Positron
A particle with the electron's mass and a positive charge. It is the electron's antiparticle; when the two meet, both vanish into light.
In 1931 Paul Dirac, having thought through the negative-energy solutions of his equation, wrote that there must be a still unknown particle with the electron's mass and the opposite charge[1]. The next year Carl Anderson, studying cosmic rays in a cloud chamber, found the track of a particle that was positively charged yet far lighter than a proton, and later named it the positron[2]. He is said to have found it without knowing of Dirac's prediction.
The positron was the first antimatter to be found. When it meets an electron the pair annihilate, sending out two rays of light, gamma rays, of fixed energy in opposite directions. A hospital PET scan introduces into the body a drug that emits positrons and detects these paired rays outside, building an image of where the drug has gathered.
Related terms
- Dirac equationThe equation describing the electron in a way that satisfies both quantum mechanics and special relativity. Written by Paul Dirac in 1928.
- Cloud chamberA device that makes the path of a charged particle visible as a trail of fine water droplets.
- Cosmic rayHigh-energy particles raining continuously on Earth from space. Most are protons, coming from the Sun and from far across the galaxy.