Positron
A positron is the antimatter equivalent of the negatively charged electron. A positron is equal to the electron in mass, but has a positive charge. In 1828, English physicist Paul Dirac advanced an equation that incorporated both quantum physics and the requirements of the theory of special relativity to provide a complete description of the electron. The equation resulted in a particle, however, that could be positively or negatively charged. On this basis, Dirac predicted the existence of the positron.
In 1932, American physicist Carl Anderson observed a new kind of particle in his cloud chamber. A particle too faint to be a proton or alpha particle entered the chamber, and then curved toward the negative area of the magnetic field that was around the chamber. It's velocity and mass indicated was the same as the electron but it swerved toward the negative pole and therefore had to carry a positive charge. Anderson realized he had discovered Dirac's antimatter particle to the electron, the positron (e+ ).
The positron has the same rest mass (0.511MeV) and the same spin as the electron, but carries the opposite electric charge. The electron has electron number +1 while the positron has -1. Both particles have their corresponding neutrinos; the electron has a neutrino with the electron lepton number +1, and the positron has its antimatter electron neutrino with electron lepton number -1.
That both particles exist in nature is a classic example of E=mc2 . Positrons are created when a high energy gamma-ray photon strikes Earth's atmosphere and decays, or when a high-energy photon itself penetrates the atmosphere and converts its energy into mass creating a positron-electron pair. In addition, some radioactive nuclei spontaneously emit a positron (along with a neutrino).
The E=mc2 transformation is reversed when an electron and positron are brought sufficiently close together. The two particles, through a process called annihilation, convert into energy. This energy can then revert to matter and antimatter so long as the matter produced exactly matches the antimatter produced. The matter-antimatter does not have to be another electron-positron pair. All energy, however, must be conserved.
As of the year 2000, positrons can be manufactured, stored, and then used in various particle reactions carried out at high energy accelerator facilities. Still, the question remains why electrons and positrons are not constantly annihilating all around us. Dirac's explanation is that there is a sea of positrons residing in energy levels that are full and not accessible to the electron. Periodically, whether spontaneous or manufactured, a positron leaves this sea providing a hole for an electron-positron annihilation to fill when the annihilation leaves behind only a puff of energy. The electron has fallen into the positron hole, releasing energy as it does so, and both the hole and the electron simply disappear from the everyday world, canceling each other out. Or if adequate energy is available, i.e. from an energetic photon, a negative-energy invisible electron may be kicked out of its hole into visibility, creating a positron-electron pair and leaving behind a hole.
In the late 1940s American physicist Richard Feynman and others put together a more complete theory called quantum electrodynamics (QED) which explains the reaction of photons, electrons, and positrons.
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