Radiation Detectors

Type of physical science: Nuclear physics

Field of study: Nuclear techniques

Many of the processes that are studied in nuclear science, physics, chemistry, biology, and medicine involve the production of radioactive particles or radiation. Without any means of detecting this radiation, scientists would be unable to study processes involving radiation production.

Overview

There are two main classifications of nuclear radiation detectors: "signal" detectors and "track" detectors. Their names are derived from the form of their output. Signal detectors produce an electrical signal, which is used to detect the presence of the radiation. Track detectors can show tracks of the nuclear particles that they detect. Examples of these types of detectors are cloud chambers or bubble chambers.

There are three main types of signal detectors: gas-filled counters, scintillation detectors, and semiconductor counters. The scintillation detectors and gas-filled counters are the oldest types of detector. Ernest Rutherford used a zinc sulfide scintillation detector when he bombarded gold foil with α particles to demonstrate the existence of the nucleus of an atom.

The gas-filled detector normally consists of a cylinder containing a noble gas compound or some other appropriate gas. The container wall serves as one of the electrodes in the detector. A metal wire that runs through the center of the detector serves as the second electrode, allowing an electrical current to run through the gas in the detector. Radiation enters into the detector through a window material, such as mylar. When the radiation enters the detector, it is capable of ionizing the gas in the detector, forming electrons and positive gas ions.

This process is the primary ionization. The electrode known as the anode attracts and collects the electrons produced in the ionization and registers a pulse.

There are three types of gas-filled detector, depending on the construction of the detector and the strength of the applied electrical field. If the field applied is too low, then the electrons and gas ions can recombine before collection of the electrons occurs at the anode.

Therefore, the radiation causes no pulse. As the field applied to the detector increases, the time available for recombination decreases, and the detector collects electrons. The saturation voltage is the applied voltage where all the electrons reach the anode. An ionization counter operates at the saturation voltage.

If the applied voltage is increased beyond the saturation voltage, the electrons are accelerated and kinetic energy increases. At sufficiently high applied voltage, the electrons can cause further ionization of gas molecules. These ionizations are called secondary ionizations. The number of secondary electrons produced is proportional to the number of primary electrons. The pulse height is still proportional to the intensity and energy of the radiation entering the detector.

Proportional counters operate in this manner.

The Geiger region is entered if the applied voltage is increased still further. In this region, the pulse heights are essentially constant, and the energy of the incident particles is so high that the secondary ionization produced is the same for all types of radiation. This secondary ionization is also independent of the energy of the incoming radiation. A Geiger tube can measure all types of radiation, but is unreliable for distinguishing between various types of radioactivity.

The principal requirement of a good counter gas is that it does not form negative ions very easily. Negative ions would collect at the anode and compete with electron collection. The most common fill gases are the noble gases. The type of radiation detected also determines the type of gas used in the counter. Helium is the best gas for electron detection, argon for X-ray detection, and krypton for high-energy γ-ray detection.

In the presence of a single fill gas, the positive ions migrate to the walls of the container, which is the electrode known as the cathode. At the wall, they acquire electrons and become neutral atoms again. The acquired electrons release their excess energy as electromagnetic energy in the form of X rays. The X rays produced cause unwanted ionizations.

One way to quench these ionizations is to add a polyatomic quench gas that ionizes at a lower energy than the fill gas. As the positive ions migrate to the cathode, they collide with molecules of quench gas. The quench gas neutralizes the positive ions and becomes ionized. These ions migrate to the cathode and acquire electrons rather than the fill gas ions; however, the excess energy of the acquired electrons results in dissociation of the quench gas molecules rather than X-ray emission.

