|Birth, life and death of a positron|
The trajectory of a positron emitted by a nucleus starts off in the same way as that of an electron. It is slowed by ionisation and deflected by collisions with heavy nuclei. Finally, however, when its speed is greatly reduced, a positron disappears in an annihilation collision with an atomic electron. The positron and the electron are mutual antiparticles and are both entirely converted into energy. The resulting 1 MeV is carried away by two gamma photons, travelling in opposite directions.