Kids Research Express

. 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

.. C. Spark Chamber In a spark chamber, incoming high-energy particles ionize the air or a gas between plates or wire grids that are kept alternately positively and negatively charged. Sparks jump along the paths of ionization and can be photographed to show particle tracks. In some spark-chamber installations, information on particle tracks is fed directly into electronic computer circuits without the necessity of photography. A spark chamber can be operated quickly and selectively. The instrument can be set to record particle tracks only when a particle of the type that the researchers want to study is produced in a nuclear reaction. This advantage is important in studies of the rarer particles; spark-chamber pictures, however, lack the resolution and fine detail of bubble-chamber pictures. D. Scintillation Counter The scintillation counter functions by the ionization produced by charged particles moving at high speed within certain transparent solids and liquids, known as scintillat