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/’98K.M.Smith

NSS Toronto 11/11/'98K.M.Smith Imaging 3 Requirements 0 Good 2-dimensional resolution(< 100 µm) 0 Linear dynamic range for low contrast (< 3%) 0 Lower dose to Patient/Sample in medical applications 0 Image processing capability (digital image) Readout Electronics Detector - Large dynamic range - 2-dimensional geometry (Pixel) - Single photon counting - High conversion efficiency for  - Low noise energies in the range keV - Digital output - Good charge collection

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

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

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

NSS Toronto 11/11/'98K.M.Smith Pixel detectors Ω3 ROIC (CERN)  Matrix of 128 rows and 16 columns –Row pitch (depth) = 50  m –Column pitch (width) = 500  m –total area = 8 x 6.35 mm 2  ENC ~ 100 e - rms  Individual pixel addressing (mask + test)  Globally adjustable threshold

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

NSS Toronto 11/11/'98K.M.Smith Image - Washer (Al) full matrix single column 500  m stepsingle column 50  m step GaAs -  3

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

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

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

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

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

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

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

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

NSS Toronto 11/11/'98K.M.Smith MEDIPIX  A true single photon counting readout chip  64 x 64 pixel matrix  pixel dim. 170 x 170  m 2  Sensitive area 1 cm 2  Individually adjust threshold  15-bit counter  Frame r/o 384  s at 10MHz

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

NSS Toronto 11/11/'98K.M.Smith Read-out Electronics Photon Counting Chip (PCC): based on ideas developed by the RD19 collaboration (CERN)  SACMOS 1  m FASELEC Technology * Matrix of 64 x 64 Pixels  Pixel size 170  m x 170  m * 1.2 cm 2 sensitive area * 1.7 cm 2 total area * 1.6 M transistors

NSS Toronto 11/11/'98K.M.Smith Pixel Design 0 Charge sensitive amplifier with leakage current compensation 0 Discriminator with globally settable threshold 0 3-bit local threshold adjustment 0 Individual pixel test and mask modes 0 Counting controlled by shutter signal 0 15-bit pseudo-random counter 0 16-bit I/O Bus 0 Readout frequency: max. 10 MHz  Readout time: 384  s

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

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

NSS Toronto 11/11/'98K.M.Smith Detector design  Material: GaAs, S.I., 200  m thick * 64 x 64 pixel matrix  square pixels of 170 x 170  m 2 * 1.2 cm 2 sensitive area

NSS Toronto 11/11/'98K.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/'98K.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’ Object Detector | n - n’| n n

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

NSS Toronto 11/11/'98K.M.Smith Summary and Conclusions 4 Successful bump-bonding of 64 x 64 pixel array to ROIC 4 Measured threshold of ~2000 e - with ~125 e - rms 4 Images of a variety of objects illuminated with 241 Am- and 109 Cd-sources 4 Correctly identified objects of low contrast (1.9 %) 4 The system enables the evaluation of the potential and limitations of the photon counting method

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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