The ISGRI CdTe gamma-ray camera: first steps

The gamma-ray telescope IBIS, on board the INTEGRAL satellite, features a c oded-mask aperture, active and passive shields and two detector arrays. The first one (ISGRI) is an assembly of 16384 CdTe detectors (4 x 4 mm large, 2 mm thick) operating at room temperature. ISGRI covers the lower part (15 keV-1 MeV) of the IBIS energy range (15 keV-10 MeV). Detectors are arranged on polycells, each including 16 crystals, connected to their front-end ele ctronics (ASICs). Each of the eight independent ISGRI modules are made of 1 28 polycells. The ASICs contain a low noise charge-sensitive preamplifier a nd feature pulse rise-time measurement in addition to the standard pulse he ight measurement. This permits to compute a charge loss correction based on the charge drift time. After application of this correction, a spectral re solution around 7.5% at 122 keV is obtained with the ASICs. Today, 16 polyc ells have been mounted on the first representative ISGRI module. This modul e has been interfaced with the entire ISGRI data-processing electronics and forms the engineering model of ISGRI. After a presentation of the scientif ic context, this paper describes the ISGRI design with particular emphasis on the detectors, the polycells performances and the module assembly. (C) 2 000 Elsevier Science B.V. All rights reserved.