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 image plane. The diffraction efficiency of the crystals is decisive for the energy bandpass, the field of view and the effective area of the crystal lens telescope. We have measured diffraction efficiencies of Ge crystals from 200 to 500 keV at the Advanced Photon Source synchrotron at Argonne National Laboratory in two experiments carried out at the sector 1 bending magnet beamline. The high brilliance and high particle energy of third-generation synchrotrons permit measurements at gamma-ray energies. The low angular divergence of synchrotron radiation is similar to that of an astronomical point source. The efficiencies ranged from 20% to 31% for diffraction of Ge (1 1 1) and (2 2 0) using crystals 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 ranging from a few are seconds to 0.4 degrees were measured. The results agree with the expectation of the Darwin model for mosaic crystals. This gives us confidence in the calculations of the crystal lens telescope 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 efficiency of the lens crystals for a source at finite distance. The efficiency increase is due to both a bending and a straining of the crystal. We measured rocking curve widths of a crystal before and after etching to determine the effect of surface damage on the diffraction efficiency. 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.