Track Detectors
. 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...