Mosaic GaAs crystals for hard x-ray astronomy

Abstract

ABSTRACT Recently the design of a Laue lens with field of view of 30 arcseconds and for x-rays in the energy range from 100 keV to 1 MeV has been proposed in which mosaic crystals are us ed as focussing elements. The pr oper mosaic angular spread is chosen as a compromise between intensity and energy resolution of the Laue lens. In the present work we consider the use of GaAs crystals as optical elements for hard x-ray astronomy. GaAs crystals have essentially the same electron density and lattice spacing as germanium, and are characterized by spontaneous formation of structures with dislocations distribution at the boundaries between perfect zones of the crystal. Because of the presence of cellular st ructures Czochralsky grown GaAs show a natural degree of mosaicity. Several GaAs ingots grown by liquid encapsulating Czochralsky technique have been characterized by high resolution x-ray diffraction. Bragg diffraction profiles have been measured along ingot axes and diameters of doped, undoped or stoichiometry deviated GaAs crystals. Full width at half maximum values ranging from 15 to 40 arcseconds depending on the position were measured close to the proposed 30 arcsecond mosaicity required for the Laue lens. Appropriate growth conditions allow the control of the dislocation density and the modification of cellular structure responsible of the crystal mosaicity so that the possibility of obtaining crystals with a given degree of mosaicity by tuning the LEC growth conditions is proposed. Keywords : hard x-ray astronomy, Laue lens, mosaic crystals, crys tal growth, GaAs cellular structures, mosaic spread of GaAs crystals.