Detectors for X-rays

Abstract

The different kinds of detectors for X-rays are reviewed. In the first section, photographic film is discussed. Although the use of photographic film has been largely supplanted by the electronic devices described in the other sections of this chapter, it is still much used in powder cameras and in the preliminary investigation of specimens. Photographic intensity measurements may be made either visually or by using a microdensitometer. Geiger counters are discussed very briefly in the second section of the chapter. Geiger-Muller counters are now obsolete for data collection, but are still used in portable monitors for X-rays. The third section of the chapter discusses proportional counters. These are one of the most common kinds of detector for powder and single-crystal diffractometry. Position-sensitive detectors using proportional counters are discussed. Scintillation and solid-state detectors are described in the fourth section of the chapter. Energy resolution, pulse-amplitude discrimination, quantum-counting efficiency and linearity, and escape peaks are discussed. The fifth section covers energy-dispersive detectors. The use of silicon and germanium detectors for energy-dispersive X-ray detection is described in terms of efficiency and resolution. Alternative detector materials are mentioned as well as the use of monochromatic detection. Position-sensitive detectors are discussed in the sixth section. The position-sensitive detection of X-rays is based on the production of electrons which result from the ionization of a gas, from the generation of electron–hole pairs in a semiconductor, or as photoelectrons in a photocathode after previous conversion of the X-rays into visible light in a phosphor. The choice of detector for a given application depends on the detection efficiency, the linearity and uniformity of response, the dynamic range, the spatial resolution, the spatial distortion, the stability and the size, weight and cost of the device. The properties of various detectors and the methods of read-out are described and compared. In the seventh section of the chapter, X-ray sensitive TV cameras are reviewed. X-ray TV camera tubes with a high sensitivity and a high resolution have been developed for the dynamic observation of crystal imperfections using a conventional rotating-target X-ray generator. They can be operated like conventional TV camera tubes, but have a beryllium window and an amorphous Se–As photoconductive layer with a PIN structure for X-ray detection; an intrinsic layer is sandwiched with p- and n-type thin layers and a high electric field is applied so that electron–hole pairs generated by an incident X-ray photon are accelerated to cause avalanche multiplication. This makes it possible to detect individual X-ray photons with a spatial resolution of 6 µm. The final section of the chapter discusses storage phosphors, also known as imaging plates. Imaging plates are two-dimensional detectors having a high detective quantum efficiency and a large dynamic range. Originally developed for diagnostic radiography, they have also proved to be very useful for X-ray diffraction experiments and have replaced conventional X-ray film in many X-ray diffraction experiments. Keywords: area detectors; counters; current ionization position-sensitive detectors; densitometry; detection; detectors; detectors for X-rays; dynamic range; energy discrimination; energy resolution; escape peaks; Fano factor; gas-filled counters; Geiger counter; image intensifiers; image processing; linearity; parallel-plate counters; phosphors; position-sensitive detectors; proportional counters; pulse-amplitude discrimination; quantum counting efficiency; scintillation detectors; semiconductor detectors; signal-to-noise ratio; solid-state detectors; spatial distortions; storage phosphors; television area detectors; television camera tubes; uniformity of response; visual estimation; X-ray diffraction; X-ray phosphors; X-ray-sensitive TV cameras