What is a positron?

   The positron is a particle smaller than an atom. It is identical to the electron in size and weight, but has a positive rather than negative charge. It was discovered by C. D. Anderson at the California Institute of Technology in 1932. He discovered positrons while studying cosmic radiation with a CLOUD CHAMBER. He named the positron for its positive charge and its similarity to the electron.
   Positrons can be formed through the change of a PROTON into a neutron inside the nucleus of an atom. Such a change occurs spontaneously in many radioactive elements. The positron can combine with a free ELECTRON and the two disappear by forming two gamma rays. This is called annihilation radiation because the electron and positron disappear. In the absence of electrons (in vacuum) the positron is stable and can live forever. In the presence of matter, such as most solids, the positron lives a very short time (one-billionth of a second).
   Positrons were predicated theoretically, before their experimental discovery, by the English physicist P. A. M. Dirac. In Dirac's theory, negative energies for electrons exist as well as the ordinary positive energies. A positron comes into existence when an electron is removed from the region of filled negative energy states. The "hole" left by the electron is the positron. To form such a hole, energy must be put into the region. When a PHOTON is absorbed in the vicinity of a charged particle, one negative electron and one positive positron are created. This process is called pair production.