Introduction

X-ray spectrometers are of two kinds. The energy-dispersive (ED) type records X-rays of all energies effectively simultaneously and produces an output in the form of a plot of intensity versus X-ray photon energy. The detector consists of one of several types of device producing output pulses proportional in height to the photon energy. The wavelength-dispersive (WD) type makes use of Bragg reflection by a crystal, and operates in ‘serial’ mode, the spectrometer being ‘tuned’ to only one wavelength at a time. Several crystals of different interplanar spacings are needed in order to cover the required wavelength range. Spectral resolution is better than for the ED type, but the latter is faster and more convenient to use. X-ray spectrometers attached to SEMs are usually of the ED type, though sometimes a single multi-crystal WD spectrometer is fitted. Electron microprobe instruments are fitted with up to five WD spectrometers, and often have an ED spectrometer as well. These two types of spectrometer are described in detail in Sections 5.2–5.4 below.

Energy-dispersive spectrometers

The modes of operation and characteristics of the detectors and associated electronics used as ED spectrometers are described in the following sections.

Solid-state X-ray detectors

In ED spectrometers the X-ray detection medium is a semiconductor (either silicon or germanium), in which the valence band is normally fully occupied by electrons.