RADIATION MEASUREMENT

Photographic emulsions The use of photographic techniques to record ionizing radiations dates back to the discovery of X rays by Rntgen in the late 1800s, but similar techniques remain important today in some applications. A photographic emulsion consists of a suspension of silver halide grains in an inert gelatin matrix and supported by a backing of plastic film or another material. If a charged particle or fast electron passes through the emulsion, interactions with silver halide molecules produce a similar effect as seen with exposure to visible light. Some molecules are excited and will remain in this state for an indefinite period of time. After the exposure is completed, this latent record of the accumulated exposure can be made visible through the chemical development process. Each grain containing an excited molecule is converted to metallic silver, greatly amplifying the number of affected molecules to the point that the developed grain is visible. Photographic emulsions used for radiation detection purposes can be classified into two main subgroups: radiographic films and nuclear emulsions. Radiographic films register the results of exposure to radiation as a general darkening of the film due to the cumulative effect of many radiation interactions in a given area of the emulsion. Nuclear emulsions are intended to record individual tracks of a single charged particle. Radiographic films Radiographic films are most familiar in their application in medical X-ray imaging. Their properties do not differ drastically from those of normal photographic film used to record visible light, except for an unusually high silver halide concentration. Thickness of the emulsion ranges from 10 to 20 micrometres, and they contain silver halide grains up to 1 micrometre in diameter. The probability that a typical incident X ray will interact in the emulsion is only a few percent, and so methods are often applied to increase the sensitivity so as to reduce the intensity of the X rays needed to produce a visible image. One such technique is to apply emulsion to both sides of the film base. Another is to sandwich the photographic emulsion between intensifier screens that consist of thin layers of light-emitting phosphors of high atomic number, such as calcium tungstate, cesium iodide, or rare earth phosphors. If an X ray interacts in the screen, the light that is produced darkens the film in the immediate vicinity through the normal photographic process. Because of the high atomic number of the screens, they are more likely to cause an X ray to interact than the emulsion itself, and the X-ray flux needed to achieve a given degree of darkening of the emulsion can be decreased by as much as an order of magnitude. The light is produced in the normal scintillation process (see below Active detectors: Scintillation and Cerenkov detectors) and travels in all directions from the point of the X-ray interaction. This spreading causes some loss of spatial resolution in X-ray images, especially for thicker screens, and the screen thickness must therefore be chosen to reach a compromise between resolution and sensitivity. technique for detecting the intensity and characteristics of ionizing radiation, such as alpha, beta, and gamma rays or neutrons, for the purpose of measurement. The term ionizing radiation refers to those subatomic particles and photons whose energy is sufficient to cause ionization in the matter with which they interact. The ionization process consists of removing an electron from an initially neutral atom or molecule. For many materials, the minimum energy required for this process is about 10 electron volts (eV), and this can be taken as the lower limit of the range of ionizing radiation energies. The more common types of ionizing radiation are characterized by particle or quantum energies measured in thousands or millions of electron volts (keV or MeV, respectively). At the upper end of the energy scale, the present discussion will be limited to those radiations with quantum energies less than about 20 MeV. This energy range covers the common types of ionizing radiation encountered in radioactive decay, fission and fusion systems and the medical and industrial applications of radioisotopes. It excludes the regime of high-energy particle physics in which quantum energies can reach billions or trillions of electron volts. In this field of research, measurements tend to employ much more massive and specialized detectors than those in common use for the lower-energy radiations. Active detectors In many applications it is important to produce a signal that indicates the presence of ionizing radiation in real time. Such devices are classified as active detectors. Many types of active detectors can produce an observable signal for an individual quantum of radiation (such as a single alpha particle or an X-ray photon). Others may provide a signal that corresponds to the collective effect of many quanta interacting in the detector within its response time. Modes of operation In many types of detectors, a single particle or quantum of radiation liberates a certain amount of charge Q as a result of depositing its energy in the detector material. For example, in a gas, Q represents the total positive charge carried by the many positive ions that are produced along the track of the particle. (An equal charge of opposite sign is carried by the free electrons that are also generated.) This charge is created over a very short time, typically less than a nanosecond, as the particle slows down and stops; it is