Radiation detector developed by C.T.R. Wilson. Its detecting medium is a supersaturated vapour (see saturation) that condenses around ions produced by the passage of energetic charged particles, such as alpha particles, beta particles, or protons. In a Wilson cloud chamber, supersaturation is caused by the cooling induced by a sudden expansion of the saturated vapour by the motion of a piston or an elastic membrane. In a diffusion chamber, the saturated vapour is cooled to supersaturation as it diffuses into a region kept cold by a coolant such as solid carbon dioxide or liquid helium.
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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 or alcohol 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). When a vertical magnetic field is applied, positively and negatively charged particles will curve in opposite directions. 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. Inspired by sightings of the Brocken spectre while working on the summit of Ben Nevis in 1894, he began to develop expansion chambers for studying cloud formation and optical phenomena in moist air. Very rapidly he discovered that ions could act as centers for water droplet formation in such chambers. He pursued the application of this discovery and perfected the first cloud chamber in 1911. In Wilson's original 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, 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 developed in 1936 by Alexander Langsdorf. This chamber differs from the expansion cloud chamber in that it is continuously sensitized to radiation, and in that the bottom must be cooled to a rather low temperature, generally as cold as or colder than dry ice. Alcohol vapor is also often used due to its different phase transition temperatures. Dry-ice-cooled cloud chambers are a common demonstration and hobbyist device; the most common fluid used in them is isopropyl alcohol, though methyl alcohol can be encountered as well. There are also water-cooled diffusion cloud chambers, using ethylene glycol.
The bubble chamber similarly reveals the tracks of subatomic particles, but as trails of bubbles in superheated liquid. Bubble chambers can be made physically larger than cloud chambers, and since they are filled with much denser material, they reveal the tracks of much more energetic particles. These factors rapidly made the bubble chamber the particle detector of choice, so that cloud chambers were effectively superseded in fundamental research by the start of the 1960s.