The Mask

The IBIS Mask Assembly is rectangular with external dimensions of mm , and consists of three main subsystems: the Coded Pattern, the Support Panel and the Peripheral Frame with the necessary interface provisions.

The Coded Pattern is a square array of size mm , made up of 95 95 individual square cells of size 11.2 11.2 mm . The mask chosen for IBIS is based on a cyclic replication of MURA (Modified Uniformly Redundant Array) of order 53. The properties of the MURA patterns are described e.g. in [11] and [13].

Approximately half of the mask cells are opaque to photons in the operational energy range of the IBIS instrument, offering a 70% opacity at 1.5 MeV. The other 50% of cells are open, i.e., with an off-axis transparency of 60% at 20 keV. Figure shows the mask pattern.

Figure: The IBIS coded mask pattern. (white: open elements, back: closed elements)

The Support Panel includes additional elements to support the coded mask pixels, providing the necessary stiffness and strength to overcome the launch environment and the in-orbit operational temperatures. This panel is done from the material known as ``nomex''. Its transparency should be taken into account in the data analysis, as it absorbs part of the flux.

Fig.  shows the cross section of the support panel.

Figure: Cross section of the support panel.

The Peripheral Frame reinforces the sandwich panel.

The mechanical interfaces with the INTEGRAL payload module provide extra Tungsten shielding to the diffuse background through the gap between the mask edges and the payload vertical walls.