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Detection and Measurement of Nuclear Radiation G. D. O'KELLEY Oak Ridge National Laboratory * Oak Ridge, Tennessee I. GENERAL INTRODUCTION During the past few years, the technique of radiation characterization has undergone a rapid transformation. New materials of construction have made possible a number of im- provements in conventional detectors, while several new detector types have been made available to the experimenter. Improvements in detectors have been accompanied by the development of more versatile and reliable electronic measur- ing equipment. Where once it was possible to record only the number of events in a certain detector, it is now quite common to record complicated spectral data. Many of the new techniques in nuclear data processing were inspired by progress in digital computer technology, which has resulted in a very desirable compatibility between modern nuclear equipment and digital com- puters. As a result, it now is possible to record and process information from a radiation detection system in a highly automated manner. In writing this monograph, the author has tried to keep in mind the needs of those nuclear chemists who, because they are not trained in electronics, find themselves confused by the bewildering array of equipment available for radiation measure- ment. The newcomer to the nuclear field is especially likely Operated for the U. S. Atomic Energy Commission by Union Carbide Nuclear Company.
to need guidance in selecting a counting system appropriate to his needs. Therefore, it was felt that a review of the nuclear radiation detection problem was in order, with particular empha- sis on new methods and their practical aspects. A description of the subject matter is given below. It will be convenient to consider first the detector, in which the radiation interacts. Each type of detector will be discussed both in terms of its principle of operation and its applicability to various problems in counting and spectrometry. The most common detectors make use of one of the two main processes by which radiation transfers energy to a stopping material, i.e., excitation and ionization. In the detectors to be discussed, molecular dissociation is of small importance; however, this effect is the basis of the chemical dosimeters which find important applications in health physics. One of the most useful and versatile detectors now in use is the scintillation counter, which uses the fluorescent light emitted when charged particles pass through certain stopping materials. The basic process here is excitation, although the interaction of the stopping medium with the incident particle may involve ionization and molecular dissociation as well. Several types of radiation detectors make use of the ioni- zation produced by the passage of charged particles. This class of detector includes ionization chambers, semiconductor radiation detectors, proportional counters, and Geiger counters. If the incident radiation consists of charged particles such as alpha particles or electrons, the ionization is produced directly (primary ionization); however, uncharged species such as gamma rays or neutrons must first interact with the detector to produce charged particles, and the ionization in this case is a secondary process. Although it is not usually necessary for an experimenter to be familiar with the details of the electronic circuits used in his equipment, it is essential that he should understand the functions and limitations of each component of his system. It is from this point of view that the discussion of auxiliary electronic instrumentation was written. No schematic diagrams are used to describe the various electronic devicesâsuch details are available elsewhere; instead, the function of each instrument is described in more general terms. Block diagrams and wave forms are used where necessary. Care has been taken
to define the more important technical terms used for describing the performance of nuclear counting equipment, in the hope that this will help the experimenter interpret published specifi- cations of commercial equipment. Other topics which were deemed appropriate to this dis- cussion of radiation measurement are low-level counting, abso- lute counting, and the mounting of radioactive sources. The subject of counting statistics was omitted, as this will be covered in a later monograph of the series. A truly complete treatment of a subject as complex as nuclear radiation detection and measurement is obviously not possible in a survey of this length; however, each section includes a reading list. The following general reviews are recommended for a more detailed description of the principles and applications of detection methods than is given here: 1. D. R. Corson and R. R. Wilson, Rev. Sci. Instr . , 19, 207 (1948); R. R. Wilson, D. R. Corson, and C. P. Baker, Particle and Quantum Detectors, Preliminary Report No. 7, National Research Council, Washington, D. C., January, 1950. 2. S. Fliigge and E. Creutz, eds., Handbuch der Physik - Encyclopedia of Physics, Vol. XLV, Springer, Berlin, 1958. 3. W. H. Jordan, Ann. Rev. Nuclear Sci., 1, 207 (1952). 4. S. A. Korff, Electron and Nuclear Counters, D. Van Nostrand Co., New York, 1955. 5. W. J. Price, Nuclear Radiation Detection, McGraw-Hill, New York, 195in 6. B. Rossi and H. H. Staub, lonization Chambers and Counters, McGraw-Hill, New York, 7. K. Siegbahn, ed., Beta- and Gamma-Ray Spectrometry, North Holland Publishing Co., Amsterdam, 1955 . 8. A. H. Snell, ed., Nuclear Instrumentation and Methods, Wiley, New York, !WZ~. 9. H. H. Staub in E. Segre, ed., Experimental Nuclear Physics, Vol. I, Part I, Wiley, New York", 1953. 10. D. H. Wilkinson, lonization Chambers and Counters, Cambridge University Press, Cambridge, 1950. 11. L. C. L. Yuan and C. S. Wu, eds., Methods of Experimental Physics, Vol. 5A, "Nuclear Physics," Academic Press, New York, 1961.
