Bent crystals as high efficiency optical elements for hard x-ray astronomy

Hard x-rays (with energies in the 70 KeV - 1 MeV range) focusing is a crucial issue for astronomy in the x-rays and gamma-ray energy range. Due to the narrow angular range of diffraction at high gamma-rays energies, perfect crystals cannot be used for such application. Mosaic crystals have been proposed, but their efficiency cannot exceed 50%. In order to increase the diffraction efficiency, bent crystals have been proposed showing good performances. For bent crystals the diffraction range is given by the total curvature of the crystal lattice planes and the diffraction eficiency can reach values close to 100%. In this work we propose a crystal bending procedure obtained by a controlled surface damaging. The damaging procedure introduces a defected superficial layer, undergoing a high compressive strain of a few tens nanometer in thickness. Several oriented silicon and gallium arsenide wafers have been treated. By using high resolution x-ray diffraction measurements in Bragg condition at low energy x-rays, the local and mean curvature radius have been determined. Curvature radii between 2 and 30 m were obtained in wafers of different thicknesses. Preliminary results on the measurements performed at high gamma energies at synchrotron and showing high diffraction efficiency are also reported.