Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998nimpa.416..493k&link_type=abstract
Nuclear Instruments and Methods in Physics Research, Sect. A, Vol. 416, No. 2 - 3, p. 493 - 504
Astronomy and Astrophysics
Astronomy
4
Gamma-Ray Astronomy: Space Instrumentation
Scientific paper
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. The authors 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 (111) and (220) using crystals with a mosaic width of a few arcseconds. Neglecting absorption, the values ranged from 31% to 49%. The continuum flux diffracted in second-order diffraction (440) was a factor of 2.4 - 2.6 less than in first-order diffraction (220). For third-order diffraction (the (333) and the (111) 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 arc seconds to 0.4° were measured. The results agree with the expectation of the Darwin model for mosaic crystals. This gives confidence in the calculations of the crystal lens telescope performance, and allows an optimization of a balloon-borne and a satellite-based telescope configuration.
Fernandez Pablo
Graber Tim
Kohnle Antje
Smither R.
von Ballmoos Peter.
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