Electronic area detectors

If the detection system is an electronic, area detector, the crystal may be mounted with a convenient crystal direction parallel to an axis about which it may be rotated under tlie control of a computer that also records the diffracted intensities. Because tlie orientation of the crystal is known at the time an x-ray photon or neutron is detected at a particular point on the detector, the indices of the crystal planes causing the diffraction are uniquely detemiined. If... 

The three-dimensional structure of protein molecules can be experimentally determined by two different methods, x-ray crystallography and NMR. The interaction of x-rays with electrons in molecules arranged in a crystal is used to obtain an electron-density map of the molecule, which can be interpreted in terms of an atomic model. Recent technical advances, such as powerful computers including graphics work stations, electronic area detectors, and... 

There are three methods of collecting high-resolution X-ray diffraction data diffractometry, photographically, and by electronic area detector. Each method has advantages and disadvantages for a particular crystalline protein, but for very accurate data acquisition beyond 2 A... 

Although in principle the time structure of the SR beam may be exploited in time resolved studies the major limiting factor is the rate at which three dimensional data may be accumulated. In this respect time resolved methods are bound to develop in tandem with the development of high count rate/fast refresh rate electronic area detectors. This applies to both monochromatic and white beam methods. For the latter the use of an integrating detector such as a CCD or image plate are the main expected improvements over film. 

The IP system seems apparently to be poorly suited to time-resolved measurements when compared with electronic area detectors such as MWPCs which permit a time-... 

Preliminary X-ray photographs (polaroid film or electronic area detector). 

Time resolved measurements, perhaps the most important application of X-ray synchrotron radiation, are only meaningful with electronic area detectors. 

Area detectors record diffraction pattern in two dimensions simultaneously. Not counting the photographic film, two t)q)es of electronic area detectors have been advanced to a commercial status, and are becoming more frequently used in modem x-ray powder diffraction analysis. 

Figure 18 A schematic representation of data collection with an electronic area detector. The reciprocal lattice plane / = 0 is shown as black dots. The direct beam and four scattered beams with their respective indices are shown three of them produce diffraction spots on the detector, while the fourth (-1,4,0) falls outside the detector area.

For macromolecular crystallography the detectors that are used can be described as zero-, one- or two-dimensional. The large number of simultaneously diffracting reflections, even for a monochromatic beam and stationary macromolecular crystal, means that a two-dimensional detector is more efficient. Electronic area detectors are now commonly... 

Electronic area detectors tend to suffer from spatial distortions. The effects can be calibrated for by placing a regular array of pinholes over the detector and illuminating it with X-ray photons from a radioactive source. The detector produces its own array of measured spot positions which deviate from the positions of the holes in the actual array. Hence,... 

Workstations for protein crystallography have been set up by the Departments of Chemistry and Biology at the Brookhaven National Laboratory. The optical system design for the Chemistry beam line is based on a spherical collimating mirror, followed by a two-crystal ( +, —) monochromator and finally by a bent cylindrical mirror (Hastings et al 1983). The instrument combines a liquid helium cryostat Huber diffractometer, for work on valence electron density studies of small molecules, with an oscillation camera for protein crystallography. The diffractometer may be upgraded to include a multichannel electronic area detector. 

The Biology Department beam line includes a station for protein crystallography (with Supper oscillation camera and FAST TV diffractometer) and a station for small angle diffraction (with a three-circle goniostat and MWPC electronic area detector). The latter station may be available for optimised anomalous dispersion crystallographic studies. The optical design for each consists of a bent pre-mirror, double crystal monochromator and bent post-mirror the mirrors have rhodium coatings (Wise and Schoenborn 1982). 

Quantitative X-ray crystal structure analysis usually involved methods (b), (c) and (e) although (d) has certainly been used. Photographic film is being replaced by use of electronic area detectors or, even more recently, the IP. 

These experiments confirmed reduced exposure times and reductions in the relative amount of radiation damage over that on a conventional source (Fourme 1978, 1979 Kahn et al 1982a). In the Soviet Union on the VEPP-3 ring some preliminary data were collected by Mokulskaya (1981) using an electronic area detector in an attempt to optimise the anomalous dispersion of a platinum derivative of pea lectin crystals. 

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