Electron-positron annihilation is the process in which a positron collides with an electron resulting in the annihilation of both particles. Electrons (or β- particles) and positrons (or β+ particles) are of equal mass but opposite charge. Positrons are the antimatter equivalent of an electron, produced from B+ decay
According to the law of conservation of energy, their masses are converted to two annihilation gamma photons with an energy of around 511 keV and moving in two opposite directions.
e− + e+ ------> γ + γ
Where e− is the electron, e+ is the positron and γ are gamma rays emitted.
511 keV is the approximate amount of energy created when an electron or positron (which each have a mass of 9.11x10-31 kg) are converted to energy according to Einstein's famous equation:
E = mc2
Where E = energy , m = particle mass and c = velocity of light.
i.e. E= 9.11x10-31 kg x (3x108 m s-1)2.
Giving E= 8.20x10-14 J which is equivalent to 511 keV.
This process is of particular importance as it is the basis of positron emission tomography.