cloud chamber

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A gas-filled device in which the path of charged subatomic particles can be detected by the formation of chains of droplets on ions generated by their passage. It is also used to infer the presence of neutral particles and to study certain nuclear reactions.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun apparatus that detects the path of high-energy particles passing through a supersaturated vapor; each particle ionizes molecules along its path and small droplets condense on them to produce a visible track