A split- and delay-unit for the European XFEL

For the European XFEL an x-ray split- and delay-unit (SDU) is built covering photon energies from 5 keV up to 20 keV. This SDU will enable time-resolved x-ray pump/x-ray probe experiments as well as sequential diffractive imaging on a femtosecond to picosecond time scale. Further, direct measurements of the temporal coherence properties will be possible by making use of a linear autocorrelation. The set-up is based on geometric wavefront beam splitting, which has successfully been implemented at an autocorrelator at FLASH. The x-ray FEL pulses will be split by a sharp edge of a silicon mirror coated with Mo/B4C multi layers. Both partial beams will then pass variable delay lines. For different wavelengths the angle of incidence onto the multilayer mirrors will be adjusted i n order to match the Bragg condition. For a photon energy of h ny = 20 keV a grazing angle of theta = 0.57° has to be set, which results in a footprint of the beam (6 sigma) on the mirror of l = 120 mm. At this photon energy the reflectance of a Mo/B4C multilayer coating with a multilayer period of d = 3.2 nm and N = 200 layers amounts to R = 0.92. In order to enhance the maximum transmission for photon energies of h ny = 8 keV and below, a Ni/B4C multilayer coating can be applied beside the Mo/B4C coating for this spectral region. Because of the different incidence angles, the path lengths of the beams will differ as a function of wavelength. Hence, maximum delays between +/- 2.5 ps at h ny= 20 keV and up to +/- 23 ps at h ny = 5 keV will be possible.