The ultimate fate of a beta particle depends upon its charge. A negatively charged beta
particle, after its kinetic energy has been spent, either combines with a positively charged ion,
or becomes a "free electron". Positrons, however, have a different fate. In spite of the fact that
they dissipate their kinetic energy just like beta particles through ionization and excitation,
they cannot exist at rest in the vicinity of the electrons. When a positron has been slowed
sufficiently, it will be attracted to the opposite charge of an electron. When the electron and
positron collide, they are both annihilated and an amount of energy equal to the sum of the
particle masses is released in the form of two photons. These photons are referred to as
"annihilation radiation". Both annihilation photons carry energy of 0.512 MeV, which is
equivalent to the rest mass of the electron or the positron. Because of this phenomenon, 0.512
MeV photons often provide a convenient means for measurement of positron-emitting
radionuclides.
Like alpha particles, betas have a