Discovering Particles

Cloud chamber

Cloud chamber

The cloud chamber, first demonstrated in 1911, was developed at the Cavendish Laboratory, University of Cambridge, by Charles T. R. Wilson. It is essentially a transparent-walled, sealed container, filled with air and some vapour at the point of condensing – a supersaturated environment. A charged particle that crosses the chamber causes ionisation along its path. The vapour condenses about the ions, and the particle’s path is traced out by what Wilson referred to as “little wisps and threads of cloud”. The mechanism at work here is the same as the one that sometimes gives rise to condensation trails (contrails) in the wake of an aeroplane.

In the original design, the chamber was filled with air and water vapour, and the supersaturated environment was achieved by first compressing and then expanding the mixture. In a later development, the diffusion cloud chamber, alcohol is used instead of water, and the base of the chamber is cooled to –79°C or lower, for example using dry ice. Alcohol vapour in the warmer upper part of the chamber cools and falls, so that a supersaturated region is formed just above the base.

A cloud chamber was used in the discovery of the first antiparticle, the positron, in 1932.

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