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Powell, Cecil Frank

particles photographic particle tracks

(1903–69) British physicist: used photographic emulsion to detect new elementary particles.

The son of a gunsmith, Powell studied at Cambridge and obtained his PhD there in 1927 for work with on condensation in cloud chambers. In that year he went to Bristol, and spent his career there. Marietta (1894–1970) in Vienna had shown that photographic emulsion is affected not only by light but also by fast particles, which leave a track. In the late 1930s Powell began to use photographic plates (later, films) to record the tracks of fast nuclear particles. These tracks (due to ionization, which leaves blackened silver grains) can be studied under a microscope and Powell showed that the mass, charge and energy of a particle can be estimated from the tracks it produces. In this way he discovered a new particle in 1947; this was the pi-meson or pion, with mass 273 times that of an electron, which had been predicted by in 1935. Since 1947 other unstable particles have been discovered by the same method, and Powell’s work marks the start of modern high-energy particle physics. He used the photographic method with ‘stacks’ of plates, both at mountain height with cosmic rays as the source of particles (as in his pion work) and carried by free balloons above the atmosphere. These balloons were of polythene sheet, filled with hydrogen: they remained stable at about 30 km altitude for some hours. Powell was awarded the Nobel Prize in 1950.

Powell, Clifton (1947–) [next]

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