When an electron collides with its antiparticle, a positron, causing both elements to be completely destroyed and in most cases resulting in the emission of gamma rays.

See also Electron, Positron and Antiparticle.

(ACD) Caused by positron decay and positron annihilation two photons are emitted each with an energy of 511 keV in opposite directions. The simultaneous detection of these two photons, by two detectors indicates that a positron annihilation occurred at the line of response (LOR), the path between the two detectors.
In PET imaging the annihilation coincidence detection is used to localize the tracer, e.g. F18.

See also Positron Decay and Electron Positron Annihilation.

Gamma rays are a form of nuclear radiation that consists of photons emitted by radioactive elements from the nucleus. This high energetic light emission is also produced from subatomic particle interaction, such as electron positron annihilation. Gamma radiation, similar to x-radiation can injure and destroy tissue, especially cell nuclei.
Gamma rays have in general very high frequencies, short wavelengths, are electrically neutral and penetrate matter. The interaction of gamma rays with matter depends on the nature of the absorber as well as the energy of the gamma rays; these interactions determine also the type and amount of shielding needed for radiation protection.

See also Radiation Safety, Lead Equivalence, Lead Apron, Leaded Glove, Glove-Box, Radioactive Decay Law and Radiation Worker.

Annihilation in general refers to the transition of a particle and its antiparticle by collision into something different, depending on their energies and based on the conservation of energy and momentum. The electromagnetic radiation emitted is the result of the annihilation (combination and disappearance) of an electron and a positron. Two gamma rays of 0.511 MeV energy, assuming very low-energy particles, are emitted perpendicular to each other.