Positron

The positron is the antiparticle of the Electron: identical mass and spin, but carrying +1 unit of Electric charge. When a positron meets an electron the pair annihilates into gamma rays — matter converted entirely to light.

Discovery

1. Paul Dirac's 1928 equation for the electron predicted a positive twin nobody had seen.
2. Carl D. Anderson photographed one in a cloud chamber in 1932, curving the "wrong" way in a magnetic field.
3. Anderson shared the 1936 Nobel Prize in Physics for it — the first antimatter ever observed.

Where you meet one

• PET scans — positron emission tomography. A tracer emits positrons; each annihilation makes two back-to-back gamma rays, and the scanner triangulates them into an image.
• Cosmic rays — Anderson's original source.
• Radioactive decay — β⁺ decay turns a proton into a neutron, a positron, and a neutrino.