The Overall Design

IBIS is a gamma-ray imager operating in the energy range 20 keV to 10 MeV, with two simultaneously operating detectors covering the full energy range, located behind a Tungsten mask which provides the encoding.

The coded mask is optimized for high angular resolution. As diffraction is negligible at gamma-ray wavelengths, the angular resolution of a coded-mask telescope is limited by the spatial resolution of the detector array. The angular resolution of a coded mask telescope is defined by the ratio between the mask element size (11.2 mm) and the mask-to-detection plane distance (3133 mm).

IBIS is made of a large number of small, fully independent pixels.

The detector features two layers, ISGRI and PICsIT: the first is made of Cadmium-Telluride (CdTe) solid-state detectors and the second of Caesium-Iodide (CsI) scintillator crystals. This configuration ensures a good broad line and continuum sensitivity over the wide spectral range covered by IBIS. The double-layer discrete-element design of IBIS allows the paths of interacting photons to be tracked in 3D if the event involves detection units of both ISGRI and PICsIT. The application of Compton reconstruction algorithms to these types of events (between few hundred keV and few MeV) allows an increase in signal to noise ratio attainable by rejecting those events unlikely to correspond to source photons inside the field of view.

The detector aperture is restricted, in the hard X-ray part of the spectrum, by passive shielding covering the distance between mask and detection plane. An active BGO scintillator VETO system shields the detector bottom as well as the four sides up to the bottom of ISGRI.

Figure 2 shows a cut-away drawing of the various components of IBIS (except the mask and tube). Figure 3 shows the distances between the different parts of the detector assembly. Figure 4 shows the spacecraft & instruments coordinate systems.

Figure 2: Cutaway drawing of the IBIS detector assembly, together with the lower part of the collimator (Hopper).

Figure 3: IBIS detector assembly in numbers.

Figure 4: Spacecraft & Instrument Coordinate Systems. Note that the -axis of the spacecraft is defined by the pointing direction.