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 in order to match the Bragg condition. For a photon energy of hν = 20 keV a grazing angle of θ = 0.57° has to be set, which results in a footprint of the beam (6σ) on the mirror of l = 345 mm. At this photon energy the reflectance of a Mo/B4C multi layer coating with a multi layer period of d = 3 nm and N = 200 layers amounts to R = 0.92. For a photon energy of hν = 5 keV a smaller size of the footprint of l = 244 mm is calculated due to the steeper grazing angle of θ = 2.28°. In order to enhance the maximum transmission for photon energies of hν = 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 +/- 3.7 ps at hν = 20 keV and up to +/- 44 ps at hν = 5 keV will be possible.
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Sebastian Roling ; Liubov Samoylova ; Björn Siemer ; Harald Sinn ; Frank Siewert, et al.
Design of an x-ray split- and delay-unit for the European XFEL
", Proc. SPIE 8504, X-Ray Free-Electron Lasers: Beam Diagnostics, Beamline Instrumentation, and Applications, 850407 (October 15, 2012); doi:10.1117/12.965547; http://dx.doi.org/10.1117/12.965547