Kids Research Express

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 clo

. 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