ISGRI: the integral soft gamma-ray imager

For the first time in the history of high energy astronomy, a large CdTe gamma-ray camera is operating in space. ISGRI is the low-energy camera of the IBIS telescope on board the INTEGRAL satellite. This paper details its design and its in-flight behavior and performances. Having a sensitive area of 2621 cm2 with a spatial resolution of 4.6 mm, a low threshold around 12 keV and an energy resolution of ∼ 8% at 60 keV, ISGRI shows absolutely no signs of degradation after 9 months in orbit. All aspects of its in-flight behavior and scientific performance are fully nominal, and in particular the observed background level confirms the expected sensitivity of 1 milli-Crab for a 106 s observation. (authors)