IBIS Instrument

IBIS Instrument

IBIS (Imager on Board the Integral Satellite) is a gamma-ray telescope with a coded mask, supplying high-angular-resolution images (source location <1 arcmin) as well as a good spectral information. The active high-energy emission sites produce a continuous spectrum, essentially induced by mechanisms involving particle acceleration, on which nuclear de-excitation and electron-positron annihilation emission lines are superimposed.

Photo of IBIS instrument detector part

This instrument, designed to supply accurate images of the sky in an energy domain ranging from 20 keV to 10 MeV, is the direct successor of the SIGMA telescope. Its design is very similar to the latter, combining a coded-mask to a set of gamma-ray detectors. The camera is actively and passively shielded from background radiation. The novelty in IBIS mostly lies in the performances of the detector set, which affords a sensitivity ten times better than SIGMA. To cover an energy domain of nearly three orders of magnitude, it was necessary to use two cameras. The first, ISGRI (Integral Soft Gamma-Ray Imager), operates at low energy (20 keV - 1 MeV). The second, PICsIT (PIxellated CsI Telescope), measures the higher energy photons (150 keV - 10 MeV). These two cameras are of a completely new type.

They are composed of a large number of independent detectors, one for each image element i.e. 128 x 128 in the case of ISGRI and 64 x 64 in the case of PICsIT. The SAp laboratory at CEA, is prime contractor for ISGRI, while PICsIT is under the responsibility of the TESRE institute in Bologna. The overall instrument was developed at the IAS in Rome. The coded-mask has a surface of nearly 1 m², is located 3.1 m above this detector set, giving a 12 arcmin angular resolution for the IBIS telescope (two sources separation limit) and a location accuracy of bright sources lower than one arcminute, i.e. similar imaging performances to SIGMA with much better sensitivity across a wider energy domain. The field of view is 20 degrees x 20 degrees at mid-sensitivity, nearly four times wider than SIGMA.

Other specific features include a large collection surface across a wide spectral domain, and the ability to produce sharp images and good resolution spectra over the entire energy domain. IBIS thus offers more capabilities on its own than any other gamma-ray instrument. These characteristics are mainly due to ISGRI which is the first gamma-ray camera with semiconductors operating at ambient temperature.