Ultrathin transmission-type bent crystals for XFEL spectral diagnostic

As the spectrum of each pulse from XFEL (X-ray free electron lasers, XFEL) undulator (often referred as pink X-ray) varied between each other, it is necessary to measure the intensity and spectrum of each pulse. The major parameters of the spectrometer are the facet of bent crystal, the Bragg angle, the transmittance (related to the material, its thickness and transmission angle), energy range and energy resolution. An ultrathin Si wafer was put above a concave lens and under a convex lens. Bending was achieved by applying the pressure, then the Si wafer with 25 mu m thickness would be regularly bent onto the concave profile until the convex substrate, Si wafer and the concave lens were overlap. The cylindrical substrates were provided with holes to let the FEL beam through the crystal, and the radius of curvature could be varied from 0.2m to 1m. The crystal orientation and miscut angle were measured by a high resolution X-ray diffractometer. Finally, the energy dispersion spectra were measured by a spectrometer built in laboratory.