Text: Positron Annihilation The positron is the antiparticle of the electron, and when a positron enters any normal matter, it will find an abundant supply of electrons with which to annihilate. The energy released by the annihilation forms two highly energetic gamma rays, and if one assumes that the momenta of the positron and electron before the annihilation, the two gamma ray photons must travel in opposite directions in order to conserve momentum. These coincident gamma rays at 180 degrees provide a useful analysis tool. For one thing, eliminating all gamma events which are not coincident at 180 degrees improves the signal-to-noise ratio of experiments using positron annihilation. Another interesting application is the use of the coincident gammas to locate the source by back projecting. This is used in medical PET scans.

See Also:

Source: