IBIS Detector and Mask

The imager on-board INTEGRAL (IBIS) is giving sharper gamma-ray images than any previous instrument. IBIS provides diagnostic capabilities of fine imaging (12 arcmin FWHM), source identification and spectral sensitivity to both continuum and broad lines over a broad (15 keV - 10 MeV) energy range. The imager examines, simultanesously with the other instruments on INTEGRAL, celestial objects of all classes ranging from the most compact galactic systems to extragalactic objects. A tungsten coded-aperture mask (located at 3.2 m above the detection plane) is optimised for high angular resolution.

As diffraction is negligible at gamma-ray wavelengths, the angular resolution obtainable with a coded mask telescope is limited by the spatial resolution of the detector array.  The spatial resolution is dependent on the number of small sensitive elements of the detector, called pixels (picture elements). IBIS has a detector with a large number of pixels, all physically distinct from one another.

The detector uses two parallel planes of pixels located one on top of the other and separated by 90 mm. The top layer (ISGRI) is made of 16 384 cadmium telluride (CdTe) pixels, covering 2600 square centimetres and each measuring 4x4x2 millimetres. This layer detects the low-energy gamma rays. The second layer (PICsIT) consists of 4096 caesium iodide (CsI) pixels, each 9x9x30 millimetres covering 3100 square centimetres. This layer captures high-energy gamma rays.

The division into two layers allows scientists to track the paths of the photons in 3D, as they scatter and interact with more than one element.

IBIS Coded Mask

IBIS was put together by a number of collaborating institutes from around the world:

• Italy (IAS Rome, ITESRE Bologna, IFCAI Palermo)
• France (CEA Saclay)
• Norway (U Bergen)
• Germany (U Tuebingen)
• Spain (U Valencia)
• USA (NASA/MSFC Huntsville)
• Poland (Space Research Centre, Warsaw)
• UK (U Southampton)

Table Showing Predicted Performance