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)
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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 Electronics Detector
- Large dynamic range dimensional geometry (Pixel)
- Single photon counting High conversion efficiency for g
- Low noise energies in the range keV
- Digital output Good charge collection
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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
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4. Photon Counting Devices Monolithic Pixel Detectors
Material budget (H.E.P.)
fabrication
cost
material choices
efficiency
application specific
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5. Photon counting devices Hybrid Pixel Sensors
separation of detector - r/o
material choice
efficiency
dynamic range
smart pixels
cost
spatial resolution
bump bonding
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6. Hybrid Pixel Detectors
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7. Detection Modes Integration Photon Counting 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
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8. Ω3 ROIC (CERN) Pixel detectors 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
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9. Ω3
500 m
28m
50m
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10. Image - Washer (Al) GaAs - 3 full matrix single column 500m step
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11. Image Quality (II)
Flood image
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12. MTF comparison
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13. detector X-Ray Diffraction Powder Method   2 X-ray beam d d sin
Powder sample
detector
detector
2d sin = n 
Bragg’s law
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14. Silicon Powder (XRD)
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15. Si-XRD (Resolution)
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16. Potassium Niobate (XRD)
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17. Potassium Niobate (XRD)
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18. Potassium Niobate
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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
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20. GaAs detector Image - Objects (Pb) Thickness 600m
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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
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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: 384 ms
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23. Performance of readout
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24. Detector performance Interesting energy range for medicine
keV
[NIST Physical Reference Data]
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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
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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)
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27. Measurement of contrast ratio
Signal Contrast Ratio:
SCR =
Signal to Noise:
SNR=
Distinguishing low contrast objects means:
Earlier recognition of tumours
Reduction of dose to patient
Incident photons
| n - n’| n n n’
Object
Detector
| n - n’| n
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28. Results of Contrast Ratio Measurements
Comparison of Experimental and Measured Contrast Ratios:
Objects with Contrast Ratio of 1.9% can be Identified
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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
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30. Image correction method
Gain map: detector X2:3
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31. Image correction
before
after
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32. Hybridized GaAs pixel detector Sens-A-Ray Si-CCD NSS Toronto 11/11/'98
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33. Source Measurements: 241Am (g-photons of 60 keV)
Steel locking nut; Steel screw
mm thick mm long; 1mm slot
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34. Pixelcell
Layout Photo
170 mm
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35. First Measurement of an Organic Sample with 109Cd source
Fish Tail irradiation g
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36. Measurements with sources: 109Cd (photons of 22 and 25 keV)
Tungsten wire,  500 mm mm thick copper mask, 300 mm
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37. Contrast Measurements
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38. Electrical performance
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39. Source Measurements: 241Am (g-photons of 60 keV)
Steel screw Steel locking nut;
6 mm long; 1mm slot mm thick
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40. Pixel cell
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41. Hybrid Pixel Detector
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42. Simulated response of ERD1 Si pixel detector
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43. Simulated response of Si pixel detector to 137 Cs
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44. Preliminary evaluation of MEDIPIX read-out chip on Glasgow LEC GaAs pixel detector
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