Rocking curve imaging of high quality sapphire crystals in backscattering geometry

We report on the characterization of high quality sapphire single crystals suitable for high-resolution X-ray optics at high energy. Investigations using rocking curve imaging reveal the crystals to be of uniformly good quality at the level of ∼10-4 in lattice parameter variations, δd/d. However, investigations using backscattering rocking curve imaging with a lattice spacing resolution of δd/d∼5×10-8 show very diverse quality maps for all crystals. Our results highlight nearly ideal areas with an edge length of 0.2-0.5 mm in most crystals, but a comparison of the back reflection peak positions shows that even neighboring ideal areas exhibit a relative difference in the lattice parameters on the order of δd/d=10- 20×10-8; this is several times larger than the rocking curve width. Stress-strain analysis suggests that an extremely stringent limit on the strain at a level of ∼100 kPa in the growth process is required in order to produce crystals with large areas of the quality required for X-ray optics at high energy.