cloud chamber

nounA particle detector used for detecting ionizing radiation. In its most basic form, it is a sealed environment containing a supersaturated vapor of water or alcohol, in which a mist can be seen where a charged particle interacts with the mixture and it ionizes it.

nounapparatus 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