ANDERSON, CARL D.

Carl David Anderson, winner of the Nobel Prize in Physics in 1936 at age 31, was born in New York City on September 3, 1905. He was the only son of Swedish immigrant parents.

The family moved to Los Angeles in 1912 where Anderson attended local public schools. In 1923 he entered the newly established California Institute of Technology (Caltech), intending to study electrical engineering. He was an outstanding student, and in 1926 he was awarded the Junior Travel Prize, which was a grant sufficient for him to spend six months traveling in Europe. During his travels, he met the eminent physicists Hendrik A. Lorentz and Heike Kamerlingh-Onnes.

In 1927 Anderson graduated with a B.S. degree in physics engineering. He continued with graduate studies in physics and received his Ph.D. magna cum laude in 1930. His thesis was on the spatial distribution of electrons ejected from gases by X rays.

Robert A. Millikan, Nobel Prize in Physics winner in 1923 and Chief Executive of Caltech, was Anderson's graduate advisor. He recommended that Anderson broaden his experience by applying for a National Research Council fellowship. Anderson approached Arthur H. Compton at the University of Chicago and was offered a position. However, Millikan had changed his mind and persuaded Anderson to return to Pasadena to work on the cosmic ray research team he was setting up.

Millikan had become very interested in cosmic radiation when he realized that this was a very-high- energy radiation striking the Earth from outer space. To support his interest he established three research groups using different techniques to observe the radiation, namely, electroscopes to study ionizing effects, Geiger counters to count cosmic ray particles directly, and cloud chambers to photograph incoming particle tracks. Anderson was responsible for the cloud chamber program.

The cloud chamber, invented by Charles T. R. Wilson, is a device consisting of a chamber, usually with a glass front and back, which contains moist air. The chamber is designed so that the pressure inside

can be suddenly dropped. This cools the air, and cloud droplets will form on any suitable nucleus. If a charged particle has passed through the chamber shortly before the pressure drop, it will leave a trail of ions that will appear as a trail of cloud droplets. By photographing the chamber at this time, the path of the particle is made visible. This device proved to be one of the most useful tools for the study of radiation phenomena.

Anderson set up his chamber in the Guggenheim Aeronautics building at Caltech, where an ample supply of electricity was available. He needed the power to operate a large electromagnet to develop a magnetic field in the chamber. Charged particles traversing the chamber would have their tracks curved by this field. By measuring the curvature, the energy of the particles could be calculated. At the time he was doing the experiments, only two elementary particles were known, namely, the electron and the proton. These differed in charge and in mass so that their tracks in the chamber could be easily distinguished.

By adding Geiger counters above and below the chamber, he was able to trigger the chamber by a pulse from the counter and thus ensure that a particle had indeed passed through the chamber. Thus almost every picture contained one or more tracks.

To his surprise Anderson found tracks that had the ion density he expected from electrons but were curved by the magnetic field as though they had a positive charge. Could these particles be previously undiscovered positively charged electrons? The alternative explanation that they were traveling in the opposite direction was of course a possibility.

To settle the matter Anderson placed a lead plate in his chamber so that the particles would have to pass through the plate. In so doing they would lose energy, and therefore the radius of curvature of the track after passing through the plate would be less than it was on entering the plate. This observation would give a definite answer to the question of the direction of travel of the particle. In 1932 he recorded the historic photograph of a track that had to be made by a positively charged electron. Surprisingly, the particle was traveling upward through the chamber.

Within a few years physicists were overwhelmed with additional new particles, constituting a veritable "zoo" of particles. Anderson contributed to this zoo by taking his cloud chamber to the top of Pikes Peak in Colorado. The trip to the top of Pike's Peak was a challenge to the experimenters and their old truck. However, they made it and obtained many photographs with their cloud chamber. These photos contained evidence for a short-lived particle with a mass that had a value that was in between the mass of electron and the mass of the proton. They suggested the name mesotron, but the particle became known as the muon.

During World War II, Anderson was associated with the Caltech project to develop barrage rockets for the Navy. He was primarily concerned with the problem of launching these rockets from aircraft. His work took him to the Normandy beachhead shortly after the invasion. He assisted in installing the rockets on various aircraft.

Data collected by Anderson and others in the 1930s showed that cosmic rays had enormous energy. For many years these rays were the only source of such radiation available for the study of high-energy particle physics. In recent years, however, the experimenters have been building ever higher energy machines, and these have now become the tools for particle research.

Carl Anderson married Lorraine Bergman in1946. It was her second marriage, and her son Marshall was adopted by Anderson. A second son David was born in 1949.

All of his professional life Anderson was at Cal- tech. He became professor of Physics in 1939 and retired in 1970. He received many honors and awards in addition to the Nobel Prize. These include member of the National Academy of Sciences 1938, Chair of its physics section 1963–1966, the Elliott Cresson medal of the Franklin Institute 1937, and the John Ericsson medal of the American Society of Swedish Engineers 1960. He received honorary degrees from three colleges: Colgate University in 1937, Temple University in 1949, and Gustavus Adolphus College of St. Peter in 1963.

Carl Anderson was a first class experimental physicist, and his discovery of the positron was one of the major discoveries in particle physics of the twentieth century. He died on January 11, 1991.

See also:Antimatter; Dirac, Paul; Muon, Discovery of; Positron, Discovery of

Bibliography

Anderson, C. D. The Discovery of Anti-matter, edited by R. J. Weiss (World Scientific, Singapore, 1999).

Anderson, C. D., and Neddermeyer, S. H. "Cloud Chamber Observations of Cosmic Rays at 4300 Meter Elevation and Near Sea Level." Physical Review 50 , 263 (1936).

Anderson, C. D., and Neddermeyer, S. H. "Nature of Cosmic Ray Particles." Review of Modern Physics 11 , 191 (1939).

Nobel Foundation. "C. D. Anderson—Nobel Lecture." <http://www.nobel.se/physics/laureates/1936/anderson-lecture.html>.

William H. Pickering