A positron is a particle of matter with the same mass as an electron but an opposite charge. It is a form of antimatter because, when a positron encounters an electron, the two completely annihilate to yield energy. The existence of the positron was predicted in 1928 by physicist Paul Dirac, and positrons were discovered experimentally in 1932 by physicist Carl Anderson. Positrons are important in a medical technology called positron emission tomography (PET), which is especially useful in brain and nervous-system scanning. A PET scanner detects and maps the radiation produced when an ingested substance emits positrons that are annihilated by surrounding electrons. The positron is not the only known antimatter particle. The proton and the neutron have antiparticles as well, known as the anti-proton and the anti-neutron, with negative and neutral electric charges, respectively