Powell, Cecil Frank

POWELL, CECIL FRANK

(b.. Tonbridge, Kent, England, 5 December 1903; d. near Bellano, Lake Como, Italy, 9 August 1969)

physics.

The son of Frank Powell, a gunsmith, and Elizabeth Caroline Bisacre, Powell was educated at Judd School, Tonbridge, and Sidney Sussex College, Cambridge. After graduating in 1925 with first-class honors in the natural science tripos, he entered the Cavendish Laboratory, Cambridge, where he was influenced by Lord Rutherford and especially by C. T. R. Wilson, under whose supervision he worked for the Ph.D.; it was awarded in 1927 for a dissertation on condensation phenomena important in the operation of the Wilson cloud chamber. Powell went to Bristol University in 1927 as research assistant to A. M. Tyndall and remained there for the rest of his career. He was appointed Melville Wills professor of physics in 1948 and Henry Overton Wills professor and director of the H. EL Wills Physics Laboratory in 1964, a post from which he retired shortly before his death. He received the Nobel Prize in physics for 1950; was fellow and Hughes medalist (1949) and Royal medalist (1961) of the Royal Society, and Lomonosov gold medalist of the Soviet Academy of Sciences (1967); held honorary doctorates from six universities; and was honorary member of four academies of science.

Following some important work with Tyndall on ionic mobility in gases, Powell began in the late 1930’s to investigate the possible use of photographic emulsions to record the tracks of fast-moving electrically charged particles. After development of the emulsion, blackened grains of silver appear in the positions of atoms that have been ionized by the fast particles, so that the particle trajectories are visible when the processed plate is examined under a suitable microscope. Powell and his collaborators transformed this method for estimating the masses, charges, and energies of the particles producing the tracks.

The power of the method was fully revealed when Powell exposed stacks of specially designed emulsion pellicles to cosmic radiation at mountain altitude. A new charged particle, the pi-meson or pion, of mass 273 times the mass of the electron, was discovered in 1947.

The existence of a particle of about this mass had been predicted by the Japanese physicist Hideki Yukawa in 1935, to explain the short-range attractive forces between neutrons and protons that hold atomic nuclei together. Physicists searching for this particle with Geiger counters or cloud chamber detectors had found a charged particle, the muon of mass 206 times the electron mass. By 1946, however, experiments had shown that the muon interacted only weakly with atomic nuclei and could not be the particle predicted by Yukawa. The pion discovered by Powell and his colleagues had the required properties, however. They showed also that it was unstable, with a lifetime of about 2 x 10^–8 second, decaying into a muon and a neutrino.

The pion was the first of a large number of unstable elementary particles found since then, many by means of the photographic emulsion method. Their study dominates high-energy physics and has led to the discovery of important symmetry properties in nature: nonconservation of parity and charge conjugation in weak interactions like beta radioactivity, and unitary symmetry in strong interactions like those concerned with nuclear forces. Powell’s work marks the beginning of elementary particle physics as it is known today.

Powell established a large international collaboration of many laboratories to study elementary particles in the cosmic radiation by means of large stacks of emulsion in free balloons, and continued to sponsor large-scale international collaboration in studying particle physics by use of accelerators. He played an important part in CERN, the laboratory of the European Organization for Nuclear Research, serving as chairman for three years of its Scientific Policy Committee. His passionate belief in science as the great transforming force of society and his deep commitment to the social responsibility of scientists led to his key role in the Pugwash Movement for Science and World Affairs, of which he was a founder, and also in the World Federation of Scientific Workers, of which he was president.

The research school that Powell built up at Bristol occupied a dominant position in particle physics during 1946-1956, prior to the building of the large accelerator laboratories, and has exerted a profound influence on the development of the field.

BIBLIOGRAPHY

Many of Powell’s writings have been brought together in Selected Papers of Cecil Frank Powell, E. H. S. Burhop, W. O. Lock, and M. G. K. Menon, eds. (Amsterdam-London, 1972). His works include The Cosmic Radiation (Stockholm, 1951), his Nobel Prize address; The Study of Elementary Particles by the Photographic Method (London, 1959), written with P. H. Fowler and D. H. Perkins; “Cosmic Radiation,” in Proceedings of the Institute of Electrical Engineers, 107B (1960), 389-394, the Kelvin lecture; “Priorities in Science and Technology in Developing Countries” in The Science of Science (London, 1964), 71-92, speech given at Pugwash Conference, Udaipur, India; “The Role of Pure Science in European Civilization,” in Physics Today, 18 (1965), 56-64, address to the Council of CERN; “Promise and Problems of Modern Science,” in Nature, 216 (1967), 543-546; and “The Nature of the Primary Cosmic Radiation,” in Scientific World, 13, (1969), 5-13, the Walther Bothe memorial lecture.

E. H. S. Burhop