International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F, ch. 7.1, pp. 144-145   | 1 | 2 |

Section 7.1.2. Evaluating and comparing detectors

S. M. Gruner,a* E. F. Eikenberryb and M. W. Tatea

aDepartment of Physics, 162 Clark Hall, Cornell University, Ithaca, NY 14853-2501, USA, and bSwiss Light Source, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
Correspondence e-mail:  smg26@cornell.edu

7.1.2. Evaluating and comparing detectors

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The DQE comprehensively characterizes the ultimate quantitative capabilities of an X-ray detector. The DQE may be determined from an analysis of the reproducibility of recorded X-ray test images of known statistics via equation (7.1.1.1[link]): given M incident X-rays per exposure, the expected incident signal-to-noise is [(M)^{1/2}]. The DQE is determined by measuring the variance in the recorded signal in repeated measurements of the test image. Repetition of this process for different values of M maps out the DQE curve. Since the DQE is dependent on the structure of the image, the integration area, the X-ray background and the long-term detector calibration, it is essential that the test images realistically simulate these features as expected in experiments. Thus, if the detector is to be used to obtain images of diffraction spots, the test images should consist of comparably sized spots superimposed on a suitable background.

A comprehensive DQE determination is nontrivial and requires specialized tools, such as test masks, uniform X-ray sources etc. Unfortunately, published DQE curves are frequently incorrect and misleading. Users can, however, set up and perform a simple DQE assessment, detailed below, which gives a great deal of information about the sensitivity and usefulness of a given area detector. Other sources of stable X-ray spots (of appropriate size and intensity) can also be used in similar tests.

The materials needed are sheet lead and aluminium, a sewing needle, a stable collimated X-ray source, X-ray capillaries filled with saturated salt solutions, an X-ray shutter with timing capability and a scintillator/phototube X-ray counting arrangement. Arrange a fluorescent X-ray source to provide a diffuse X-ray signal. An X-ray capillary filled with a saturated solution of iron chloride makes a suitable source for a copper anode machine. Next, make an X-ray-opaque metal mask by punching a clean pinhole with a sewing needle in a lead sheet. The size of the hole should be representative of an X-ray spot, say 0.3 mm in diameter. The mask should be firmly and reproducibly secured a few cm from the fluorescent source at a wide angle to the incident beam. Using a scintillator/phototube combination, measure the number of X-rays per second emerging through the hole at a given X-ray source loading. A sufficient number of X-rays per measurement (say 105) is necessary to obtain accurate statistics (0.3%). This measurement should be repeated to verify the stability of the source.

This spot can now be recorded by the detector in question, using different integration times to vary the dose. 20 measurements at each integration time should give a reliable measure of the standard deviation in the signal. It is vital to move the position of the spot on the detector face for each exposure, taking care to move only the detector without disturbing the remainder of the experimental setup. Only by moving the detector is the fidelity of the calibrations tested. One subtlety is that the sensitivity of many detectors varies with the angle of incidence of the X-rays, so that it will be necessary to vary both the position and angle of the detector between exposures.

By using a wide range of integration times, both the sensitivity of the detector at low doses and the ultimately achievable measurement accuracy can be examined. These data may also highlight specific problems a detector might have, such as nonlinearity.

The DQE can be measured for a spot in the presence of a background if the lead pinhole mask is now replaced with a pinhole in a semitransparent aluminium foil. Choose the foil thickness to yield an appropriate background level, say 20% of the pinhole intensity. The uncertainty in the measurement of the spot intensity now results from the total counts in the integration area in addition to the uncertainty in determining the background. A wide PSF is especially harmful in this case, since many more pixels must be integrated to encompass the spot.

These evaluation procedures test only limited aspects of the detector, but in doing so, much is learned not only about the detector, but also about the degree to which the vendor is willing to work with the user, which is clearly of interest. The ultimate test for a crystallographer is whether a detector delivers good data in a well understood experimental protocol. Usually, values of Rsym, the agreement of integrated intensities from symmetry-related reflections, are evaluated as a function of resolution. Low values of Rsym suggest good quality data. A much more stringent test can be made by comparing anomalous difference Patterson maps based on the Fe atom in myoglobin (Krause & Phillips, 1992[link]). The limitation in these crystallography-based evaluations is that they tend to rely on robust, strongly diffracting crystals, which allow accumulation of good X-ray statistics even with insensitive detectors. Weakly diffracting and radiation-sensitive crystals are less forgiving.

References

Krause, K. L. & Phillips, G. N. Jr (1992). Experience with commercial area detectors: a `buyer's' perspective. J. Appl. Cryst. 25, 146–154.








































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