Grown and artificial mosaic GaAs crystals for hard X-ray astronomy

In order to increase the collection efficiency of a lens for hard X-ray energies, mosaic GaAs crystals to be used as optic elements in Laue diffraction are proposed. In fact, GaAs crystals show a natural degree of mosaicity due to the spontaneous formation during the growth of cellular structures with dislocations at the boundaries. Several GaAs samples grown by LEC method have been characterized by means of high-resolution X-ray diffraction. Mosaicity values ranging from 10 to 25 arcsec have been measured. Since proper growth conditions allow to control and modify both the dislocation density and the cellular structure responsible of the mosaic spread, the possibility of obtaining crystals with a given degree of mosaicity by tuning the LEC growth conditions is proposed. A complementary strategy to increase the Darwin width of the diffraction curve based on curved crystals has also been proposed. The lattice curvature was achieved by introducing a compressive stress on the crystal surface. Curvature radii between 2 and 8m were easily obtained in wafer crystals 500 ?m thick.