A scintillation counter consists of a transparent phosphor scintillating material coupled to a photomultiplier tube to count the electrons ejected from the scintillator. A scintillating material produces light when some form of radiation impinges on it. The amount of light produced by the scintillator is proportional to the amount of radiation impinging on it. A reflector surrounds the scintillator, except where the scintillator attaches to the photomultiplier tube. A photomultiplier tube contains a material that ejects electrons when light impinges on it. The number of electrons ejected is proportional to the intensity of the light that shines on the photoelectric material. The electrons then pass through a series of dynodes. When a photoelectron hits a dynode, it ejects a secondary electron. A photomultiplier tube may have ten or more dynodes, resulting in photomultiplication of the original photoelectron. In this process, the original radioactivity causes light to be emitted. The light that reaches the photomultiplier tube causes electron ejection. The dynodes multiply the number of ejected electrons. The electrons produced flow through an electrical circuit, causing a voltage drop across a resistor.

The original radiation is ultimately recorded as a voltage reading in this circuit.

The third type of signal detector is the semiconductor counter. A metal is a substance that contains electrons in the highest energy band of a substance, the conduction band. A nonmetal contains electrons in the valence band, but not in the conduction band. There is an energy gap between the valence and the conduction band. A semiconductor does not normally have any electrons in the conduction band. In a semiconductor, the spacing between the valence band and the conduction band is smaller than that of a nonmetal. It is easier to impart energy to an electron in the valence band, and promote it to the conduction band. When radiation strikes a semiconducting material, such as silicon or germanium, an electron can be promoted from the valence band to the conduction band. This results in the formation of a "hole" in the valence band. If a potential is applied across the semiconductor, the electrons produced in this manner will migrate to the anode, and the holes will migrate toward the cathode; the result is an electrical pulse.

Cloud chambers are one type of track detector. Operation of the cloud chamber requires use of a supersaturated gas sample in the chamber. In one type of cloud chamber, the diffusion type, the supersaturated gas is obtained by moving the gas from a warmer to a cooler area of the chamber. A decrease in temperature causes a lowering of the amount of gas required for saturation. Thus, the gas enters a supersaturated condition. Supersaturated gases condense very easily, and charged particles entering the chamber act as a condensation center. Methyl and ethyl alcohol work very well in cloud chambers.

Another type of track detector, the bubble chamber, employs a superheated liquid in its operation. The superheated liquid is kept under pressure. When charged particles pass through a superheated medium, bubbles form along the path of the particles. The bubble chamber can employ hydrogen, deuterium, helium, xenon, diethyl ether, propane, freon, and other materials as the superheated medium.

Besides the "track" type counters and the "signal" type detectors, photographic film can be employed to detect radiation, particularly X-ray and γ radiation. The film produces a permanent record of the radiation event. Photographic emulsions can detect and track charged particles. The charged particles will produce images on the photographic material of the path they travel. A researcher can obtain information about the mass, energy, and charge of the particles by analyzing the characteristics of the paths produced on the emulsion.

Applications

The track counters are used mainly to study high-energy physics phenomena. The signal detectors are of more general interest to researchers in fields that employ radioactive isotopes, including chemistry, geology, biology, and medicine. Many of these applications involve the use of radioactive isotopes as tracers.

All three types of signal detectors can be used to detect many of the same types of radioactivity. The decision to use one or another of these detectors depends on the relative importance of counting efficiency, ease of operation, and other factors. Usually, the semiconductor detectors are the easiest to operate. Semiconductor detectors are becoming increasingly important because of improvements in semiconductor technology.

Geiger, proportional, scintillation, and semiconductor counters all can be used to detect charged particles such as α and β particles very efficiently. Flow-type proportional counters are good detectors of &α;-particle radiation. They have the advantage of a large area sensitive to the radiation. Scintillation counters employing thallium-activated cesium iodide or silver-activated zinc sulfide are also appropriate &α;-particle detectors. Scintillation counters also have the advantage of a large area of sensitivity, but they have poor-to-moderate energy resolution. Semiconductor detectors, on the other hand, have good energy resolution but a very limited area of sensitivity.

Gas-filled counters can be used to detect β particles. They have the advantage of simple operation over a wide range of energies, but they yield only an intensity measurement, and thus cannot be used to resolve the energies of the β particles. Scintillation counters have large areas of sensitivity to the β particles, but provide only very moderate energy resolution.

