They do this to study electrons, negatively charged particles that exist in all atoms and thus in all matter.
Energy is released in the form of electromagnetic radiation, or gamma rays.
By characterizing the gamma rays through spectroscopy, scientists can determine the properties of the electrons. The annihilation rate gives them the density of the electrons, and the energy and angle of the annihilation supply information about electron momentum. Solid-state physicists have used this spectroscopy technique for over 20 years to study the properties of many metals and alloys. Scientists have also discovered that measurement of positron annihilation is an excellent tool for studying the structure of microscopic defects on and inside metals and other materials. With the increasing use of exceedingly small electronic devices and thin films in the semiconductor and polymer industries has come the challenge of detecting proportionately small material defects.