Area detector types

FIG. 33 X-Ray Diffraction Patterns of Ammonium Dodecane 1-Sulfonate. 2-D (a) and 3-D plots (b) of oriented samples. Both pictures show the presence of a nonordered smectic phase, since the diffuse, weak, wide-angle diffraction indicates only an average distance between the molecules and the sharp, intense small angle reflections a very well defined layer distance. The reflections are perpendicular to each other, so the structure should correspond to an orthogonal smectic A type. The pictures were obtained using an x-1000 area detector from Siemens. 

X-ray detectors also come in several varieties (1) single-photon counters which yield accurate results but require up to several weeks to acquire the 10,000 -100,000 (lO lO ) reflections necessary to compile a complete data set for a protein crystal (2) image plates that operate much like photographic film but are 10 times more sensitive (3) area detectors, electronic devices that detect X-ray photons on a two-dimensional surface. Both fluorescent-type detectors, image plates and fast area detectors, are more sensitive at the shorter wavelengths of X-ray radiation from synchrotron sources. 

Area detectors combine the accuracy and wide dynamic range of scintillation counting the simultaneous measurement of many reflections, as with image plates and the advantage of direct collection data by computer, without a separate scanning step. One type of area detector is the multiwire or gas-proportional detector. As an example, the Mark I detector at the University of California, San Diego, as diagrammed in Fig. 4.19, consists of two perpendicular sets of parallel wires in a flat box filled with an inert gas. A window of beryllium permits entry of X rays from the front of the detector. 

The collimated, monochromatic neutron beam is delivered to the sample on a diffractometer, and diffraction is detected by an area detector (Chapter 4, Section III.C). The most common type is a multiwire area detector that uses helium-3 as the active gas, according to this reaction ... 

The area detector - is an electronic device for measuring many diffracted intensities at one time. It is an electronic substitute for film, and is now used, where possible, for crystals of biological macromolecules. It is a position-sensitive detector, and is coupled to an electronic device for recording the data in computer-readable form. The data so recorded include the intensity of a Bragg reflection (diffracted beam) and its precise direction (as a location on the detector). Both types of information are needed for each Bragg reflection so that I(hkl), and sinO/X can be determined. 

Three types of area detectors are presently used in macromolecular crystallographic research (1) multiwire proportional counters, (2) television area detectors, and (3) imaging plates. A multiwire proportional counter consists of an anode between two cathodes these consist of arrays of parallel wires and are arranged perpendicular to each other. The chamber is filled with a gas, consisting partly of xenon which is ionized by the incident X rays, an effect recorded by the detecting device. Television area detectors contain a fluorescent phosphor that produces visible light when hit by an X-ray beam. After intensification, the photons are detected by a television photocathode. 

X-Ray detectors may be classified as point, linear or area, depending on whether they record the diffraction pattern in zero, one or two spatial dimen -sions. Point detectors must be scanned to measure the diffraction pattern, whereas linear or area detectors can be fixed. Point detectors are easily compatible with post-sample optical elements. Linear and area detectors allow the data to be acquired much faster, but as more open systems they are prone to detecting parasitic scatter from the air or sample environment. Both linear and area detectors are types of position sensitive detector (PSD). 

Detector Type Pixel area/pm2 Number of pixels (area/mm2 j Max. quantum (10% limits) Full well capacity/e Read-out noise (e rms)... 

San Diego, MWPC refers both to the regional/national area detector data collection facility there (for a description of the various systems and the developments over the years see Xuong et al (1978, 1985) and Hamlin et al (1981)) and to the identical type of system supplied commercially (from about 1986) by San Diego Multi-wire Systems . 

A disadvantage of the back end monochromator is its inability to separate Ka 2 from Kai. For very precise unit cell determinations this separation is desirable. For this purpose a front end or primary beam monochromator is used (Figure 12c). This monochromator is the only type that can be used with area detectors. Their disadvantages include a high intensity loss, the need for precise alignment and nonremoval of fluorescent radiation. 

Detector type Number of LST planes Gamma-ray converters Number of lead converters Geometric area Pick-up... 

Area smoke detectors are generally installed in buildings and accommodation areas. Where this is not practical—say in the galley area—other types of fire detector should be used. Actuation of a single smoke detector will initiate a fire alarm. If additional detectors sound an alarm, the equipment in the area of the fire and HVAC systems will be shut down. 

Three types of detector are available for measuring the X rays diffracted by a crystal Point detectors, linear and area detectors. The film method involves a two-dimensional detector, which can provide simultaneous information about every point in the region of reciprocal space investigated. The measurements are not immediately available in digital form, but powerful computer-controlled photometers can digitize them. 

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