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 200–1.3 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° 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 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 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 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.