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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.