Limitations of single-axis diffraction

As we have seen in Chapter 1, we need something near a plane wave in order to see the finest details of the specimen stracture. A single-axis diffractometer utilises a beam that is very far from a plane wave. Thus, single-crystal rocking curves are broadened due to the beam divergence, and the spectral width of the characteristic X-ray lines. 

for a typical case of h=QA mm, 5=1 mm, a=500 mm. 500 arc seconds, far above the width of the rocking curve for highly perfect crystals, which is typically a few arc seconds. 

This rises to 10 if both the K 1 and K 2 ines are diffracted by the specimen. The effect this has upon the rocking curve depends on the dispersion of the whole system of beam conditioner and specimen, and ranges from zero to very large. This will be discussed below, in section 2.6. 

We therefore need to limit the divergence and wavelength spread of the beam incident upon the specimen. Beam conditioners are used to collimate and to 

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