Design of an asymmetric monochromator
During the feasibility measurements we used a silicon 111 asymmetrical monochromator, due to the large Darwin width of the monochromator crystal and the large angle difference between the reflection angle of the silicon 111 plane and the diamond 220 reflection plane, the measured diamond rocking curve is much larger than the intrinsic diamond 220 Darwin width which is around 7 micro radians. Therefore, for the future measurements, we should consider using a new monochromator which has smaller Darwin width and better match of the refection angle between the monochromator and the diamond reflection plane. Silicon (331) and (333) both have small Darwin width and their reflection angle are quite close to the diamond 220 reflection angle. Hence, the new monochromator could be built by using the silicon 333 or silicon 331 reflections. Since the distance from the source to the diamond crystal is short, we have to use an asymmetric monochromator to increase beam size and reduce the beam divergence. For some practical reasons, the surface of the monochromator has to be along a plane of the crystal. That means we can not choose an arbitrary angle between the monochromator surface and the reflection plane to design this monochromator. The following table gives the possible planes that could be used to be the surface plane.
