Section 7.1.1. Commonly used detectors: general considerations

This chapter summarizes detector characteristics and provides practical advice on the selection of crystallographic detectors. Important types of detectors for crystallographic applications are summarized in Section 7.1.2 and listed in Table 7.1.1.1. To be detected by any device, an X-ray must be absorbed within a detective medium through electrodynamic interactions with the atoms in the detecting layer. These interactions usually result in an energetic electron being liberated which, by secondary and tertiary interactions, produces the signal that will be measured, e.g. luminescence in phosphors, electron–hole pairs in semiconductors or ionized atoms in gaseous ionization detectors. As we shall see, there are many schemes for recording these signals. The various detector designs, as well as the fundamental detection processes, have particular advantages and weaknesses. In practice, detector suitability is constrained by the experimental situation (e.g. home laboratory X-ray generator versus synchrotron-radiation source; fine-slicing versus large-angle oscillations), by the sample (e.g. whether radiation damages it readily) and by availability. An assessment of detector suitability in a given situation requires an understanding of how detectors are evaluated and characterized. Some of the more important criteria are discussed below.

The detective quantum efficiency (DQE) is an overall measure of the efficiency and noise performance of a detector (Gruner et al., 1978). The DQE is defined as where S is the signal, N is the noise, and the subscripts o and i refer to the output and input of the detector, respectively. The DQE measures the degradation owing to detection in the signal-to-noise ratio. For a signal source that obeys Poisson statistics, the inherent noise is equal to the square root of the number of incident photons, so that the incident signal-to-noise ratio is just . The ideal detector introduces no additional noise in the detection process, thereby preserving the incident signal-to-noise ratio, i.e. . Real detectors always have because some noise is always added in the detection process. The DQE automatically accounts for the fact that the input and output signals may be of a different nature (e.g. X-rays in, stored electrons out), since it is a ratio of dimensionless numbers.

A single number does not characterize the DQE of a system. Rather, the DQE is a function of the integrated dose, the X-ray spot size, the length of exposure, the rate of signal accumulation, the X-ray energy etc. Noise in the detector system will limit the DQE at low dose, while the inability to remove all systematic nonuniformities will limit the high-dose behaviour.

The accuracy, ρ, measures the output noise relative to the signal, i.e. . For a Poisson X-ray source, it follows that the accuracy and the DQE are related by This allows the determination of the number of X-rays needed to measure a signal to a given accuracy with a detector of a given DQE. The accuracy for an ideal detector is , e.g. 100 X-rays are required to measure to 10% accuracy, and 10⁴ X-rays are needed for a 1% accuracy. Nonideal detectors always require more X-rays than the ideal to measure to a given accuracy.

Spatial resolution refers to the ability of a detector to measure adjacent signals independently. The spatial resolution is characterized by the point spread function (PSF), which, for most detectors, is simply the spread of intensity in the output image as a result of an incident point signal. An alternative measure of resolution is the line spread function (Fujita et al., 1992). Although a detector might have a narrow PSF at 50% of the peak level, poor performance of the PSF at the 1% level and below can severely hamper the ability to measure closely spaced spots. It is important to realize that the PSF is a two-dimensional function, which is often illustrated by a graph of the PSF cross section; therefore, the integrated intensity at a radius R pixels from the centre of the PSF is the value of the PSF cross section times the number of pixels at that radius. Often the wings of the PSF decay slowly, so that considerable integrated signal is in the image far from the spot centre. In this case, a bright spot can easily overwhelm a nearby weak spot. Another consequence is that bright spots appear considerably larger than dim ones, thereby complicating analysis.

The stopping power is the fraction of the incident X-rays that are stopped in the active detector recording medium. In low-noise detectors, the DQE is proportional to the stopping power. A detector with low stopping power may be suitable for experiments in which there is a strong X-ray signal from a specimen that is not readily damaged by radiation. On the other hand, even a noiseless detector with a low stopping power will have a low DQE, because most of the incident X-rays are not recorded.

Unfortunately, many definitions of dynamic range are used for detectors. For an integrating detector, the dynamic range per pixel is taken to be the ratio of the saturation signal per pixel to the zero-dose noise per pixel for a single frame readout. For photon counters, the dynamic range per pixel refers to the largest signal-to-noise ratio, i.e. the number of true counts per pixel that are accumulated on average before a false count is registered. In practice, the dynamic range is frequently limited by the readout apparatus or the reproducibility of the detector medium. For example, the large dynamic range of storage phosphors is almost always limited by the capabilities of the reading apparatus, which constrains the saturation signal and limits the zero-dose noise by the inability to erase the phosphor completely. The number of bits in the output word does not indicate the dynamic range, since the number of stored bits can only constrain the dynamic range, but, obviously, cannot increase it.

The dynamic range is sometimes given with respect to an integrated signal that spans more than one pixel. For a signal S per pixel which spans M pixels, the integrated signal is MS, and, assuming the noise adds in quadrature, the noise is , yielding a factor of larger dynamic range. For most detectors, the noise in nearby pixels does not add in quadrature, so this is an upper limit.

The characteristics of a detector may be severely compromised by practical considerations of nonlinearity, reproducibility and calibration. For example, the optical density of X-ray film varies nonlinearly with the incident dose. Although it is possible to calibrate the optical density versus dose response, in practice it is difficult to reproduce exactly the film-developing conditions required to utilize the highly nonlinear portions of the response. A detector is no better than its practical calibration. This is especially true for area detectors in which the sensitivity varies across the face of the detector. The proper calibration of an area detector is replete with subtleties and constrained by the long-term stability of the calibration. Faulty calibrations are responsible for much of the difference between the possible and actual performance of detectors (Barna et al., 1999).

The response of a detector may be nonlinear with respect to position, dose, intensity and X-ray energy. Nonuniformity of response across the active area is compensated by the flat-field correction. Frequently, nonuniformity of response varies with the angle of incidence of the X-ray beam to the detector surface, which is a significant consideration when flat detectors are used to collect wide-angle data. Although this may be compensated by an energy-dependent obliquity correction, few detector vendors provide this calibration. An X-ray image may also be spatially distorted; this geometric distortion can be calibrated if it is stable.

Other important detector considerations include the format of the detector (e.g. the number of pixels across the height and width of the detector). The format and the PSF together determine the number of Bragg orders that can be resolved across the active area of the detector. Robustness of the detector is also important: as examples, gas-filled area detectors may be sensitive to vibration of the high-voltage wires; detectors containing image intensifiers are sensitive to magnetic fields; or the detector may simply be easily damaged or lose its calibration during routine handling. Some detectors are readily damaged by too large an X-ray signal. Count-rate considerations severely limit the use of many photon counters, especially at synchrotron-radiation sources. Detector speed, both during exposure and during read out, can be important. Some detector designs are highly flexible, permitting special readout modes, such as a selected region of interest for use during alignment, or operation as a streak camera.

Ease of use is especially important. A detector may simply be hard to use because, for example, it is exceptionally delicate, requires frequent fills of liquid nitrogen, or is physically awkward in size. A final, often compelling, consideration is whether a detector is well integrated into an application with the appropriate analysis software and whether the control software is well interfaced to the other X-ray hardware.