X-ray diffraction efficiency of bent GaAs mosaic crystals for the LAUE project

In a Laue lens made by single crystals oriented to diffract parallel x-rays at the lens focus, the energy and angular resolution are limited by the crystal size and by the crystal mosaicity. The use of extended crystals bent according to the lens curvature provides a better focusing, with the resolution given essentially by the crystal mosaicity. With this approach a crystal mosaicity as low as 15-25 arcseconds, well below the mosaicity value of copper crystals, was found suitable for the new design of the Laue lens. The reflectivity and transmission profiles and the integrated intensity have been measured in flat and bent GaAs and Si crystals prepared by the method of surface damaging by using sandpaper of different grain size. The surface grinding induces a local lattice strain which produces a self standing bent crystal. Bent crystals with radius of curvature lower than a critical value given by the extinction length behave as perfect mosaic crystals or strongly bent perfect crystals, maximizing the diffraction efficiency at high x-ray energies. It is found that the surface grinding does not affect the crystal diffraction efficiency, the damage thickness being limited to a few tens microns near the crystal surface.