The positron is the anti-particle of the electron. This means that the positron has the same mass and magnitude of charge as the electron, it just has the opposite charge from the electron.

The positron was the first antiparticle to be predicted and discovered. It was postulated in 1930 to account for the possible negative energy states of an electron that was predicted by the relativistic electron theory (1928). Carl Anderson established the existence of the positron in 1932 while researching cosmic rays.

The positron is a stable particle, however it will react with an electron (found in ordinary matter). When a positron and electron meet they annhilate two 511 keV gamma rays 180 degrees apart (necessary to conserve momentum). This is used in Positron Emission Tomography to produce detailed maps for medical use.

If the positron and electron are high energy enough, the collision may form two mesons. In this sequence, the collision forms either a very high energy photon, or a Z-particle. This particle carries large amounts of energy and decays into a charmed and anti-charmed quark. These two quarks will then separate from the residual energy and form a down and anti-down quark bound to the anti-charmed or charmed quark (respectively) to form D+ and D- mesons.

                             c
                        c    | D- Meson
e-                 c    |    d
    (bang) -> Z -> | -> * -> 
e+                 c    |    d
                        c    | D+ Meson
                             c

Positrons are occasionally emitted (known as positive beta decay) in a radio-active nucleus that has too many protons. In this situation, the proton becomes a neutron and emits a neutrino and positron. (When a neutron decays into a proton it emits an anti-neutrino and electron). This is used in Positron Emission Tomography which, as mentioned above is often used in the medical field.

Positrons are also created in an electron-positron pair from a photon with sufficient energy. When a photon has energy greater than the rest mass of the pair (in this case 1.022 MeV) it is possible that it will form the pair. This often happens with x-rays and gamma-rays interacting with normal matter.

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