Particle Detectors

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Particle Detectors, instruments used to detect and study fundamental nuclear particles (see Atom; Nuclear Energy). These detectors range in complexity from the well-known portable Geiger counter to room-sized spark and bubble chambers.

A. IONIZATION CHAMBER

One of the first detectors to be used in nuclear physics was the ionization chamber, which consists essentially of a closed vessel containing a gas and equipped with two electrodes at different electrical potentials. The electrodes, depending on the type of instrument, may consist of parallel plates or coaxial cylinders, or the walls of the chamber may act as one electrode and a wire or rod inside the chamber act as the other. When ionizing particles of radiation enter the chamber they ionize the gas between the electrodes. The ions that are thus produced migrate to the electrodes of opposite sign (negatively charged ions move toward the positive electrode, and vice versa), creating a current that may be amplified and measured directly with an electrometer—an electroscope equipped with a scale—or amplified and recorded by means of electronic circuits.

Ionization chambers adapted to detect individual ionizing particles of radiation are called counters. The Geiger-Müller counter is one of the most versatile and widely used instruments of this type. It was developed by the German physicist Hans Geiger from an instrument first devised by Geiger and the British physicist Ernest Rutherford; it was improved in 1928 by Geiger and by the German American physicist Walther Müller. The counting tube is filled with a gas or a mixture of gases at low pressure, the electrodes being the thin metal wall of the tube and a fine wire, usually made of tungsten, stretched lengthwise along the axis of the tube. A strong electric field maintained between the electrodes accelerates the ions; these then collide with atoms of the gas, detaching electrons and thus producing more ions. When the voltage is raised sufficiently, the rapidly increasing current produced by a single particle sets off a discharge throughout the counter. The pulse caused by each particle is amplified electronically and then actuates a loudspeaker or a mechanical or electronic counting device.

B. TRACK DETECTORS

C. OTHER TYPES OF DETECTORS

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