15 October 1997 Recent progress in focusing gamma rays
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The Argonne/Toulouse collaboration is developing a crystal lens diffraction telescope for use as an astrophysical detector in the energy range of 200 keV to 1.3 MeV. The lens consists of 8 rings of diffraction crystals that all focus a narrow band of energies on a common HPGe detector. The inclination angle of these crystals controls the energy band being focused and will need to be adjusted over a range of 0.5 to 1.5 degrees with arcsecond precision to cover this energy band. At Argonne National Laboratory, a new lens frame was constructed and the inner ring was equipped with 16 Ge crystals of 1 cm3 size. The orientation of each crystal could be adjusted using a piezo-based picomotor in combination with a noncontact eddy-current sensor. The sensors have 0.1 - 0.2 arcsecond resolution; the motors have a step size of 0.05 - 0.2 arcseconds. By changing the crystal inclination and the distance of the detector from the lens, we were able to focus the 662 keV radiation from a 137Cs source at 24.75 m as well as line energies at 276, 303, 356, and 383 keV from a 133Ba source at 24.45 m. The sensor and system stability were demonstrated by alternately focusing line energies. We were able to simulate scans in energy of a spaceborne instrument as well as the enlargening of the energy repone by a slight detuning of the lens crystals. At the Advanced Photon Source (APS) Facility, an experiment to directly measure the diffraction efficiency of lens crystals from 200 - 500 keV using a beam with 3 arcsecond divergence was carried out. A double-crystal monochromator using two 3-mm-thick Ge crystal in Laue geometry was realized. The experimental results imply diffraction efficiencies for an astrophysical point source of 38% to 41% over the energy range for the crystals used.
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A. Kohnle, A. Kohnle, Robert K. Smither, Robert K. Smither, Timothy J. Graber, Timothy J. Graber, Peter von Ballmoos, Peter von Ballmoos, "Recent progress in focusing gamma rays", Proc. SPIE 3114, EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy VIII, (15 October 1997); doi: 10.1117/12.283788; https://doi.org/10.1117/12.283788

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