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Proceedings Paper

Recent progress in focusing gamma rays
Author(s): A. Kohnle; Robert K. Smither; Timothy J. Graber; Peter von Ballmoos
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Paper Abstract

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.

Paper Details

Date Published: 15 October 1997
PDF: 11 pages
Proc. SPIE 3114, EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy VIII, (15 October 1997); doi: 10.1117/12.283788
Show Author Affiliations
A. Kohnle, Ctr. d'Etude Spatiale des Rayonnements (France)
Robert K. Smither, Argonne National Lab. (United States)
Timothy J. Graber, Univ. of Chicago (United States)
Peter von Ballmoos, Ctr. d'Etude Spatiale des Rayonnements (France)

Published in SPIE Proceedings Vol. 3114:
EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy VIII
Oswald H. W. Siegmund; Mark A. Gummin, Editor(s)

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