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DSM/DAPNIA Service d’Astrophysique

DSM/DAPNIA Service d’Astrophysique. Characterisation and 3D modelling of pixelated Cd(Zn)Te detectors for space applications in the X-ray domain. Characterization. Bulk and surface leakage currents Spectra. Bob Dirks bdirks@cea.fr. Modelization.

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DSM/DAPNIA Service d’Astrophysique

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  1. DSM/DAPNIA Service d’Astrophysique Characterisation and 3D modelling of pixelated Cd(Zn)Te detectors for space applications in the X-ray domain Characterization • Bulk and surface leakage currents • Spectra Bob Dirks bdirks@cea.fr Modelization • Monte Carlo simulation of interactions inside bulk Cd(Zn)Te with GEANT4 • Numerical modelling using Matlab • Modelling of read-out electronics

  2. DSM/DAPNIA Service d’Astrophysique Detectors at CEA/Saclay Bob Dirks bdirks@cea.fr Examples of 64 pixels CZT arrays, 2 and 6 mm thick, from eV-Products (USA)

  3. DSM/DAPNIA Service d’Astrophysique Detectors at CEA/Saclay Left: example of 256 pixels CZT arrays, 6 mm thick from eV-Products (USA) Bob Dirks bdirks@cea.fr Right: example of 256 pixels Schottky arrays, 0.5 mm thick from ACRORAD (Japan)

  4. DSM/DAPNIA Service d’Astrophysique Mechanical set-up Bob Dirks bdirks@cea.fr Modular upper stage 256 coax lines

  5. DSM/DAPNIA Service d’Astrophysique Mechanical set-up Cooled detector and electronics Bob Dirks bdirks@cea.fr Service area

  6. DSM/DAPNIA Service d’Astrophysique Mechanical set-up Bob Dirks bdirks@cea.fr

  7. DSM/DAPNIA Service d’Astrophysique Characterisation Very first 64 channel acquisition:CdZnTe detector, 2 mm thick, 64 pixels Bob Dirks bdirks@cea.fr

  8. DSM/DAPNIA Service d’Astrophysique LO connected to ground by chassis at the back of Keithley 6517A i1 i2 HI LO HI cathode GR - Keithley 6517A LO A A CdTe ground + anode V ground Characterisation Leakage current : The design and functionality of the IDeF- X ASIC read-out electronics depends directly on the direction and value of this current. Bob Dirks bdirks@cea.fr Measurement method 1 Schottky diodes Current expected to be of the same order as in pixelated CdZnTe detectors

  9. DSM/DAPNIA Service d’Astrophysique Characterisation Measurement method 1 Results Bob Dirks bdirks@cea.fr

  10. DSM/DAPNIA Service d’Astrophysique A A Characterisation Measurement method 2 Total current in this way we obtained the mean current per pixel Keithley 6517A I tot= Σ I pixel I tot= Σ I pixel HI LO I GR I GR anode HI HI GR GR Keithley 6517A LO LO ground CdZnTe kathode kathode Bob Dirks bdirks@cea.fr Shielding V ground Leakage current of pixels:~6nA/64 pixels = 94 pA/pixelguard ring ~10 nA at 250 V @ T=23°C

  11. DSM/DAPNIA Service d’Astrophysique Characterisation Measurement method 3 Direct measurement of current per pixel Use of dedicated electronics circuit of highly resistive material Relays interactively controlled to choose pixel to be measured, other pixels connected to ground Bob Dirks bdirks@cea.fr Top view of leakage current measurement circuit. Keithley 6517A HI LO I GR I GR anode HI HI Guard ring pix Keithley 6517A LO LO CdZnTe ground kathode kathode Shielding V ground Side view

  12. DSM/DAPNIA Service d’Astrophysique Characterisation Measurement method 3 First results Bob Dirks bdirks@cea.fr

  13. DSM/DAPNIA Service d’Astrophysique Modelisation Hit position: (x,y,z)Energy: E (keV) GEANT4: Monte Carlo simulation of interactions inside bulk Matlab:-3D modelling of detector geometry-charge carrier transport-simulation of charge induction Bob Dirks bdirks@cea.fr Induced current at pixel electrodes as function of time Matlab:Modelling of read-out electronics using transfer function Full detector simulation!

  14. DSM/DAPNIA Service d’Astrophysique Modelisation γ, 70 keV Bob Dirks bdirks@cea.fr CdZnTe, 2mm GEANT4: Above: simulation of particle interaction inside a piece of CdZnTe material of 2 mm thickness. Left: user interface with GEANT4 and electronics parameter input and action buttons

  15. DSM/DAPNIA Service d’Astrophysique Modelisation Matlab:The program developed by IReS is adapted to our CdTe/CdZnTe detectors. Different are modelled to research the influence of pixelsize, interpixel space, thickness etc. Bob Dirks bdirks@cea.fr

  16. DSM/DAPNIA Service d’Astrophysique Modelisation Matlab:Weighting field of different geometries… Bob Dirks bdirks@cea.fr

  17. DSM/DAPNIA Service d’Astrophysique Modelisation Matlab:Important parameter for electronics is the charge induction on neighbouring pixels Bob Dirks bdirks@cea.fr Small-pixel effect Slow hole contribution can be ignored! Only e- signal induction: see plot.

  18. DSM/DAPNIA Service d’Astrophysique Modelisation Electronics response The numerical impulse response function of IDeF-X read-out electronics and induced current are used to complete the modelisation Bob Dirks bdirks@cea.fr Not yet implemented…very soon!

  19. DSM/DAPNIA Service d’Astrophysique Questions Questions & Remarks: • When calculating the weighting field for geometries with large interpixel gap, what potential has to be chosen for the area between pixels? • How to increase the resolution? Matlab has a maximum matrix size. • etc. Bob Dirks bdirks@cea.fr

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