Fixed exit double-crystal monochromator with one single cooled crystal

The performance of a fixed exit double-crystal monochromator in terms of stability and reproducibility of the outgoing X-ray beam becomes the crucial point at modern synchrotron beamlines dealing with high-resolution X-ray optics. Due to the high heat load the monochromator crystals have to be cryogenically cooled. The cooling loop of the second crystal may have an impact on the performance of the monochromator. We therefore suggest to use a Si1−xGex single crystal as the first cooled crystal of the monochromator. The second crystal is held at room temperature. To verify the proposed solution an experiment was performed where the lattice parameters of pure Si and SiGe crystals as a function of the temperature were measured.