cloud chamber

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cloud chamber,

device used to detect elementary particleselementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
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 and other ionizing radiation. A cloud chamber consists essentially of a closed container filled with a supersaturated vapor, e.g., water in air. When ionizing radiation passes through the vapor, it leaves a trail of charged particles (ions) that serve as condensation centers for the vapor, which condenses around them. The path of the radiation is thus indicated by tracks of tiny liquid droplets in the supersaturated vapor. The cloud chamber was invented c.1900 by C. T. R. Wilson. In the type devised by him, which is often called the Wilson cloud chamber, air or another gas is saturated with water vapor and enclosed in a cylinder fitted with a transparent window at the top and a piston or other pressure-regulating device at the bottom. When the pressure in the chamber is suddenly reduced, e.g., by lowering the piston, the gas-vapor mixture is cooled, producing supersaturation. Cloud chambers of this design are sometimes called the pulsed type, since they do not maintain a continuous state of supersaturation of the vapor. A more recent design is the diffusion cloud chamber. In this device a large temperature difference is maintained between the top and bottom of the chamber, usually by cooling the bottom of the chamber with dry ice. The gas in the chamber, usually air, is saturated with a vapor, usually alcohol; the air-vapor mixture cools as it diffuses toward the cool bottom, becoming supersaturated. If the gas is kept saturated with a fresh supply of vapor, e.g., by an alcohol-soaked pad inside the top of the chamber, the operation of the chamber can be essentially continuous. One disadvantage of the cloud chamber is the relatively low density of the gas, which limits the number of interactions between ionizing radiation and molecules of the gas. For this reason physicists developed other particle detectorsparticle detector,
in physics, device for detecting, measuring, and analyzing particles and other forms of radiation entering it. Such devices play an important role not only in basic research, as in the study of elementary particles, but also in numerous applications of
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, notably the bubble chamberbubble chamber,
device for detecting charged particles and other radiation by means of tracks of bubbles left in a chamber filled with liquid hydrogen or other liquefied gas. It was invented in 1952 by Donald Glaser.
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 and the spark chamberspark chamber,
in physics, device for recording the passage of elementary particles produced by reactions in a particle accelerator. Particles pass through a stack of metal plates or wire grids that are maintained with high voltage between alternate layers.
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Cloud Chamber


(also Wilson Chamber), a device for observing the tracks of charged particles. Invented by C. Wilson in 1912. The operation of a cloud chamber is based on the phenomenon of the condensation of supersaturated vapor—that is, on the formation of small droplets of liquid at some condensation centers, such as the ions that form along the track of a charged particle. The droplets reach visible size and can be photographed. The particles under investigation may either be emitted by a source within the chamber or may enter the chamber from without through a window that is transparent to them. The nature and properties of particles under investigation are established by the magnitude of the particles’ path and momentum. The momentum is measured by the curvature of the particle tracks under the influence of the magnetic field, in which a cloud chamber is usually placed.

In studying low-energy particles, the chambers are filled with gas at less than atmospheric pressure; for the study of high-energy particles, a chamber is filled with gas to pressures measured in dozens of atmospheres. The size and shape of the chambers and the materials used in their walls vary greatly.

The cloud chamber played an important role in the study of the structure of matter. For several decades the cloud-chamber method was virtually the only visual method for detecting nuclear radiation. In recent years, however, the cloud chamber has given way to bubble and scintillation chambers.


Printsipy i melody registratsii elementarnykh chastits. Moscow, 1963. (Translated from English.)


cloud chamber

[′klau̇d ‚chām·bər]
A particle detector in which the path of a charged particle is made visible by the formation of liquid droplets along the trail of ions left by the particle as it passes through the gas of the chamber. Also known as expansion chamber; fog chamber.

cloud chamber

Physics an apparatus for detecting high-energy particles by observing their tracks through a chamber containing a supersaturated vapour. Each particle ionizes molecules along its path and small droplets condense on them to produce a visible track