Bragg signals

Polymers invariably form helical structures, and the helix symmetry is denoted by u, indicating that there are u repeat units in V turns of the helix. The helix pitch is denoted by P and the molecular repeat distance is c = vP. X-ray diffraction patterns from non-crystalline specimens contain diffracted intensities restricted to layer lines that are spaced by 1/c. On a diffraction pattern from a polycrystalline specimen, diffraction signals, or Bragg reflections, occur only at discrete positions on the layer lines, the positions being related to the lateral dimensions of the unit cell of the crystal. The meridian (vertical axis) of the diffraction pattern is devoid of diffracted intensity unless the layer line number J, is a multiple of u, so that u can be determined straightforwardly. The diffracted intensities can be calculated using standard expressions (2), for model structures (i.e. given the atomic coordinates). 

Figure 18.3 Schematic diagram of tin ion chamber that drifts the ionization perpendicular to the particle s path is shown. In this case the anode is segmented and the relative rate of ionization along the path can be determined. The device also contains a Frisch grid between the anode and chamber to improve the pulse-shape response of the device, (c) The schematic version of a detector that drifts the ionization along the particle s path, called a Bragg counter, is shown. The time distribution of the output signal will contain information on the relative rate of ionization all along the particle s path.

The Bragg-Brentano type of diffractometer is composed of an x-ray tube with a metallic anode that supplies x-rays that are scattered from the sample and focused at the slit before hitting the detector. In some cases, a monochromator capable of yielding a monochromatic x-ray beam is added. The sample is rotated, relative to the x-ray at angles from 0° to 90° with the help of a goniometer, where the powdered sample is placed on the sample holder. Electronic equipment is used to amplify and filter signal pulses from the detector. 

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