REALIZATION OF A TUNABLE CRYSTAL LENS AS AN INSTRUMENT TO FOCUS GAMMA-RAYS

The Argonne/Toulouse collaboration is developing a crystal lens diffra ction telescope for use as an astrophysical detector in the energy ran ge of 200 keV to 1.3 MeV. The lens consists of eight rings of diffract ion 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 Argo nne National Laboratory, a new lens frame was constructed, and the inn er ring was equipped with 16 Ge crystals of 1 cm(3) size. The orientat ion of each crystal was adjusted using a piezo-based picomotor in comb ination 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 arcsec onds. By changing the crystal inclination and the distance of the dete ctor 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 sta bility were demonstrated by alternately focusing different line energi es. We were able to simulate scans in energy of a spaceborne instrumen t, as well as the enlargening of the energy response by a slight detun ing of the lens crystals. (C) 1998 Elsevier Science B.V. All rights re served.