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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