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Cloud Chamber

  Also called Wilson chamber , a cloud chamber is a historic device, used to make charged tracks (originally cosmic rays in pre-accelerator times) visible over a large volume. To this effect, a chamber was filled with a gas, in fact, a mixture of vapour in equilibrium with liquid, and a non-condensating gas; this mixture was brought into a supersaturated state by expansion. Condensation started around the ions generated by passing charged particles, and the resulting droplets were photographed. In a way, the cycle is just the opposite of that in a bubble chamber, its successor. The cycle of decompression and recompression was long, several minutes; the evaporation of droplets is slow, so they were grown to a size which made them fall to the chamber bottom by gravity. The sensitive state lasted long enough (a fraction of a second) for the chamber to be triggerable by external means (e.g. arrangements of scintillation counters).

A similar principle of using supersaturation to make visible droplets appear along particle trajectories, was used in the diffusion chamber ; the expansion was replaced by cooling: a gas in equilibrium was continuously diffused into a cooled volume. Diffusion chambers were permanently sensitive, as the droplets moved out of the visible volume together with the gas.

Rudolf K. Bock, 9 April 1998