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Progress with GaAs Pixel Detectors

Progress with GaAs Pixel Detectors. K.M.Smith University of Glasgow Acknowledgements: RD8 & RD19 (CERN Detector R.&D. collaboration) XIMAGE (Aixtron, I.M.C., Metorex, Freiburg, Glasgow, K.T.H.) MEDIPIX (CERN, Freiburg, Glasgow, Pisa)

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Progress with GaAs Pixel Detectors

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  1. Progress with GaAs Pixel Detectors K.M.Smith University of Glasgow Acknowledgements: RD8 & RD19 (CERN Detector R.&D. collaboration) XIMAGE (Aixtron, I.M.C., Metorex, Freiburg, Glasgow, K.T.H.) MEDIPIX (CERN, Freiburg, Glasgow, Pisa) IMPACT (B.N.F.L., E.E.V., Oxford Instr., R.A.L., Glasgow, Imperial College, Leicester, UMIST) NSS Toronto 11/11/’98 K.M.Smith

  2. Imaging • Requirements • Good 2-dimensional resolution(< 100 µm) • Linear dynamic range for low contrast (< 3%) • Lower dose to Patient/Sample in medical applications • Image processing capability (digital image) Readout ElectronicsDetector - Large dynamic range - 2-dimensional geometry (Pixel) - Single photon counting - High conversion efficiency for g - Low noise energies in the range 5 - 100 keV - Digital output - Good charge collection K.M.Smith

  3. Why Single Photon Counting? • Linear and extendable dynamic range • Energy threshold 1) Compton suppression 2) Large signal-to-noise ratio 3) Insensitive to leakage current • Local threshold tuning ( for each pixel): - can also be used for gain equalisation • Asynchronous counting Minimum dead time K.M.Smith

  4. Photon Counting DevicesMonolithic Pixel Detectors • Material budget (H.E.P.) • fabrication • cost • material choices • efficiency • application specific K.M.Smith

  5. Photon counting devicesHybrid Pixel Sensors • separation of detector - r/o • material choice • efficiency • dynamic range • smart pixels • cost • spatial resolution • bump bonding K.M.Smith

  6. Hybrid Pixel Detectors K.M.Smith

  7. Detection Modes Integration spatial resolution cheap experience dynamic range detection efficiency r/o speed cost (if custom made) charge integration Photon Counting individual particle counting choice of active media detection-r/o separated efficiency dynamic range “smart” pixels spatial resolution bump bonding cost K.M.Smith

  8. Pixel detectors Ω3 ROIC (CERN) • Matrix of 128 rows and 16 columns • Row pitch (depth) = 50 mm • Column pitch (width) = 500 mm • total area = 8 x 6.35 mm2 • ENC ~ 100 e- rms • Individual pixel addressing (mask + test) • Globally adjustable threshold K.M.Smith

  9. Ω3 500 m 28m 50m K.M.Smith

  10. Image - Washer (Al) full matrix single column 500m step GaAs - 3 single column 50 m step K.M.Smith

  11. Image Quality (II) Flood image K.M.Smith

  12. MTF comparison K.M.Smith

  13. X-Ray Diffraction 2d sin = n  Powder Method   2 X-ray beam d d sin detector Powder sample detector Bragg’s law K.M.Smith

  14. Silicon Powder (XRD) K.M.Smith

  15. Si-XRD (Resolution) K.M.Smith

  16. Potassium Niobate (XRD) K.M.Smith

  17. Potassium Niobate (XRD) K.M.Smith

  18. Potassium Niobate K.M.Smith

  19. MEDIPIX • A true single photon counting readout chip • 64 x 64 pixel matrix • pixel dim. 170 x 170 m2 • Sensitive area 1 cm2 • Individually adjust threshold • 15-bit counter • Frame r/o 384 s at 10MHz K.M.Smith

  20. Image - Objects (Pb) GaAs detector Thickness 600m K.M.Smith

  21. Read-out Electronics Photon Counting Chip (PCC): based on ideas developed by the RD19 collaboration (CERN) • SACMOS 1mm FASELEC Technology • Matrix of 64 x 64 Pixels • Pixel size 170 mm x 170 mm • 1.2 cm2 sensitive area • 1.7 cm2 total area • 1.6 M transistors K.M.Smith

  22. Pixel Design • Charge sensitive amplifier with leakage current compensation • Discriminator with globally settable threshold • 3-bit local threshold adjustment • Individual pixel test and mask modes • Counting controlled by shutter signal • 15-bit pseudo-random counter • 16-bit I/O Bus • Readout frequency: max. 10 MHz • Readout time: 384ms K.M.Smith

  23. Performance of readout K.M.Smith

  24. Detector performance Interesting energy range for medicine 10 - 100 keV [NIST Physical Reference Data] K.M.Smith

  25. Detector design • Material: GaAs, S.I., 200 mm thick • 64 x 64 pixel matrix • square pixels of 170 x 170 mm2 • 1.2 cm2 sensitive area K.M.Smith

  26. Electrical performance of system System = detector flip-chip bonded to readout chip Bonded detector settings: • min. mean threshold: ~2000 e- • trimmed threshold rms: ~125 e- • noise: ~200 e- (Note: a photon of 20 keV produces about 4700 e- in GaAs) K.M.Smith

  27. Measurement of contrast ratio Object | n - n’| n Detector n’ n | n - n’| n Incident photons Signal Contrast Ratio: SCR = Signal to Noise: SNR= Distinguishing low contrast objects means: • Earlier recognition of tumours • Reduction of dose to patient K.M.Smith

  28. Results of Contrast Ratio Measurements Comparison of Experimental and Measured Contrast Ratios: Objects with Contrast Ratio of 1.9% can be Identified K.M.Smith

  29. Summary and Conclusions • Successful bump-bonding of 64 x 64 pixel array to ROIC • Measured threshold of ~2000 e- with ~125 e- rms • Images of a variety of objects illuminated with 241Am- and 109Cd-sources • Correctly identified objects of low contrast (1.9 %) • The system enables the evaluation of the potential and limitations of the photon counting method K.M.Smith

  30. Image correction method Gain map: detector X2:3 K.M.Smith

  31. Image correction before after K.M.Smith

  32. Hybridized GaAs pixel detector Sens-A-Ray Si-CCD K.M.Smith

  33. Source Measurements: 241Am (g-photons of 60 keV) Steel locking nut; Steel screw 300 - 500 mm thick 6 mm long; 1mm slot K.M.Smith

  34. Pixelcell Layout Photo 170 mm K.M.Smith

  35. First Measurement of an Organic Sample with 109Cd source g Fish Tail irradiation K.M.Smith

  36. Measurements with sources: 109Cd (photons of 22 and 25 keV) Tungsten wire,  500 mm 300 mm thick copper mask, 300 mm K.M.Smith

  37. Contrast Measurements K.M.Smith

  38. Electrical performance K.M.Smith

  39. Source Measurements: 241Am (g-photons of 60 keV) Steel screw Steel locking nut; 6 mm long; 1mm slot 300 - 500 mm thick K.M.Smith

  40. Pixel cell K.M.Smith

  41. Hybrid Pixel Detector K.M.Smith

  42. Simulated response of ERD1 Si pixel detector K.M.Smith

  43. Simulated response of Si pixel detector to 137 Cs K.M.Smith

  44. Preliminary evaluation of MEDIPIX read-out chip on Glasgow LEC GaAs pixel detector K.M.Smith

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