Detector

The ISGRI CdTe and PICsIT CsI(Tl) detectors are layered with respect to each other, with PICsIT below ISGRI with respect to the coded mask (and hence the astronomical source).

Figure: ISGRI and PICsIT division in modules and submodules

• Upper Detector Layer: ISGRI

  Cadmium Telluride (CdTe) is a semiconductor operating at ambient temperature. 0 20 C is the optimum range. With their small area, the CdTe detectors are ideally suited to build an image with good spatial resolution.

  The CdTe layer is made of 8 identical Modular Detection Units each having 32 64 pixels (see Figure ). Total sensitive area of the detector is 2621 cm .

• Lower Detector Layer: PICsIT

  Caesium Iodide is a scintillation crystal. The CsI(Tl) layer is divided into eight rectangular modules of 16 32 detector elements (see Figure ). In each module, there are two independent semi-module each one with its independent Front End Electronics. Total sensitive area of the detector is 2994 cm .

• Noisy Pixels

  Figure: Schematic view of the PICsIT layer. Each module number is indicated. The dotted lines represent the division in semimodules whose number is indicated at the top. The black pixels are the dead ones. The ( , ) coordinates are the IBIS ones both ranging from 0 to 63. X-axis is directed toward the source located above the page. The -axis is pointing positively to the sun.

  
  It is possible that with the time some of the pixels of the detector may become out of order and start to produce outputs not triggered by an income photon, i.e., to become ``noisy''. If the particular pixel countrate is too high relatively to the module countrate, then the on-board electronics switch it off. In ISGRI, the noisy pixels can recover after being switched off for some time and disabled pixels are periodically reset to check their status.

  In PICsIT, pixels cannot be recovered so easily. PICsIT pixel will remain off once killed. Only if half of the detector (or so) will be off, an attempt will be made to turn pixels on. The current situation is shown on Figure . Overall the killed pixels are less than 1%.