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Macro Pixel Detector – A Combination of DEPFET and Silicon Drift Detector



  Macro Pixel Detector – A Combination of DEPFET and
Silicon Drift Detector
  Silicon detectors have the potential of measuring simultaneously and precisely position and energy of single photons in the X-ray regime. Examples of spectroscopic imagers are CCDs and (active) pixel sensors. Typical cell sizes of these detectors range from 20 µm by 20 µm to 100 µm by 100 µm. This results in a large number of pixels per unit area, long readout time and/ or large power consumption as well as handling of large data volume.   Artist’s impression of SIMBOL-X
Artist’s impression of SIMBOL-X, the detector spacecraft (left) and the mirror spacecraft (right) are in autonomous formation flight at a focal length of 20 m.
  Space based experiments in particular have severe restrictions on these requirements. However, such a precise position measurement is often not even desired, as the spatial resolution is limited by other detector components, e.g. the point spread function of the X-ray optics.  
  A new class of MPI HLL developed devices combines a SDD with the integrated readout device DEPFET to a macro pixel detector (MPD). The use of the SDD allows for scaling the pixel dimension to the optics’ resolution in the range from 100 µm to 1 cm, while the central DEPFET brings in its properties of signal charge storage, low noise readout on demand, fast column-parallel readout, and random accessible pixels. The Figure to the left shows the layout of an MPD cell. Two variants of MPD are in development for applications in astrophysics and planetary research both to be launched in 2013.  
The Mercury Imaging X-ray Spectrometer (MIXS) system on board ESA’s BepiColombo Mercury planetary orbiter (MPO) will remotesense the atomic composition of the Mercury surface by applying the techniques of X-ray Fluorescence analysis (Link to external siteXRF and particle induced X-ray emission (PIXE) with solar coronal X-rays and protons providing the primary sources of illumination. The MPD in the focal plane of a Wolter-I micro-channel plate optics has a pixel size of 300 µm by 300 µm and a format of 64 pixels by 64 pixels.
Simbol-X is a joint hard X-ray mission with French, Italian and German contributions. To provide the exceptional focal length of 20 m required by the high-energy cut-off at 80 keV Simbol-X will be the first mission to consist of a detector spacecraft and a mirror spacecraft in autonomous formation flight (figure on the right). The focal plane instrumentation is a stack of a low-energy MPD covering the energy range from 0.5 to 20 keV and a high-energy pixelated CdZnTe-detector sensitive from 5 keV on.
  To match the telescope‘s angular resolution of 15 arcsec the MPD’s pixel size has been chosen to be 625 µm by 625 µm. The format of 128 pixels by 128 pixels corresponds to a monolithic sensitive area of 8 cm by 8 cm and a field of view of 12 arcmin.
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Layout plot of an MPD cell for the SIMBOL-X low energy detector consisting of a central DEPFET readout device and an SDD with six drift rings