matter, producing gamma rays by the process of annihilation. They are emitted in positive beta decay of proton-rich radioactive nuclei and are formed in pair production.
* * *also called positive electronpositively charged subatomic particle having the same mass and magnitude of charge as the electron and constituting the antiparticle of a negative electron. The first of the antiparticles to be detected, positrons were discovered by Carl David Anderson (Anderson, Carl David) in cloud-chamber studies of the composition of cosmic rays (cosmic ray) (1932). The discovery of the positron provided an explanation for a theoretical aspect of electrons predicted by P.A.M. Dirac (Dirac, P.A.M.). The Dirac wave equation (quantum mechanics) (1928), which incorporated relativity into the quantum mechanical description for the allowable energy states of the electron, yielded seemingly superfluous negative energy states that had not been observed. In 1931 Dirac postulated that these states could be related to a new kind of particle, the antielectron.Stable in a vacuum, positrons quickly react with the electrons of ordinary matter by annihilation to produce gamma radiation (gamma ray). Positrons are emitted in the positive beta decay of proton-rich (neutron-deficient) radioactive nuclei and are formed in pair production, in which the energy of a gamma ray in the field of a nucleus is converted into an electron-positron pair. They are also produced in the decays of certain short-lived particles, such as positive muons (muon). Positrons emitted from man-made radioactive sources are used in medical diagnosis in the technique known as positron emission tomography (PET).
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