Sample thickness and uniformity

The second important factor is the absorption of x-rays by the sample. In the Bragg-Brentano geometry the sample should be completely opaque to x-rays. Assuming that the absorption of 99.9 % of the incident beam intensity represents complete opacity, then the beam intensity should be reduced by a factor of 1000 and the following equation can be written (also see Eq. 2.8 in Chapter 2)  

After solving Eq. 3.2 with respect to t, the minimum sample thickness (in mm) can be estimated from 

Conversely, transmission geometry requires that the sample is minimally absorbing. This is usually not a problem if the studied specimen is a molecular substance. However, when the material is a dense alloy or intermetallic compound containing heavy elements, the preparation of a high quality specimen for transmission powder diffraction may be problematic. With flat transmission samples the best approach is to try to arrange no more than a single layer of particles mounted on the film. When cylindrical specimens are employed, the radius of the capillary should be reduced to a practical minimum. Unfortunately, these measures usually reduce the 

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