Istituto dei materiali per l'elettronica ed il magnetismo     
Buffagni E., Ferrari C., Zanotti L., Zappettini A. As grown and artificial mosaic GaAs crystals for hard x-ray astronomy. In: Channeling 2010 - 4th International Conference on Charged and Neutral Particles Channeling Phenomena (Ferrara, 3-8 October 2010).
In order to increase the collection efficiency at x-ray energies from 100 keV to 1 MeV, mosaic crystals as optic elements in Laue diffraction are proposed with mosaicity of the order of a few tens of arcseconds. The mosaicity of a nearly perfect crystal made of small perfect domains is defined as the average misalignment of the crystal domains orientation. A suitable mosaic angular spread depends on the desired 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 "cellular structures" with dislocations distribution at the boundaries between perfect zones of the crystal. Because of the presence of cellular structures Czochralsky grown GaAs shows a natural degree of mosaicity. Several GaAs ingots grown with the encapsulating Czochralski method have been characterized by high resolution x-ray diffraction. Bragg diffraction profiles have been measured along ingot axes and diameters of undoped or Te doped GaAs crystals. Full width at half maximum values ranging from 15 to 40 arcseconds and depending on the position were measured: The measured values are 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 mosaic spread so that it is in principle possible to obtain crystals with a given degree of mosaicity. A complementary strategy to increase the Darwin with of the diffraction curve is by using "artificial mosaic" crystals as in curved crystals. This lattice curvature was obtained by introducing a compressive stress on the crystal surface by damaging the surface of several GaAs and Si crystals after a treatment with fine sandpaper. Radii of curvatures between a few to several tens meters were easily obtained in wafers crystals with thickness form 0.5 to 1 mm. Based on this approach we propose a modified mosaic Laue lens made of a stack of this bent crystals.
Subject Laue lens
X-ray astronomy
Bent crystals
GaAs crystals

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