Particle Detectors

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

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.

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IV

Other Types of Detectors

Many other types of interactions between matter and elementary particles are used in detectors. Thus in semiconductor detectors, electron-hole pairs that elementary particles produce in a semiconductor junction momentarily increase the electric conduction across the junction. The Cherenkov detector, on the other hand, makes use of the effect discovered by the Russian physicist Pavel Alekseyevich Cherenkov in 1934: A particle emits light when it passes through a nonconducting medium at a velocity higher than the velocity of light in that medium (the velocity of light in glass, for example, is lower than the velocity of light in vacuum). In Cherenkov detectors, materials such as glass, plastic, water, or carbon dioxide serve as the medium in which the light flashes are produced. As in scintillation counters, the light flashes are detected with photomultiplier tubes.

Neutral particles such as neutrons or neutrinos can be detected by nuclear reactions that occur when they collide with nuclei of certain atoms. Slow neutrons produce easily detectable alpha particles when they collide with boron nuclei in borontrifluoride. Neutrinos, which barely interact with matter, are detected in huge tanks containing perchloroethylene (C₂CI₄, a dry-cleaning fluid). The neutrinos that collide with chlorine nuclei produce radioactive argon nuclei. The perchloroethylene tank is flushed at regular intervals, and the newly formed argon atoms, present in minute amounts, are counted. This type of neutrino detector, placed deep underground to shield against cosmic radiation, is currently used to measure the neutrino flux from the sun. Neutrino detectors may also take the form of scintillation counters, the tanks in this case being filled with an organic liquid that emits light flashes when traversed by electrically charged particles produced by the interaction of neutrinos with the liquid's molecules.

The detectors now being developed for use with the storage rings and colliding particle beams of the most recent generation of accelerators are bubble-chamber types known as time-projection chambers. They can measure three-dimensionally the tracks produced by particles from colliding beams, with supplementary detectors to record other particles resulting from the high-power collisions. The Fermi National Accelerator Laboratory's CDF (Collision Detector Fermilab) is used with its colliding-beam accelerator (see Particle Accelerators) to study head-on particle collisions. CDF's three different systems can capture or account for nearly all of the subnuclear fragments released in such violent collisions.

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