Berg-Barrett method

Orientation and quality of single crystals Berg-Barrett Method... 

One much-used method of X-ray diffraction topography is the Berg-Barrett method, where the crystal is set to reflect X-rays at the Bragg angle for some plane. Geometric resolutions of about 1 pm can be achieved, and single dislocations can be resolved. The eontrast on the film is due to variations in the reflecting power due to imperfeetions in the crystal. 

Fig. 8-32 Berg-Barrett (B-B) method, (a) Plan view. The source is the focal spot on the x-ray tube target, seen end on from the specimen, so that the source appears square, (b) Perspective view.

X-Ray Method. For small crystals totally irradiated in the beam the existence and relative proportion of domains can be assessed depending on sensitivity of the instrumental resolution and the angular differences between adjacent domains. This varies from a few minutes of arc by normal singlecrystal techniques ( ) to a few seconds by improved versions of the Berg-Barrett technique [( ), also standard texts on X-ray diffraction techniques]. By suitable choice of X-ray wavelength it is also possible to distinguish between 180° domains by utilizing the breakdown of Friedel s law... 

There are three experimental methods (Figure 8). The back-reflection Berg-Barrett camera is useful in examining imperfections near the crystal surface at depths of 10-100 um. A well-collimated monochromatic X-ray beam incident on the crystal is diffracted by planes [hkl) and recorded on film. Because of extinction effects, the diffracted intensity from imperfect regions exceeds that of the perfect regions. Dislocations appear as bands of contrasting darkness, 5-50 pro. wide. Film and crystal are moved in synchronization to map out... 

Fig. 8. Three X-ray topographic methods (a) back-reflection Berg-Barrett, (b) transmission Berg-Barrett, (c) Borrmann anomalous transmission. In the Laue case X-ray interferometer (d) two coherent beams are produced by the beam splitter the transmission mirror recombines the beams to form one interference pattern at the analyzer.

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