Carl Anderson and the Discovery of the Positron

Resources with Additional Information · Discovery of the Positron and Muons

The Positron
Positron Single Track Antiparticle of Electron from California Institute of Technology
Courtesy of Lawrence Berkeley
National Laboratory, © 2010
The Regents of the University of
California, Lawrence Berkeley
National Laboratory

Carl David Anderson discovered the positron in 1932. Anderson, then a postdoc in the physics department at California Institute of Technology (Caltech) was photographing the track of a cosmic ray particle in a cloud chamber. The track had an unusual curvature, and he deduced that it could only be produced by a particle "carrying a positive charge but having a mass of the same order of magnitude as that normally possessed by a free negative electron."1 He called this positively-charged electron a positron – the first identified antiparticle. For his accomplishment, Anderson shared the Nobel Prize for Physics in 1936.

Carl David Anderson graduated from Caltech with a B.Sc. and a PhD in physics and Engineering in 1927 and 1930, respectively.2 His doctoral thesis and subsequent work as a postdoc dealt with X-rays. But soon cosmic rays were discovered by physicist Victor Hess (with whom Anderson would later share the Nobel Prize), and Anderson began research on these high energy particles.

A cloud chamber was the experimental piece of equipment of choice to study cosmic rays. … Anderson designed and built his own cloud chamber and surrounded it with an electromagnet which caused the ionized particles to move on circular trajectories. When he examined the cosmic rays within the cloud chamber, he noticed that there appeared to be showers of both positively and negatively charged particles, as determined by the direction of curvature of the track. Moreover, the mass of the positively charged particles was smaller than a proton's.3 In order to prove that the particles were indeed positively charged, and not merely negatively charged electrons moving in the opposite direction, he put a lead plate in the chamber and captured his evidence in a now famous photograph. In the picture, a particle is seen approaching the metal plate from the bottom. When it hits the plate, it loses energy but continues to curve in the direction appropriate for a positively charged electron, which he later called a positron.A

A few years after discovering the positron, Anderson and Seth Neddermeyer discovered mu-mesons (or muons). "During World War II Anderson was closely associated with the Caltech rocket research and development effort, … [which was] funded by the U. S. Office of Scientific Research and Development (OSRD)" B, a predecessor of DOE.

A Edited excerpts from Carl Anderson: Discovery of the Positron
1 The Positive Electron, The Physical Review, Volume 43, Number 6, pp. 491-49
2 Nobel Lectures, Physics 1922-1941
3 "Carl David Anderson." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 29 Jul. 2009 .

B Edited excerpts from OAC Finding Aid for the Carl D. Anderson Papers 1923-1987


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Resources with Additional Information

Additional information about Carl David Anderson and the discovery of the positron is available on the Web.

Documents:

The Positive Electron, The Physical Review, Volume 43, Issue 6, pp. 491-494; March 1933

Positrons from Gamma-Rays, The Physical Review, Volume 43, Issue 12, p. 1034; June 1933

Cosmic-Ray Positive and Negative Electrons, The Physical Review, Volume 44, Issue 5, pp. 406-416; September 1933

Note on the Nature of Cosmic-Ray Particles, The Physical Review, Volume 51, Issue 10, pp. 884-886; May 1937

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