Bragg-Brentano focusing

The so-called Bragg-Brentano focusing geometry is the most commonly used technique in modern powder diffractometry. It is briefly discussed later in this chapter and in greater depth in the next chapter. 

For the most commonly used Bragg-Brentano focusing geometry (see Chapter 3), the overall correction is generally a sum of six factors ... 

Here the surface represents the probability to find the reciprocal lattice point (100) in the diffractometer coordinate system assuming Bragg-Brentano focusing geometry. The Z-axis is perpendicular to the sample, and X- and 7-axes are located in the plane of the sample. 

Figure 3.8. The schematic of the Bragg-Brentano focusing geometry using a flat sample when the self-focused diffracted beam is registered by the detector after reflection from the sample. F - focus of the x-ray source, DS - divergence slit, RS - receiving slit, D - detector, 0 -Bragg angle.

Fig. 14. Focusing schemes in powder diffraction (a) conventional para-focusing Bragg-Brentano diffractometer (b) parallel-beam diffractometer using a...

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. 

Both flat and cylindrical transmission samples are commonly used in combination with position sensitive or image plate detectors. The major disadvantage of the transmission geometry arises from the fact that self-focusing of the diffracted beam is not as precise as in the Bragg-Brentano... 

The Bragg-Brentano two-circle diffractometer has a Seemann - Bohlin focusing circle, on which the X-ray tube anode, the counter tube window, and the specimen are located (Fig. 27). Only the specimen is focusing. 

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