Two gamma photons emitted back-to-back
Positrons emitted by a beta-plus marker disappear after travelling a few millimetres through the body, by annihilating with an atomic electron in a process whereby both particles cease to exist. The two times 511,000 electronvolts which are released are carried away by two gamma photons emitted back-to-back. In PET, the travel paths of both photons are traced back to the source point - a distance which is usually comparatively small. This is why fluorine 18, with its comparatively long half-life, is a frequently-used marker