Instrumentation for Gamma-Ray Spectroscopy

The subject of ttis chapter is the function and operation of the components of a gamma-ray spectroscopy system. In Chapter 3 it was shown that the output pulse amplitude from most gamma-ray detectors is proportional to the energy deposited by the gamma ray. The pulse-height spectrum from such a detector contains a series of full-energy peaks superimposed on a continuous Compton background. Although the spectrum can be quite complicated (for example, see Figure 1.10 of Chapter 1) and thereby dHficult to analyze, it contains much useful information about the energies and relative intensities of the gamma rays emitted by the source. The information that is important for the quantitative nondestructive assay (NDA) of nuclear material is contained in the full-energy peaks. The purpose of the electronic equipment that follows the detector is to acquire an accurate representation of the pulse-height spectrum and to extract the desired energy and intensity information from that spectrum. This chapter provides a relatively brief introduction to the wide variety of instrumentation used in the gamma-ray spectroscopy of nuclear materials. It emphasizes the function of each component and provides information about important aspects of instrument operation. For a detailed description of instrument operation, the reader should refer to the instruction manuals provided with each instrument. Because of the rapidly advancing state-of-the-art of gamma-ray spectroscopy quipment, the best sources of current information are often the manufacturers and users of the instruments. Although the manufacturers are clearly the best source of information about the electronic capabilities of their equipment, those active in the application of gamma-ray spectroscopy to NDA are usually the best source of information on effective assay procedures and the selection of equipment for a given application. Books and reports on gamma-ray spectroscopy equipment are often out of date soon after they are published. Gamma-ray spectroscopy systems can be divided into two classes according to whether they use single-channel analyzers (SCAS) or multichannel analyzers (MCAS). Figures 4.1 and 4.2 show block diagrams of the two classes. Both systems begin with a detector, where the gamma-ray interaction produces a weak electrical signal that is proportional to the deposited energy. Section 4.2 discusses the process of