The monochromators

After the collimation, the beam needs to be filtered tightly in energy to ensure the final collimation in the expanded beam. This is obtained via a 4-fold diffraction on silicon crystals, cut parallel to the (220) planes.
At each diffraction, only photons with the energy fulfilling the Bragg condition are reflected: in this way, the K_α1 fluorescence line at 4.511 keV can be isolated from the source spectrum; iterating the diffraction 4 times makes the passing beam more monochromatic. The crystals are organized into 2 pairs, each of them cut out from a single silicon block aiming at the alignment of their crystalline planes, a geometry known as channel-cut crystal (CCC). The two CCCs can be rotated to de-tune the Bragg energy slightly and so improve the monochromation to the required tier.