Scintillation counters are not very efficient at detecting lower-energy β particles, emitted by some important radioactive isotopes such as carbon 14. Semiconductor counters provide the best energy resolution for β particles.

Gas-filled counters normally are inappropriate for detecting γ rays or X rays.

Scintillation counters employing inorganic scintillants, such as thallium-activated sodium iodide, are very good for the detection of γ rays and X rays. Semiconductor counters are also very good for the detection of γ rays and X rays. A semiconductor detector employing a germanium semiconductor, doped with lithium, has very good energy resolution. The disadvantages of these detectors are the small area of sensitivity and the fact that these detectors need to be cooled with liquid nitrogen.

The nuclear fission of large nuclei results in the production of neutrons, which can be detected easily by scintillation counters. The type of scintillant used will depend on the energy of the neutrons. Polyester disks are good for detecting thermal neutrons, whereas certain plastics are better for detecting fast neutrons. Some inorganic scintillants, such as europium-activated lithium iodide, can detect slow neutrons. Specialized gas-filled counters are also capable of detecting neutrons. One version of these counters employs a fill gas of boron trifluoride. The boron nuclei absorb the neutrons and decay by α emission. The counter detects the α particles produced in these decays. Semiconductor counters also rely on a nuclear reaction between the neutrons and substances such as boron or lithium. Thus, it is a charged particle produced in the nuclear reaction that undergoes detection.

Scintillation counters and certain semiconductor counters can even distinguish between various types of radioactivity by the shape of the pulse produced. This can be very advantageous, when radioactive nuclei emit several different types of radioactivity simultaneously. Through electronic means, the detector can be tuned to detect only one type of radiation.

Photographic film is sometimes used to detect radiation. Medical and dental X rays employ film in this way. A film badge is a piece of film in a holder of some type, used to detect exposure to radioactivity. The film badge is very compact, which makes it convenient for researchers using radioactive isotopes to monitor their exposure to radioactivity.

Context

The development of radiation detectors parallels the discovery of the different forms of radiation. Some of the earlier discoveries, such as the discovery of the electron and the α particle, used cathode-ray tubes and did not involve any radiation detectors. In 1895, Wilhelm Rontgen discovered and named X rays. At almost the same time, Antoine-Henri Becquerel accidentally discovered radioactivity, when he noticed that when an object containing uranium came near a photographic plate, an image of that object formed on the plate. On January 20, 1896, the first medical use of X rays occurred.

Ernest Rutherford employed a crude scintillation counter, made of zinc sulfide mounted on the end of a telescope, in some of his experiments with α particles. In these experiments, he bombarded gold foil with α particles and detected the scattering angles of the α particles. The flashes of light produced had to be observed with the naked eye through the telescope. Hans Geiger, a colleague of Rutherford, developed a gas-filled counter in 1928 and employed this detector to study α particle interactions with thin foils. The gas-filled detectors replaced the scintillation detector until 1944, when the scintillation counter was used with a photomultiplier tube. With the development of a variety of inorganic and organic compounds that could be used as scintillating materials, the scintillation counter became very popular.

The last type of counter developed was the solid-state semiconductor detector. In 1945, a "crystal counter" was constructed. These detectors began to find routine application in radiation detection in the early 1960's. Because of improvements in the area of semiconductor design, and their ease of use, these detectors have become increasingly popular. The applications of these solid-state detectors continue to grow as the number of uses of radioactive isotopes grows.

The uses of radioactive isotopes are numerous. Even if the use of radioactive isotopes in tracer studies, radio dating, and other applications were absent, radiation detectors would still have an important application in determining levels of radiation present in the environment. A combination of natural and man-made sources causes these levels of radiation. Burning coal releases naturally occurring radioactive isotopes into the environment. These sources of radioactivity can be monitored with the help of radiation detectors.

