Recording of Diffraction Intensities

Generating X rays and directing them on to the macromolecular crystal is only half of the data collection process. Equally important is recording the diffraction intensities produced by the interaction of the X rays with the atoms distributed about the Bragg planes in the 

The multiwire devices, though workhorses for about 10 years, were an intermediate in the development of the detectors we use today. They provided excellent data, but even faster, more reliable, more accurate devices became available in the late 1980s. The instruments that we use today fall into two categories those based on what are called image plates, and those based on charge coupled devices (CCD) detectors. While CCD detectors are now exclusively used at synchrotrons because of their almost instantaneous readout of data, necessary with high intensity sources with short exposure times, most university and private laboratories continue to use detectors based on image plates. 

FIGURE 7.4 A multi-image plate X-ray detector. The system contains two image plates on arotating carrousel so that one can be recording data while the second is read and translated into intensities. (Courtesy of Rigaku America Corp.) 

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Fig. 9.9. Schematic of the experimental configuration used to obtain the diffraction patterns with parallel recording of diffraction intensities as a function of thickness.

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... 

It is possible from measurements of a limited number of reciprocal-lattice vectors and observations of systematic absences of reflections to deduce not only the shape and dimensions of the unit cell, but also the complete symmetry of the crystal structure. The determination of the complete structure now requires the contents of the unit cell to be deduced from the intensities of the reflections . These are usually determined by using diffractometers rather than film to record the diffraction. A diffractometer is usually a device that allows the recording of the intensity of scattering in any particular direction in space. Modern types, using CCD arrays, can determine the intensity over a range of directions for one setting of the instrument. This greatly speeds up the collection of data but leads to some complication in terms of the need to calibrate the different... 

Crystalline materials can be identified by rapid computerized powder diffraction techniques. The principle of this technique [6,7] is that the crystallites within a sample, placed in a collimated x-ray beam, reflect x-rays at specific angles and intensities. The diffraction pattern can be recorded photographically, using a camera, e.g. a Debye-Scherrer camera, or using a powder diffractometer. Chemical analysis depends on the fact that each chemical composition and crystallographic structure produces a unique angular distribution of diffracted intensity. Analysis is based on comparison of the diffractometer scan with known standards. Typical applications of the powder diffraction technique to polymers would be the identification of mineral fillers in engineering resins, the nature of crystalline contaminants and determination of crystalline phases in a material. 

Because of the difficulty of obtaining satisfactory photometer records of electron diffraction photographs of gas molecules, we have adapted and extended the visual method to the calculation of radial distribution curves, by making use of the values of (4t sin d/2)/X obtained by the measurement of ring diameters (as in the usual visual method) in conjunction with visually estimated intensities of the rings, as described below. Various tests of the method indicate that the important interatomic distances can be determined in this way to within 1 or 2% (probable error). 

When a diffracted X-ray beam hits a data collection device, only the intensity of the reflection is recorded. The other vital piece of information is the phase of the reflected X-ray beam. It is the combination of the intensity and the phase of the reflections that is needed to unravel the contributions made to the diffraction by the electrons in different parts of the molecule in the crystal. This so-called phase problem has been a challenge for theoretical crystallographers for many decades. For practical crystallography, there are four main methods for phasing the data generated from a particular crystal. 

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