Multiwire proportional counters

Area detector stationary monochromatic multiwire proportional counter stationary... 

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. 

Hamlin, R. Multiwire proportional counters for use in area X-ray diffractometers. Trans. Amer. Cryst. Assn. 18, 95-123 (1982). 

The area detector is an electronic device for measuring many diffracted intensities at one time. It is a two-dimensional, position-sensitive detector that records the intensity of a Bragg reflection (diffracted beam) and its precise direction (as a location on the detector) it acts like an electronic substitute for film. This detection device is now used extensively for crystals of biological macromolecules. Such a detector may involve a multiwire proportional counter coupled to an electronic device or a television imaging system both devices permit a recording of the data in a computer-readable form. Alternatively, imaging plates may be used. These have phosphorescent material layered on them and store information on the extent of X-ray exposure until scanned bv a laser, when the intensity and location of the light then emitted is recorded. 

The multiwire proportional counter filled with Ar/CO (7/3) is still active. The spatial resolution increased slightly to about 4 mm (2 mm at 7, = 1.5 A). 

If the scattering patterns are anisotropic due to a preferred orientation of molecules and crystals, it is often desirable to use two-dimensional position sensitive counters. Such area detectors, based on the principles described above, are the multiwire proportional counters (MWPC). A number of parallel tightened anode wires are mounted in front of a cathode structure, the latter consisting of two layers of parallel metal strips which are oriented horizontally in one layer and vertically in the other. One possibility of reading out the strips is to connect the strips of each... 

Various multiwire proportional counters (MWPC) have been designed for use as two-dimensional position-sensitive detectors. An example of such a design is illustrated in Figure 2.17. Here a plane of fine, equally spaced parallel wires functions... 

Figure 2.17 Schematic of a two-dimensional position-sensitive multiwire proportional counter (MWPC). (The spacing between electrode planes is not to scale.)...

The feasibility of this kind of modulation imaging has been confirmed by a balloon experiment (Lu Z.G. et al, 1994). The hard X-ray telescope HAPI-4, constructed in collaboration of IHEP/Beijing, MEPI/Moscow and AIT/Tuebingen, consists of multiwire proportional counter and phoswish scintillators with a sensitive area 1600 cm and slat coUimator of 3° X 3°... 

The Digital Autoradiograph (DAR) LB 287, a two-dimensional, position-sensitive, multiwire proportional counter detector, was discontinued by Berthold, but the instrument is still being used in many analytical laboratories, and papers are published regularly citing its use. The DAR is described in Chapter 13 of the previous edition of this book. 

Comparisons of the various TLRC methods are summarized in Tables 13.1 (Shul-man, 1983 Shulman and Weaner, 1991) and 13.2 (Clark and Klein, 1996). Linear analyzers are still widely used in many laboratories because they offer reasonable speed, resolution, sensitivity and quantitative accuracy that is adequate for many applications and are less expensive than the newer detection instruments. When the highest level of sensitivity, quantification, and resolution are required, use of a multiwire proportional counter or phosphor imager is in order despite their high price. The field appears to be moving rather quickly toward much wider use of phosphor imagers, and it is likely this trend will continue at the expense of proportional counters. 

The one-dimensional PSPC is well suited to the study of powder-type systems (though wasteful of photons) where the diffraction pattern consists of rings. For single-crystal materials with spot patterns, a two-dimensional area detector is required. In these cases, a multiwire proportional counter... 

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