Principal terms

ALPHA PARTICLE: radiation consisting of a helium nucleus; contains two neutrons and two protons and has a positive charge twice that of the negative charge of an electron

ANION: an ion having a negative electrical charge; forms by the addition of an electron or electrons to a neutral substance and is attracted to the electrode called the anode

BETA PARTICLE: an electron emitted from the nucleus of an atom; it has a negative electrical charge and a small mass compared with protons and α particles

CATION: an ion having a positive electrical charge; it forms by the removal of an electron or electrons from a neutral substance and is attracted to the electrode called the cathode

CONDUCTION BAND: the high-energy band in a solid where electrons are free to move about in the entire solid

FISSION: the process whereby an unstable nucleus splits into two smaller nuclei, several neutrons, and energy

GAMMA RAY: high-energy electromagnetic radiation; it has neither mass nor electrical charge

NEUTRON: a particle having zero electrical charge, almost equal in mass to the proton

PRIMARY IONIZATION: the original ionization of a substance in a detector, caused by the original radiation entering the detector

RADIOACTIVE ISOTOPE: an unstable isotope of an element that is capable of decaying by emitting some form of radiation; it usually converts to a more stable isotope through this decay or a series of these decays

SECONDARY IONIZATION: any subsequent ionization caused by electrons created in the original primary ionizations, or electrons created by other secondary ionizations; causes gas multiplication

Bibliography

Deme, Sandor. SEMICONDUCTOR DETECTORS FOR NUCLEAR RADIATION MEASUREMENT. New York: John Wiley & Sons, 1971. A complete work on various aspects of semiconductor detectors. Contains a chapter on the interaction of nuclear radiation with matter, extensive coverage of applications of semiconductor detectors, and a comprehensive table comparing the three types of radiation detectors in terms of their detection efficiencies for various forms of radiation.

Glasstone, Samuel. SOURCEBOOK ON ATOMIC ENERGY. 2d ed. Princeton, N.J.: D. Van Nostrand, 1958. A good reference book on many different aspects of atomic energy, radiochemistry, and nuclear physics. The chapter on measurement of nuclear radiation contains a concise, easy-to-understand discussion of detectors.

Kleinknecht, K. DETECTORS FOR PARTICLE RADIATION. New York: Cambridge University Press, 1986. A 206-page text based on a series of lectures given by Professor Kleinknecht at Dortmund University. A complete yet not overpowering treatment of nuclear radiation detection. Contains much material on track detectors.

L'Annunziata, M. F. RADIONUCLIDE TRACERS: THEIR DETECTION AND MEASUREMENT. New York: Academic Press, 1987. An extensive treatise, written as a practical manual for the user of radioactive isotopes. The first chapter outlines the basic principles of radioactivity and the various interactions that can take place between radioactivity and matter. Includes a detailed table in the appendix, which lists some of the important radiation characteristics for many useful radioactive isotopes.

Ouseph, P. J. INTRODUCTION TO NUCLEAR RADIATION DETECTORS. New York: Plenum Press, 1975. Provides a survey of developments in semiconductor detectors along with a discussion of various aspects of gas and scintillation counters.

Ouseph, P. J., and M. Schwartz. "LXXV. Nuclear Radiation Detectors." JOURNAL OF CHEMICAL EDUCATION 51, no. 3 (1974): A139. A short article discussing and comparing gas-filled counters, scintillation counters, and semiconductor counters. A good reference for the reader wanting to obtain more information about radiation detectors without reading some of the more technical references listed here. Does not discuss track detectors.

Ouseph, P. J. "LXXV. Nuclear Radiation Detectors." JOURNAL OF CHEMICAL EDUCATION 51, no. 4 (1974): A209. A continuation of the previous article.

Washtell, C. C. H. AN INTRODUCTION TO RADIATION COUNTERS AND DETECTORS. New York: Philosophical Library, 1960. A well-written monograph on gas-filled counters and scintillation counters. Contains a very informative table of phosphors used in scintillation counters. Easy to read.

The Structure of the Atomic Nucleus

Detectors on High-Energy Accelerators

Electrons and Atoms

Nuclear Reactions and Scattering

Radiation: Interactions with Matter

Radioactive Nuclear Decay and Nuclear Excited States