Istituto dei materiali per l'elettronica ed il magnetismo     
Ferrari C., Zanotti L., Zappettini A., Arumainathan S. Mosaic GaAs crystals for hard x-ray astronomy. In: Conference 7077: Advances in X-Ray/EUV Optics and Components III (San Diego, CA, USA, 11 August 2008). Abstract, p. 438. (SPIE Optics + Photonics 2008). A.M. Khounsary, C. Morawe, S. Goto, Eds., 2008.
Recently the design of a Laue lens with fi eld of view of 30 arcseconds for x-rays in the energy range from 100 keV to 1 MeV has been proposed. Mosaic crystals are used as focussing elements in order to increases the total diffracted intensity. The mosaic angular spread must be carefully defi ned as a compromise between intensity and energy resolution of the Laue lens. According to the abovementioned criteria copper and germanium crystals with typical mosaicity of 30 arcseconds have been studied for this purpose. In the present work we propose 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, but because of the spontaneous formation of "cellular structures" with dislocations at the boundaries between perfect zones of the crystal, Czochralsky grown GaAs crystals have the advantage of presenting some degree of mosaicity. Several GaAs ingots grown with the encapsulating Czochralski method have been characterized by high resolution x-ray diffraction. Bragg diffraction profi les 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 dependent on the position were measured. They are close to the proposed 30 arcsecond mosaicity required for the Laue lens. By controlling the growth conditions the dislocation density and cellular structure responsible of the crystal mosaicity can be in principle varied in a large range. The possibility of obtaining crystals with a given degree of mosaicity by tuning the Czochralski growth conditions is presented.
Subject GaAs crystals

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