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

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 - 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 NSS Toronto 11/11/'98 K.M.Smith

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 NSS Toronto 11/11/'98 K.M.Smith

Photon Counting Devices Monolithic Pixel Detectors Material budget (H.E.P.) fabrication cost material choices efficiency application specific NSS Toronto 11/11/'98 K.M.Smith

Photon counting devices Hybrid Pixel Sensors separation of detector - r/o material choice efficiency dynamic range smart pixels cost spatial resolution bump bonding NSS Toronto 11/11/'98 K.M.Smith

Hybrid Pixel Detectors NSS Toronto 11/11/'98 K.M.Smith

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 NSS Toronto 11/11/'98 K.M.Smith

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 NSS Toronto 11/11/'98 K.M.Smith

Ω3 500 m 28m 50m NSS Toronto 11/11/'98 K.M.Smith

Image - Washer (Al) GaAs - 3 full matrix single column 500m step NSS Toronto 11/11/'98 K.M.Smith

Image Quality (II) Flood image NSS Toronto 11/11/'98 K.M.Smith

MTF comparison NSS Toronto 11/11/'98 K.M.Smith

detector X-Ray Diffraction Powder Method   2 X-ray beam d d sin Powder sample detector detector 2d sin = n  Bragg’s law NSS Toronto 11/11/'98 K.M.Smith

Silicon Powder (XRD) NSS Toronto 11/11/'98 K.M.Smith

Si-XRD (Resolution) NSS Toronto 11/11/'98 K.M.Smith

Potassium Niobate (XRD) NSS Toronto 11/11/'98 K.M.Smith

Potassium Niobate (XRD) NSS Toronto 11/11/'98 K.M.Smith

Potassium Niobate NSS Toronto 11/11/'98 K.M.Smith

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 NSS Toronto 11/11/'98 K.M.Smith

GaAs detector Image - Objects (Pb) Thickness 600m NSS Toronto 11/11/'98 K.M.Smith

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 NSS Toronto 11/11/'98 K.M.Smith

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 NSS Toronto 11/11/'98 K.M.Smith

Performance of readout NSS Toronto 11/11/'98 K.M.Smith

Detector performance Interesting energy range for medicine 10 - 100 keV [NIST Physical Reference Data] NSS Toronto 11/11/'98 K.M.Smith

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 NSS Toronto 11/11/'98 K.M.Smith

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) NSS Toronto 11/11/'98 K.M.Smith

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 NSS Toronto 11/11/'98 K.M.Smith

Results of Contrast Ratio Measurements Comparison of Experimental and Measured Contrast Ratios: Objects with Contrast Ratio of 1.9% can be Identified NSS Toronto 11/11/'98 K.M.Smith

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 NSS Toronto 11/11/'98 K.M.Smith

Image correction method Gain map: detector X2:3 NSS Toronto 11/11/'98 K.M.Smith

Image correction before after NSS Toronto 11/11/'98 K.M.Smith

Hybridized GaAs pixel detector Sens-A-Ray Si-CCD NSS Toronto 11/11/'98 K.M.Smith

Source Measurements: 241Am (g-photons of 60 keV) Steel locking nut; Steel screw 300 - 500 mm thick 6 mm long; 1mm slot NSS Toronto 11/11/'98 K.M.Smith

Pixelcell Layout Photo 170 mm NSS Toronto 11/11/'98 K.M.Smith

First Measurement of an Organic Sample with 109Cd source Fish Tail irradiation g NSS Toronto 11/11/'98 K.M.Smith

Measurements with sources: 109Cd (photons of 22 and 25 keV) Tungsten wire,  500 mm 300 mm thick copper mask, 300 mm NSS Toronto 11/11/'98 K.M.Smith

Contrast Measurements NSS Toronto 11/11/'98 K.M.Smith

Electrical performance NSS Toronto 11/11/'98 K.M.Smith

Source Measurements: 241Am (g-photons of 60 keV) Steel screw Steel locking nut; 6 mm long; 1mm slot 300 - 500 mm thick NSS Toronto 11/11/'98 K.M.Smith

Pixel cell NSS Toronto 11/11/'98 K.M.Smith

Hybrid Pixel Detector NSS Toronto 11/11/'98 K.M.Smith

Simulated response of ERD1 Si pixel detector NSS Toronto 11/11/'98 K.M.Smith

Simulated response of Si pixel detector to 137 Cs NSS Toronto 11/11/'98 K.M.Smith

Preliminary evaluation of MEDIPIX read-out chip on Glasgow LEC GaAs pixel detector NSS Toronto 11/11/'98 K.M.Smith