Tables for
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 2.1, pp. 45-46

Section Detector parameters

A. Kerna*

aBruker AXS, Östliche Rheinbrückenstrasse 49, Karlsruhe 76187, Germany
Correspondence e-mail: Detector parameters

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There are many ways to characterize the properties and performance of an X-ray detector.

Ideally, in a given detector operated under appropriate conditions, (1) each photon will produce a detectable signal and (2) the signal recorded is proportional to the number of photons detected. If both conditions are fulfilled then the detector has unit quantum efficiency. The detective quantum efficiency (DQE) may be defined as the squared ratio of the output signal-to-noise ratio to the input signal-to-noise ratio, expressed as a percentage. A detector's DQE is generally less than 100% because there is always detector noise and not every photon is detected. The DQE thus depends on the characteristics of the detector (e.g. transmission of the detector window, count rates and dead time, etc.) and varies with the X-ray energy for the same detector.

The detector linearity determines the accuracy of intensity measurements and depends on the ratio between the photon count rate and the rate of signals generated and registered by the detector. In any detector it takes some time to process the conversion of an individual photon to a voltage pulse, which is related to the detector dead time: photons arriving while the detector is still processing the previous photon conversion may be lost. The detector dead time is related to the physical characteristics of the detector, e.g. the drift time in a gas-ionization detector, or the read-out time of the counting electronics, e.g. the shaping time of the amplifier. The effect of dead time becomes a substantial issue at high photon count rates, when the dead time becomes a significant part of the average time separation between two arriving photons, leading to increasing intensity losses at higher count rates. Detectors can be categorized as being non-paralysable or paralysable with respect to dead time. A non-paralysable detector is dead for a fixed time after each count, but not influenced by photons arriving during the dead time. Counting losses increase with increasing count rates, but the true count rate of a nonparalysable detector can be corrected unless the maximum observed count rate is equal to the inverse of the dead time. In a paralysable detector, a second photon arriving within the dead time can not be counted but will extend the dead time up to a point where the detector will be incapable of collecting any counts at all (saturation point). Modern detectors can stand the count rates obtained in powder diffraction experiments using fixed-target X-ray sources. At very high count rates, e.g. those obtained in thin-film experiments such as reflectometry, it may be necessary to attenuate the beam. Sophisticated instruments are equipped with an electronic feedback system and automatic absorbers (see Section[link]) to ensure that detector saturation is avoided.

The dynamic range of a detector may be defined as the range between the smallest detectable photon count rates (determined by inherent detector noise such as readout and dark noise) to the largest acceptable photon count rates (determined by the dead time).

Energy resolution is the ability of a detector to resolve two photons that have different energies. Energy resolution is typically characterized by the size of the detector energy window, ΔE, in electron volts, as determined by the full width at half maximum (FWHM) of the detector-efficiency curve as a function of energy, with the detector and counting electronics set to a specific wavelength. Another frequently used expression for energy resolution is the ratio of the detector energy window size to the energy of the monochromatic X-ray beam, E, expressed as ΔE/E.

The proportionality of the detector determines how the size of the generated voltage pulse is related to the energy of the absorbed X-ray photons, and electronic methods (pulse-height selection) can be used to discriminate between different energies. An accurate proportionality thus allows the use of energy discrimination as a form of monochromatization, where the energy is filtered by the detector rather than by an optical element such as a metal filter, crystal or mirror; see Section[link]. Signals corresponding to photons with too high or too low energies are discarded.

The size and weight of detectors may impose several practical constraints, see also Section[link]. For large detectors the accessible angular range may be limited owing to collision issues. For heavy detectors a horizontal goniometer may be preferred over a vertical goniometer (unless horizontal specimen positioning is imperative) in order to minimize the goniometer load.

X-ray detectors may be broadly classified as counting detectors or integrating detectors. Counting (digital) detectors are able to detect and count individual photons. The number of pulses counted per unit time is proportional to the incident X-ray flux. Integrating (or analogue) detectors accumulate photon-induced signals for a given period of time, prior to the integrated signal being read out and converted into an (analogue) electrical signal. The signal size is proportional to the flux density of the incident X-rays.

Counting and integrating detectors each have their clear advantages and disadvantages. Counting detectors normally have a greater dynamic range than integrating detectors, while integrating detectors normally have better spatial resolution (Section[link]). Energy resolution is only possible for counting detectors. Readout and dark noise are usually higher for integrating detectors. Integrating detectors are not limited by the photon count rate as there is no dead time; nevertheless, the measurement time has to be kept sufficiently small to avoid saturation.

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