The two gamma rays emitted back to back during the positron annihilation are detected almost simultaneously by two opposite scintillators. This coincidence is a very strong signature that distinguishes them from other photons. Specific electronic circuits "coincidence" circuits pick up gamma pairs. On the figure, it is requested that the signals coming from the scintillators A and B coincide within 12 billionths of a second (nanosecond). The straight line joining the centers of detectors A and B is an approximation of the actual line of flight of the two gamma rays.
|A.Aurengo/ Hôpital Pitié-Salpêtrière|