The ISGRI CdTe gamma-ray camera:: first steps

The gamma-ray telescope IBIS, on board the INTEGRAL satellite, features a coded-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 electronics (ASICs). Each of the eight independent ISGRI modules are made of 128 polycells. The ASICs contain a low noise charge-sensitive preamplifier and feature pulse rise-time measurement in addition to the standard pulse height measurement. This permits to compute a charge loss correction based on the charge drift time. After application of this correction, a spectral resolution around 7.5% at 122 keV is obtained with the ASICs. Today, 16 polycells have been mounted on the first representative ISGRI module. This module has been interfaced with the entire ISGRI data-processing electronics and forms the engineering model of ISGRI. After a presentation of the scientific context, this paper describes the ISGRI design with particular emphasis on the detectors, the polycells performances and the module assembly. (C) 2000 Elsevier Science B.V. All rights reserved.