Dr. Andreas Schropp - DESY, Hamburg

Titel: Magnified X-Ray Phase-Contrast Imaging at XFELs

Abstract: Current x-ray sources of the fourth generation, also denoted by x-ray free-electron lasers (XFELs), produce intense and short x-ray pulses opening up completely new scientifc possibilities for the investigation of fast dynamical processes [1, 2]. In this way snapshots of the state of matter can be recorded at specifc moments in time. However, some experiments require as well high spatial resolution in order to visualize physical processes occurring on short length scales. While the temporal resolution is determined by the pulse length, the spatial resolution in x-ray imaging is mainly limited by the properties of the x-ray optics and diffraction effects.

We built an x-ray microscope for the Matter in Extreme Conditions (MEC) endstation of the Linac Coherent Light Source (LCLS), which is especially adapted to the XFEL environment, and used the setup to carry out different experiments based on magnifed x-ray phase-contrast imaging. The method allows one to image a sample with high spatial resolution and high sensitivity to small density changes within a sample. The x-ray microscope is based on a set of Beryllium compound refractive lenses (Be-CRLs) creating a secondary x-ray source closely in front of a sample. The sample is positioned in the divergent x-ray beam and a CCD detector records a magni ed image of the illuminated area at a larger distance further downstream. In a pump-probe scheme an optical laser hits the sample from the side, which initiates a shock wave propagating into the material, and the XFEL pulse probes the state of the material shortly after. In this talk, we report on these experiments, demonstrate the new capabilities at the example of imaging of shock waves in different materials and outline the analysis procedure including XFEL nano-beam characterization using ptychography [3, 4].

References
[1] Chapman, H. N. et al. Femtosecond x-ray protein nanocrystallography. Nature 470, 73-U81 (2011).
[2] Berrah, N. et al. Double-core-hole spectroscopy for chemical analysis with an intense X-ray femtosecond laser. PNAS 108, 16912-16915 (2011).
[3] Schropp, A. et al. Full spatial characterization of a nanofocused x-ray free-electron laser beam by ptychographic imaging. Scientifc Reports 3, 1633 (2013).
[4] Schropp, A. et al. Scanning coherent x-ray microscopy as a tool for xfel nanobeam characterization. Proc. SPIE 8849, 88490R (2013).