Abstract
Next-generation high-brilliance X-ray photon sources call for new X-ray optics. Here we demonstrate the possibility of using monolithic diamond channel-cut crystals as high-heat-load beam-multiplexing narrow-band mechanically stable X-ray monochromators with high-power X-ray beams at cutting-edge high-repetition-rate X-ray free-electron laser (XFEL) facilities. The diamond channel-cut crystals fabricated and characterized in these studies are designed as two-bounce Bragg reflection monochromators directing 14.4 or 12.4 keV X-rays within a 15 meV bandwidth to 57Fe or 45Sc nuclear resonant scattering experiments, respectively. The crystal design allows out-of-band X-rays transmitted with minimal losses to alternative simultaneous experiments. Only ≲2% of the incident ∼100 W X-ray beam is absorbed in the 50 µm-thick first diamond crystal reflector, ensuring that the monochromator crystal is highly stable. Other X-ray optics applications of diamond channel-cut crystals are anticipated.
Highlights
Hard x-ray free-electron lasers (XFELs) in the selfseeding mode [1, 2] generate fully coherent brilliant x-ray beams with a well-defined and narrow spectrum, typically with a bandwidth of 0.2 − 1 eV [3,4,5,6] and a pulse duration of a few tens of femtoseconds.When operated in a high-repetition-rate ( MHz) pulse sequence mode with properly tapered undulators, self-seeded XFELs may deliver x-rays with a pulse energy of 10 mJ/pulse ( 7 × 1012 photons/pulse of10-keV photons) and a time-averaged power of more than 100 W, corresponding to a time-averaged spectral flux of 1017 photons/s in a bandwidth of 1 eV [7]
About 1% of the XFEL radiation power in the small bandwidth can be directed to the ultra-high-resolution monochromator and associated spectroscopic experiments
Such a diamond monolithic channel-cut crystal can function as a two-bounce 800 Bragg reflection monochromator, deflecting 14.4 keV x-rays within a 15-meV bandwidth to 57Fe nuclear resonant scattering experiments, while the rest of the x-rays in the 1-eV XFEL bandwidth are transmitted through the thin drumhead crystal and could be transported to a simultaneous experiment 2
Summary
Hard x-ray free-electron lasers (XFELs) in the selfseeding mode [1, 2] generate fully coherent brilliant x-ray beams with a well-defined and narrow spectrum, typically with a bandwidth of 0.2 − 1 eV [3,4,5,6] and a pulse duration of a few tens of femtoseconds. Diamond is a material of choice in such cases, as it is superior to silicon because of its unrivaled radiation hardness, an order of magnitude higher transparency to x-rays, an order of magnitude higher stiffness [16, 17], orders of magnitude higher thermal conductivity [18, 19], small thermal expansion [20, 21] and almost 100% reflectivity in Bragg diffraction even in backscattering [22, 23] This unique combination of outstanding properties makes diamond the most promising.
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