Preparation of bent crystals as high efficiency optical elements for hard x-ray astronomy

Curved crystals can be used as optical elements of a x-ray lens for hard x-ray astronomy in order to increase the diffraction efficiency. In this work we propose to obtain bent crystals by a controlled surface damaging, which introduces defects in a superficial layer of few tens nanometers in thickness undergoing a highly compressive strain. Several (100) oriented silicon and gallium arsenide wafer crystals 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 of each sample have been determined. Curvature radii between 3 and 70 m were easily obtained in wafers of different thicknesses. In particular, spherical curvatures have been determined in silicon samples whereas GaAs treated samples showed an elliptical curvature with the major axes corresponding to the < 011 > crystallographic directions. The characterization of GaAs samples performed in the Laue geometry at gamma-ray energy of 120 keV confirmed the increase of the diffraction efficiency in the bent crystals.