Cloud Chamber

Cloud Chamber, instrument used to track the paths of charged subatomic particles released by radioactive substances. Physicists learn about a particle’s charge, mass, and energy by observing its path as it moves through a cloud chamber.

A cloud chamber is an airtight container, varying in size from a few square centimeters or inches to a room large enough for a person to enter. The container has at least one glass window through which the particle tracks can be observed and photographed. In order to trace the paths of particles, which are invisible to the naked eye, cloud chambers cool a gas saturated with water vapor, methyl alcohol vapor, or ethyl alcohol vapor so the gas becomes supersaturated—so full of vapor that liquid would begin to condense out of the gas under normal conditions. The vapor in the gas will not condense, however, unless there are foreign particles in the gas that the vapor can condense around.

As a charged particle passes through the supersaturated gas in the cloud chamber, the particle interacts with atoms and molecules of the gas and vapor. This interaction turns the gas particles into ions, atoms or molecules with a positive or negative electric charge. The supersaturated vapor condenses around these ions in the gas, forming a visible string of liquid droplets that illuminate the pathway of the particle.

Scottish physicist and Nobel Prize winner Charles T. R. Wilson invented the cloud chamber in 1911. He had experimented with optical phenomena created by light shining through supersaturated water vapor since the 1890s. In 1896 he discovered that supersaturated vapor condensed around the ions created by X rays just as it did around any other foreign body.

The cloud chamber played a key role in the early discoveries about particle physics. It enabled physicists to study the behavior of individual atoms, to photograph the actual paths of ionizing particles, and to analyze the complicated interactions that take place between charged particles and individual atoms.

Track Detectors

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Detectors that enable researchers to observe the tracks that particles leave behind are called track detectors. Spark and bubble chambers are track detectors, as are the cloud chamber and nuclear emulsions. Nuclear emulsions resemble photographic emulsions but are thicker and not as sensitive to light. A charged particle passing through the emulsion ionizes silver grains along its track. These grains become black when the emulsion is developed and can be studied with a microscope.

A. Cloud Chamber

The fundamental principle of the cloud chamber was discovered by the British physicist C. T. R. Wilson in 1896, although an actual instrument was not constructed until 1911. The cloud chamber consists of a vessel several centimeters or more in diameter, with a glass window on one side and a movable piston on the other. The piston can be dropped rapidly to expand the volume of the chamber. The chamber is usually filled with dust-free air saturated with water vapor. Dropping the piston causes the gas to expand rapidly and causes its temperature to fall. The air is now supersaturated with water vapor, but the excess vapor cannot condense unless ions are present. Charged nuclear or atomic particles produce such ions, and any such particles passing through the chamber leave behind them a trail of ionized particles (see Ionization) upon which the excess water vapor will condense, thus making visible the course of the charged particle. These tracks can be photographed and the photographs then analyzed to provide information on the characteristics of the particles.

Because the paths of electrically charged particles are bent or deflected by a magnetic field, and the amount of deflection depends on the energy of the particle, a cloud chamber is often operated within a magnetic field. The tracks of negatively and positively charged particles will curve in opposite directions. By measuring the radius of curvature of each track, its velocity can be determined. Heavy nuclei such as alpha particles form thick and dense tracks, protons form tracks of medium thickness, and electrons form thin and irregular tracks. In a later refinement of Wilson's design, called a diffusion cloud chamber, a permanent layer of supersaturated vapor is formed between warm and cold regions. The layer of supersaturated vapor is continuously sensitive to the passage of particles, and the diffusion cloud chamber does not require the expansion of a piston for its operation. Although the cloud chamber has now been supplanted almost entirely by the bubble chamber and the spark chamber, it was used in making many important discoveries in nuclear physics.

B. Bubble Chamber

The bubble chamber, invented in 1952 by the American physicist Donald Glaser, is similar in operation to the cloud chamber. In a bubble chamber a liquid is momentarily superheated to a temperature just above its boiling point. For an instant the liquid will not boil unless some impurity or disturbance is introduced. High-energy particles provide such a disturbance. Tiny bubbles form along the tracks as these particles pass through the liquid. If a photograph is taken just after the particles have crossed the chamber, these bubbles will make visible the paths of the particles. As with the cloud chamber, a bubble chamber placed between the poles of a magnet can be used to measure the energies of the particles. Many bubble chambers are equipped with superconducting magnets instead of conventional magnets (see Superconductivity). Bubble chambers filled with liquid hydrogen allow the study of interactions between the accelerated particles and the hydrogen nuclei.

C. Spark Chamber

D. Scintillation Counter