MEASUREMENT OF DIFFRACTION EFFICIENCIES RELEVANT TO CRYSTAL LENS TELESCOPES

The Toulouse/Argonne collaboration is working on a novel detector for gamma-ray astronomy based on a crystal lens that uses Bragg reflection of Ge crystals to concentrate photons onto a small detector in the im age plane. The diffraction efficiency of the crystals is decisive for the energy bandpass, the field of view and the effective area of the c rystal lens telescope. We have measured diffraction efficiencies of Ge crystals from 200 to 500 keV at the Advanced Photon Source synchrotro n at Argonne National Laboratory in two experiments carried out at the sector 1 bending magnet beamline. The high brilliance and high partic le energy of third-generation synchrotrons permit measurements at gamm a-ray energies. The low angular divergence of synchrotron radiation is similar to that of an astronomical point source. The efficiencies ran ged from 20% to 31% for diffraction of Ge (1 1 1) and (2 2 0) using cr ystals with a mosaic width of a few are seconds. Neglecting absorption , the values ranged from 31% to 49%. The continuum flux diffracted in second-order diffraction (4 4 0) was a factor of 2.4-2.6 less than in first-order diffraction (2 2 0). For third-order diffraction (the (3 3 3) and the (1 1 1) planes), the flux was reduced by a factor of 6.8. The peak efficiency and the rocking curve integral for mosaic widths r anging from a few are seconds to 0.4 degrees were measured. The result s agree with the expectation of the Darwin model for mosaic crystals. This gives us confidence in the calculations of the crystal lens teles cope performance, and allows an optimization of a balloon-borne and a satellite-based telescope configuration.Measurements were carried out with a wedged crystal, that was used to increase the diffraction effic iency of the lens crystals for a source at finite distance. The effici ency increase is due to both a bending and a straining of the crystal. We measured rocking curve widths of a crystal before and after etchin g to determine the effect of surface damage on the diffraction efficie ncy. The mean decrease in rocking curve width due to the etching was 1 .7 are sec in the center of the crystal and 6.9 are sec at the crystal edge. (C) 1998 Elsevier Science B.V. All rights reserved.