The cloud chamber, also known as the Wilson chamber, is used for detecting particles of ionizing radiation. In its most basic form, a cloud chamber is a sealed environment containing a supercooled, supersaturated water vapour. When an alpha particle or beta particle interacts with the mixture, it ionises it. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation). The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle. These tracks have distinctive shapes (for example an alpha particle's track is broad and straight, while an electron's is thinner and shows more evidence of deflection). For more detailed track shape information see bubble chamber.
Charles Thomson Rees Wilson (1869-1959), a Scottish physicist, is credited with inventing the cloud chamber in 1900. In Wilson's original cloud chamber the air inside the sealed device was saturated with water vapor, then a diaphragm is used to expand the air inside the chamber (adiabatic expansion). This cools the air and water vapor starts to condense. When an ionizing particle passes through the chamber, the water vapor condenses on the resulting ions and the trail of the particle is visible in the vapor cloud. A diagram of Wilson's apparatus is given here. Wilson, along with Arthur Compton, received the Nobel Prize for Physics in 1927 for his work on the cloud chamber. This kind of chamber is also called a pulsed chamber because the conditions for operation are not continuously maintained.
The diffusion cloud chamber was later developed in 1936 by Alexander Langsdorf. This chamber differs from the expansion cloud chamber in that it is continuously sensitized to radiation and that the bottom must be cooled to a rather low temperature, generally as cold as or colder than dry ice. Alcohol vapour is also often used due to its different phase transition temperatures.
A refinement of the design is the bubble chamber.