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

Track Detectors

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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 scintillating materials, causing flashes of visible light (see Luminescence). The gases argon, krypton, and xenon produce ultraviolet light, and hence are used in scintillation counters. A primitive scintillation device, known as the spinthariscope, was invented in the early 1900s and was of considerable importance in the development of nuclear physics. The spinthariscope required, however, the counting of the scintillations by eye. Because of the uncertainties of this method, physicists turned to other detectors, including the Geiger-Müller counter. The scintillation method was revived in 1947 by placing the scintillating material in front of a photomultiplier tube, a type of photoelectric cell. The light flashes are converted into electrical pulses that can be amplified and recorded electronically.

Various organic and inorganic substances such as plastic, zinc sulfide, sodium iodide, and anthracene are used as scintillating materials. Certain substances react more favorably to specific types of radiation than others, making possible highly diversified instruments. The scintillation counter is superior to all other radiation-detecting devices in a number of fields of current research. It has replaced the Geiger-Müller counter in the detection of biological tracers (see Isotopic Tracer) and as a surveying instrument in prospecting for radioactive ores. It is also used in nuclear research, notably in the investigation of such particles as the antiproton (see Proton), the meson Elementary Particles, and the neutrino. One such counter, the Crystal Ball, has been in use since 1979 for advanced particle research, first at the Stanford Linear Accelerator Center and, since 1982, at the German Electron Synchrotron Laboratory (DESY) in Hamburg, Germany. The Crystal Ball is a hollow crystal sphere, about 2.1 m (7 ft) wide, that is surrounded by 730 sodium iodide crystals.