Section 7.1.3.2.1. Photon-counting detectors

Commonly used photon counters include scintillator/photomultiplier combinations, gas-filled counters and reverse-biased semi-conductor detectors.

Scintillator/photomultipliers usually consist of a relatively thick crystal of a scintillator coupled to a high-gain photomultiplier tube. These detectors are generally designed to serve as point photon counters with moderate energy resolution. In order to perform this function, several constraints must be met:

Scintillator/phototube combinations are relatively trouble-free and often have near-unity DQE. Their main limitations are count rates well below 10⁶ photon s⁻¹ and the lack of spatial resolution. Even so, such detectors are still preferred in many applications where the data are effectively zero- or one-dimensional.

Reverse-biased semiconductor detectors are designed to have a thick depletion zone in which charge can be efficiently collected and conveyed to an amplifier. X-rays that stop in the depletion zone produce electron–hole pairs; these are separated by the depletion zone field and the electrons are swept to the input of a low-noise amplifier. Single-photon counting can be readily achieved, even for low-energy X-rays, especially if the detector is cooled to minimize thermally generated charge. These detectors are typically fabricated as silicon diodes, but germanium and gallium arsenide are also used (Hall, 1995). Until recently, these devices were generally configured as point detectors or strip detectors consisting of a linear array of narrow sensitive regions, forming a one-dimensional detector (Ludewigt et al., 1994). Two-dimensional arrays of square pixels are being developed, e.g. see the description of pixel array detectors below. In another device, the silicon drift detector, potentials are arranged in the silicon to funnel signals from a large area to a low-noise collection point (Rehak et al., 1986). Such devices are being developed for both linear and area applications.

By increasing the electric field strength in an appropriately designed p-n junction in silicon, avalanche multiplication of the X-ray-induced electrons can be obtained as they move toward the anode where they are collected. This gives rise to a very high linear signal gain, with high speed and low noise. Arrays of such avalanche photodiodes as large as elements, each square, have been fabricated (Gramsch et al., 1994; Farrell et al., 1994).

Gas discharge (wire) counters make use of the ionization produced when an X-ray is stopped in the high-atomic-number gas, usually xenon, that fills the detector. A strong electric field between a fine anode wire and a cathode plane accelerates the products of the primary ionization to produce an ionizing multiplication (either a proportional or an avalanche discharge, depending on the field strength) that is detected as a charge pulse on one or both of the electrodes. The discharge is quenched by the presence of a few per cent of a second gas, e.g. methane or carbon dioxide. Gas discharge detectors have been configured in zero-, one- and two-dimensional versions and continue to be widely used in some applications. The venerable Geiger counter is in this class and is used for radiation monitoring and beam alignment in home laboratories. Properly designed gas discharge counters have very low noise, but the quantum efficiency depends critically on design, gas and X-ray energy.

Linear wire detectors have been used to record small-angle X-ray scattering. The localization of the X-ray event along the length of the detector is often performed by measuring the difference in arrival time of the charge pulses at the two ends of one of the electrodes (Barbosa et al., 1989). The pulses are stretched to permit this measurement. One design uses a resistive anode wire to perform this function, whereas others configure the cathode plane as a delay line. Various two-dimensional arrangements of crossed planes of wires, broadly classified as multiwire proportional counters (MWPCs), have been widely used in crystallography, and some types have been commercially successful (Hamlin et al., 1981; Blum et al., 1987).

The design of MWPC area detectors has had difficulty keeping up with improvements in X-ray sources, particularly the high fluxes available at storage rings, and the shift toward use of higher-energy X-rays. The electric discharge at the heart of the technology has an inherent dead time associated with it. Added to this inherent dead time are the pulse propagation and processing times which limit the counting rate for a given wire. Thus, MWPCs are subject to a severe count-rate limitation. A second limitation of MWPCs has been their large pixel size and the relatively small number of pixels across the detector face, as well as parallax effects. These problems have been addressed by changes in the detector geometry (e.g. spherical drift chambers; Charpak, 1982), by microfabrication on glass substrates of the wires comprising the back plane of the detector, and by dividing the active area into small zones, each of which is read out independently. Robustness of MWPCs has also been a problem.

The dead time can be reduced by reducing the thickness of the detector. However, reducing the detector thickness reduces the X-ray stopping power. Increasing the gas pressure not only improves the quantum efficiency, but also helps to reduce the dead time further. Unfortunately, high gas pressure complicates the design of the front window of the detector. Despite these problems, two-dimensional gas-detector prototype modules with 200 µm square pixels have been constructed that are expected to have a local linear count-rate limit of 7 MHz mm⁻² and a quantum efficiency above 80% at energies used in crystallography (see Sarvestani et al., 1998).