A Hard X-Ray Split-And-Delay Unit For The Hed Experiment At The European Xfel

For the High Energy Density (HED) experiment [1] at the European XFEL [2] an x-ray split-and delay-unit (SDU) is built covering photon energies from 5 keV up to 20 keV [3]. This SDU will enable time-resolved x-ray pump / x-ray probe experiments [4,5] as well as sequential diffractive imaging [6] 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 [7,8]. The set-up is based on geometric wavefront beam splitting, which has successfully been implemented at an autocorrelator at FLASH [9]. The x-ray FEL pulses are split by a sharp edge of a silicon mirror coated with multilayers. Both partial beams will then pass variable delay lines. For different photon energies the angle of incidence onto the multilayer mirrors will be adjusted in order to match the Bragg condition. For a photon energy of h nu = 20 keV a grazing angle of theta = 0.57 degrees has to be set, which results in a footprint of the beam (6 sigma) on the mirror of l = 98 mm. At this photon energy the reflectance of a Mo/B4C multi layer 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 nu = 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 nu = 20 keV and up to +/- 23 ps at h nu = 5 keV will be possible.