Sources and the Production of X Rays

We can see the diffraction pattern with our own eyes when we collect X-ray data because we obtain the image, the pattern of diffraction spots, on the face of our detector or film. We can t directly see the families of planes in the actual crystal, but we know, through the Ewald construction, how the diffraction pattern is related to the crystal orientation, and hence to the dispositions of the planes that pass through it. We also know from Ewald how to move the crystal about its center, once we know its orientation with respect to our laboratory coordinate system, in order to illuminate various parts of reciprocal space. In data collection we watch the diffraction pattern, not the crystal, and let the pattern of intensities guide us. 

The crucial aspects of data collection are (1) producing a finely collimated beam of monochromatic X rays, (2) getting the crystal out of the mother liquor from which it was grown and centered in the X-ray beam, (3) reorienting it systematically in the X-ray beam in a continuous fashion, and (4) recording the intensities of the emitted X-ray reflections. After that, assuming you have done these things well, you can retire to your computer. 

Once the data have been recorded, there are a host of statistical measures that provide criteria for evaluating the quality of the data. These serve as important guides to possible weaknesses in the data set, give estimates of the useful resolution of the data, and yield quantitative measures of precision. These are normally provided as a matter of course by the data-processing programs that correct, scale, and merge the measured intensities into a comprehensive set. 

FIGURE 7.1 Schematic diagram of a conventional, broad focus, sealed X-ray tube. The anode material, usually of a pure element, determines the characteristic X-ray spectrum that is produced. 

Currently there are three commonly used X-ray sources the first two, sealed X-ray tubes and rotating anode sources, are found in most protein crystallography laboratories. The third source of X rays is synchrotrons, which are available only at specialized facilities, generally national laboratories. X rays produced by synchrotrons, which have a number of unique and highly desirable features, are generated by a completely different principle than that described above for conventional sealed tubes and rotating anode sources. 

---