Realization of a tunable crystal lens as an instrument to focus gamma rays

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 eight 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 0.5-1.5 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 cm(3) size. The orientation of each crystal was 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 Cs-137 source at 24.75 m, as well as line energies at 276, 303, 356, and 383 keV from a Ba-133 source at 24.45 m. The sensor and system stability were demonstrated by alternately focusing different line energies. We were able to simulate scans in energy of a spaceborne instrument, as well as the enlargening of the energy response by a slight detuning of the lens crystals. (C) 1998 Elsevier Science B.V. All rights reserved.