The Collimator

In order to maintain the low energy response of IBIS despite the dithering needed for SPI, the collimation baseline consists of a passive lateral shield that limits the solid angle (and therefore the cosmic gamma-ray background) viewed directly by the IBIS detector in the full field of view up to a few hundreds of keV. The tube collimation system is implemented with three different devices:

• The Hopper: four inclined walls starting from the detector unit with a direct interface to the IBIS detector mechanical structure. The hopper is not physically connected to the payload module structure.

• The Tube: The Tube is formed by four payload module walls shielded with glued Lead foils.

• The additional side shielding on the mask. Four strips of 1 mm thick Tungsten provide shielding from the diffuse background in the gaps between the mask edges and the top of the tube walls.