ElektronPositronAnnihilation

ElektronPositronAnnihilation refers to the process in which an electron and its antiparticle, the positron, annihilate and release energy primarily as photons. In vacuum, the dominant channel yields two gamma photons of 511 keV each, emitted back-to-back to conserve momentum, for a total energy of 1.022 MeV.

If the electron and positron form a bound state called positronium, the decay modes depend on the state. Para-positronium decays into two photons (each 511 keV) with a lifetime of about 125 picoseconds. Ortho-positronium decays into three photons with a total energy of 1.022 MeV and a lifetime of about 142 nanoseconds in vacuum. In matter, these lifetimes are modified by interactions with surrounding atoms, including processes such as pick-off annihilation.

In matter, the positron loses energy through interactions with electrons and nuclei until it either directly annihilates with an electron or forms positronium. The surrounding material influences the relative probabilities of direct annihilation versus positronium formation and alters the observed energy and angular distributions through Doppler broadening and other effects.

Applications and significance include positron annihilation spectroscopy, used to study defects and voids in solids, and medical imaging through positron emission tomography (PET), which detects the coincidence of 511 keV photons to image metabolic processes. The phenomenon also provides tools for fundamental studies of quantum electrodynamics and the behavior of bound states in different environments.