Eighty years ago today, the journal Physical Review published a paper by physicist Carl Anderson announcing the discovery of the positron.
The positron is the antimatter counterpart of the electron. The two particles have identical masses but opposite charges. When an electron and a positron interact, they annihilate in a burst of energy, producing two gamma rays.
In the early 1930s, Anderson and his mentor, Robert Millikan, were using a cloud chamber to measure high-energy cosmic rays.
A cloud chamber’s sealed cavity contains a supersaturated vapour, usually water or alcohol, which condenses around ion trails left behind by fast-moving charged particles, allowing them to be seen as they pass through. Physicists can deduce the charge of a particle from the way it curves when the chamber is subjected to a magnetic field.
In August of 1932, Anderson photographed the track of a high-energy particle with a mass about the same as an electron’s but with a positive charge. By measuring both the energy the particle lost in crossing a lead plate within the chamber and the length of the track on the other side of the lead, he determined an upper limit for the particle’s mass. He found it to be of the same order of magnitude as the electron’s mass.
Anderson had observed a new kind of particle, which he named the positron. It was soon to be identified as the first antiparticle, the antielectron.
Anderson’s detailed findings were published on 15 March 1933. Although the scientific community expressed skepticism, the positron fitted with Paul Dirac's prediction in 1931 of the antielectron. Anderson’s result was soon confirmed - Patrick Blackett and Giuseppe Occhialini found clear corroborating evidence in March 1933.
Today, positrons have a wide variety of applications in particle-physics research and beyond. They have been instrumental in electron-positron colliders in which bunches of electrons and positrons are smashed into each other at nearly the speed of light, allowing physicists to discover new elementary particles. The largest such machine was the Large Electron-Positron Collider, predecessor to the Large Hadron Collider at CERN. Positrons are the basis for positron emission tomography (PET) scanners, which are used to create three-dimensional images of processes inside the body.
Carl Anderson’s discovery in a cloud chamber continues to make tracks 80 years later.
Read the paper: “The positive electron” by Carl Anderson, Physical Review 13 March 1933, vol. 43, p491.