then collected over a much longer period of time, ranging from a few nanoseconds to several microseconds. In a gas or a semiconductor, the charge is collected through the motion of individual charge carriers in the electric field that is established within the detector. As these moving charges represent an electric current, detector response to a single quantum of radiation can then be modeled as a momentary burst of current that begins with the stopping of the charged particle and ends once all the charge carriers have been collected. If the detector is undergoing continuous irradiation, a sequence of these current bursts will be produced, one for each interacting quantum. In most applications the time of arrival of each quantum of radiation is randomly distributed. For purposes of this discussion, it is assumed that the average time between events in the detector is long compared with the charge collection time. Each burst of current is then distinct, and the integral or area under the current versus time profile for each burst is the charge Q formed for that event. Because the amount of energy deposited may be different for individual events, each of these current pulses may represent a different total charge Q. Furthermore, the charge collection time may also be variable, so the length of each of these current bursts may be different. Additional reading General works on radiation detection and measurement include Ralph E. Lapp and Howard L. Andrews, Nuclear Radiation Physics, 4th ed. (1972), a good fundamental coverage of sources, interactions, and other aspects of ionizing radiation; Nicholas Tsoulfanidis, Measurement and Detection of Radiation (1983), a good general coverage of the field intended for an undergraduate reader, including a detailed treatment of methods of data analysis; National Council on Radiation Protection and Measurements, A Handbook of Radioactivity Measurements Procedures (1985), an extensive report and excellent textbook-like coverage of many of the important procedures in radionuclide measurements; W.B. Mann, A. Rytz, and A. Spernol, Radioactivity Measurements: Principles and Practice, International Journal of Radiation Applications and Instrumentation, part A, Applied Radiation and Isotopes, 39(8):717937 (1988), a thorough coverage of topics of particular interest in radionuclide metrology; and Glenn F. Knoll, Radiation Detection and Measurement, 2nd ed. (1989), a widely used comprehensive textbook. E. Fenyves and O. Haiman, The Physical Principles of Nuclear Radiation Measurements (1969; originally published in German, 1965), is a rigorous treatment of the theory of various detection processes, with less emphasis on specific devices. Frank H. Attix, William C. Roesch, and Eugene Tochilin (eds.), Radiation Dosimetry, 2nd ed., 3 vol. (196669); and Kenneth R. Kase, Bengt Bjrngard, and Frank H. Attix (eds.), The Dosimetry of Ionizing Radiation, 3 vol. (198590), are comprehensive collections of chapters by various authors on all aspects of radiation dosimetry and instrumentation for dose measurement.General discussion of radiation detectors is found in Jack Sharpe, Nuclear Radiation Detectors, 2nd ed. rev. (1964), a short monograph emphasizing basic detector mechanisms, with good coverage of gas-filled detectors; Geoffrey G. Eichholz and John W. Poston, Principles of Nuclear Radiation Detection (1979, reissued 1985), general coverage at the undergraduate level; and P.N. Cooper, Introduction to Nuclear Radiation Detectors (1986), an introductory-level short monograph with brief descriptions of the major instruments used in radiation measurements and dosimetry.Several detectors are examined in the following works: scintillation detectors in J.B. Birks, The Theory and Practice of Scintillation Counting (1964), detailed coverage of all aspects of scintillation counting, from the basic scintillation mechanisms to practical aspects of counters; E. Schram, Organic Scintillation Detectors: Counting of Low-energy Beta Emitters (1963), short, specialized coverage of organic scintillation mechanisms and specific devices; and Stephen M. Shafroth (ed.), Scintillation Spectroscopy of Gamma Radiation (1967), a collection of individual articles on various aspects of practical scintillation spectroscopy; ionization detectors in Bruno B. Rossi and Hans H. Staub, Ionization Chambers and Counters (1949), a classic text that is still a useful source of detailed analysis; and Denys Haigh Wilkinson, Ionization Chambers and Counters (1950, reissued 1970), still a valuable reference; and semiconductor detectors in G. Dearnaley and D.C. Northrop, Semiconductor Counters for Nuclear Radiations, 2nd ed., rev. and enlarged (1966), a specialized treatment; and Klaus Debertin and Richard G. Helmer, Gamma- and X-ray Spectrometry with Semiconductor Detectors (1988), a thorough monograph on all aspects of high-resolution photon spectroscopy, with an excellent collection of reference data. Other specific detector types are discussed by Robert L. Fleischer, P. Buford Price, and Robert M. Walker, Nuclear Tracks in Solids (1975), a thorough description of techniques useful in track-etch detectors; R.H. Herz, The Photographic Action of Ionizing Radiations in Dosimetry and Medical, Neutron, Auto-, and Microradiography (1969), a thorough description of radiographic techniques for various types of ionizing radiation; and Konrad Kleinknecht, Detectors for Particle Radiation (1986), an excellent specialized coverage of the gas-filled and position-sensitive devices of primary interest in high-energy particle physics. Glenn F. Knoll