Crystal bending by surface damaging in mosaic GaAs crystals for the LAUE project

Curved crystals used as optical elements of a Laue lens for hard x- and gamma-ray astronomy have a larger diffraction efficiency with respect to perfect flat crystals. In this work we show how to achieve the bending of the crystals by a controlled surface damaging which introduces defects in a superficial layer of few tens micrometers in thickness undergoing a highly compressive strain. Several silicon, gallium arsenide and germanium wafer crystals have been treated. The local and mean curvature radii of each sample have been determined by means of high resolution x-ray diffraction measurements in Bragg condition at low energy (8 keV). (100) oriented silicon and (111) oriented germanium samples showed spherical curvatures, whereas (100) oriented GaAs treated samples evidenced an elliptical curvature with major axes corresponding to the <011< crystallographic directions. Curvature radii between 3 and 70 m were easily obtained in wafers with thicknesses up to 2 mm. Several 3x1x0.2 cm3 GaAs crystals (100) oriented with a radius of curvature of 40 m were prepared for the Laue Lens. Using a x-ray tube set at a distance of 20 m from the crystal for the first time the focusing of the (022) diffracted beam at a distance of 20 m was observed. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.