|
Anatomy => Isotopic Tracer => Particle detectors
Particle detectors
INTRODUCTION
Particle Detectors, instruments used to detect and study fundamental nuclear particles . These detectors range in complexity from the well-known portable Geiger counter to room-sized spark and bubble chambers.
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.
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.
° 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 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.
° 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 . Bubble chambers filled with liquid hydrogen allow the study of interactions between the accelerated particles and the hydrogen nuclei.
° 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.
° 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 . 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 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 , 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.
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 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.
Back
|
