Section 6.1.3.3. Spectral composition

Any X-rays outside the wavelength band used for generating the desired X-ray pattern contribute to the radiation damage of the sample and to the X-ray background. In the interests of resolving neighbouring diffraction spots in the pattern, one would require the wavelength spread, , in the incident radiation to be less than 5 × 10⁻³. For the Cu Kα doublet and the doublet nature of the line usually does not matter. On the other hand, the value of is 0.1, so the Kβ component must be eliminated by means of a β-filter (a 0.15 mm-thick nickel foil for copper radiation) or by reflection from a crystal monochromator to avoid the appearance of separate Kβ diffraction spots. The dispersion produced by a crystal monochromator is small enough to be ignored in most applications.

In synchrotron beam lines, the bandpass is usually determined by the divergence of the beam and is of the order of 10⁻⁴. This is a smaller bandpass than is required for most purposes, and intensity can be gained by widening the bandpass by the use of an asymmetric-cut monochromator in spatial expansion geometry (Nave et al., 1995; Kohra et al., 1978). The intensity outside the monochromator bandpass is usually totally negligible.