Multiwire detectors

More recent versions of this design feature an added focusing system, called a radial drift chamber, in front of the detector. Gas ionization occurs within this drift chamber, which is composed of a set of concentric charged rings sandwiched between two lens-shaped metal grids. The cluster of electrons is then focused by the drift chamber, making its direction normal to the surface of the multiwire detector. 

A second type of image-plate detector employs gadolinium oxide, which absorbs a neutron and emits a gamma ray, which in turn exposes the image plate. Image plates have higher spatial resolution but lower efficiency than multiwire detectors. 

As already mentioned, area (or position sensitive) detectors were first developed for neutron crystallography, since neutrons are scarce and expensive and it is important both to shorten the experiment and to use aU neutrons scattered by a sample in various directions, rather than waste all but one reflection at a time. Besides 2D scintillation detectors, there are banana ID multiwire detector and 2D detectors with two mutually perpendicular sets of parallel wires. The latter design is usefiil for time-of-ffight experiments because of ideal time resolution once an ioiuzation discharge induces current in one or more wires, we instantly know both the place and time of its arrival. 

Area Detectors. A two-dimensional or area detector attached to a powder diffractometer can gready decrease data collection time. Many diffraction appHcations require so much time with a conventional detector that they are only feasible if an area detector is attached to the iastmment. The Siemens General Area Detector Diffraction System (GADDS) uses a multiwire area detector (Fig. 17). This detector measures an x- and ajy-position for each x-ray photon detected. The appHcations foUow. 

G. Charpak (ficole Superieure de Physique et Chemie, Paris, and CERN Geneva) invention and development of particle detectors, in particular the multiwire proportional chamber. 

As area detectors (other than multiwire systems) are not energy discriminating devices, apotential source of error lies in the contamination of the data with harmonics of the assumed wavelength of the primary beam. The importance of this effect has been estimated for molybdenum Ka radiation using a graphite monochromator [1],... 

Cork, C., Hamlin, R., Vernon, W. and Xuong, N. H. (1975). Xenon-fhled multiwire area detector for X-ray diffraction. Acta Crystallogr. A 31, 702-703. 

Howard, A. J., Nielsen, C. and Xuong, N. H. (1985). Software for a diffractometer with multiwire area detector. Method Enzymol. 114,452-472. 

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. 

Figure 4.1 9 Exploded view of detector wires in multiwire area 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 basic principle of operation of gas filled detectors, be it multiwire proportional chambers or linear devices, with a single anode wire, is the mechanism of gas amplification. This kind of detectors have — as its name implies, a filling of an... 

Photon counting devices are required in order to arrive at the necessary accuracy. The noise of the detector has to be lower than 4 orders of magnitude with respect to the signal. Our experience with multiwire proportional chambers may let us believe that this rather stringent condition can be fulfilled. The simultaneous measurement of the whole scattering pattern by position sensitive area detectors is absolutely mandatory... 

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. 

Blum, M., Metcalf, P., Harrison, S. C., and Wiley, D. C. A system for collection and on-line integration of X-ray diffraction data from a multiwire area detector. J. Appl. Cryst. 20, 235-242 (1987). 

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