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Performance limits of a 55 m m pixel CdTe detector

G.Pellegrini, M. Lozano , R. Martinez, M. Ullan Centro Nacional de Microelectronica, Barcelona, 08193, Spain M. Chmeissani , M. Maiorino, G. Blanchot, J. Garcia C. Puigdengoles Institut de Física d'Altes Energies, UAB Campus, 08193 Bellaterra, Spain.

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Performance limits of a 55 m m pixel CdTe detector

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  1. G.Pellegrini, M. Lozano , R. Martinez, M. Ullan Centro Nacional de Microelectronica, Barcelona, 08193, Spain M. Chmeissani , M. Maiorino, G. Blanchot, J. Garcia C. Puigdengoles Institut de Física d'Altes Energies, UAB Campus, 08193 Bellaterra, Spain Performance limits of a 55mm pixel CdTe detector

  2. Objectives of the research • CdTe is one of the most widely studied material in the field of X-ray detection for energies above 10keV, due to its high absorption efficiency. • CdTe detectors for imaging applications are usually fabricated in thick substrates and with pixel size smaller than 100μm. • However, reducing the pixel size might lead to an increase in the charge shared among neighboring pixels due to the low mobility of the major carriers. • The operation of a pixel detector is strongly influenced by the ratio of the pixel size to the thickness of the detector. • We present charge sharing results from simulation and experimental data obtained with a specific purpose circuitry using Medipix-II with CdTe detectors.

  3. X-ray absorption Mammography General radiography

  4. Charge sharing problem • Charge sharing depends strongly on the position of the interaction of the incident X-ray. • Since detectors are illuminated from the back contact, low energy x-rays will interact “far” from the pixel side. Charges generated in point 1 will be shared between different pixels. Charges generated in point 2 will be collected only in the central pixel. • Mean free path* = 80m for a 20 KeV X-ray in CdTe. 80% of the X-rays interact in the first 120m *Average distance traveled before the interaction take place

  5. 55m 15m 45m CdTe detector • Solder bumps • CdTe pixel electrodes from ACRORAD

  6. MEDIPIX-II chip Single photon counting mode • Chip Specifications • 55 µm pixel pitch • 65536 (256×256) pixels (14.1×14.1 mm active area) • 14 DAC settings • Very good energy resolution (0.75keV) • 1 Mbit data • Serial & Parallel read-out bus (input bus is Serial only) • Maximum estimated communication clock: 80 MHz • Pixel Specifications • 55×55 µm area • Dynamic range: -1 ÷ 8000 • 3-bits (8 levels) threshold adjustment Schematic view of a detector pixel bump bonded to Medipix II chip

  7. X-ray Imaging Image taken with Medipix2 chip with CdTe detector with low temperature bump bonding. Microfocus X-ray tube: 40 keV, 10 µA, 1 sec. Pixel size 55 µm Detector bias –100 V (electron collection)

  8. Experimental setup • 3×3 pixels matrix at the bottom of MPX2 chip (pixels 120,0 to 122,2) • Spectrum 241Am source • Bias voltage :100V • Chip analog outputs, got via DEar-MaMa read-out system, weredigitised and read through GPIB bus • Collected 500 events per bias value • Events analyzed: • values 3s above noise • peak in the central pixel • NOTE: the chip threshold level is NOT involved in this operation • M. Chmeissani et al., [1]”, Proc. of the 21st IEEE Instr. and Meas. Tech. Conf., Vol. 1, pp. 787-791, Como, 18-20 May 2004, Italy, ISBN 0-7803-8249-8

  9. Charge sharing measurements • The maximum charge collected in the central pixel is 60% at 400V.

  10. Software package • Electrical simulators: • 3D simulator : ISE TCAD software • Monte Carlo simulator: Geant4 • Parameters: • Carrier carrier scattering • Recombination • High field saturation • No k-shell effect • No traps added to the model.

  11. Simulation results • Charge collection efficiency is calculated for pixel 5 generating the charge in different points and at various depths.

  12. Integrated charge • Minuit fit Current density profile Charge cloud profile Bias =100 V x-ray generated in point 1 and at a depth of 100um from the back contact

  13. Charge sharing comparison Average relative charge sharing for the 3x3 matrix simulated and measured experimentally. • Simulation results combining DESSIS and Geant4 Monte Carlo simulation. • Experimental result using an Americium source. The detector was biased at –100V.

  14. Conclusions • The data collected with the pixilated CdTe coupled to Medipix-II chip and the simulated results by DESSIS are in good agreement , thus one can use such a model to optimize the design of the pixilated CdTe detector for photon counting readout ASICS. • A 1mm thick CdTe with small pixel pitch will have poor performance when coupled to Photon Counting (PC) ASIC. Either it does not trigger, if the threshold is relatively too high, or many pixels count the same photon if the threshold is very low.

  15. CdTe spectroscopy

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