<title>In-flight calibration requirements for the PICsIT high-energy imaging detector</title>

Abstract

IBIS is the high energy imagin telescope onboard the ESA satellite INTEGRAL, which will be launched in September 2001. The detection p;lane of IBIS comprises two position sensitive layers: ISGRI and PICsIT. PICsIT consists of a 64 X 64 unit array of approximately equals 0.8 cm 2 crystals operating in the energy range between 150 keV and 10 MeV. Due to the low intrinsic signal-to-noise ratio of the cosmic sources in the gamma-ray domain, INTEGRAL observing times will be very long, lasting about 10 5 -10 6 s. Moreover, the image formation principle on which PICsIT works is that of coded aperture imaging in which the entire detection plane contributes to each decoded sky pixel. For these two reasons, the spatial and temporal uniformity in gain, linearity and energy resolution of the individual detection units is of paramount importance for fully exploiting the capabilities of the instrument. In IBIS this is accomplished by having onboard a low-intensity tagged radioactive source constantly illuminating the entire detection plane with 511 and 1275 keV energy photons. Herein we describe the scientific rationale and requirements of the in-flight calibration system from the point of view of the high energy detector PICsIT, and the impact on the PICsIT scientific performance as a function of the overall calibration accuracy achieved during the flight.