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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

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
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


Why single photon counting
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


Photon counting devices monolithic pixel detectors
Photon Counting DevicesMonolithic Pixel Detectors

  • Material budget (H.E.P.)

  • fabrication

  • cost

  • material choices

  • efficiency

  • application specific

K.M.Smith


Photon counting devices hybrid pixel sensors
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



Detection modes
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


Pixel detectors
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


Ω3

500 m

28m

50m

K.M.Smith


Image washer al
Image - Washer (Al)

full matrix

single column 500m step

GaAs - 3

single column 50 m step

K.M.Smith


Image quality ii
Image Quality (II)

Flood image

K.M.Smith


Mtf comparison
MTF comparison

K.M.Smith


X ray diffraction
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







Medipix
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


Image objects pb
Image - Objects (Pb)

GaAs detector

Thickness

600m

K.M.Smith


Read out electronics
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


Pixel design
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



Detector performance
Detector performance

Interesting energy range for medicine

10 - 100 keV

[NIST Physical Reference Data]

K.M.Smith


Detector design
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


Electrical performance of system
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


Measurement of contrast ratio
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


Results of contrast ratio measurements
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


Summary and conclusions
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


Image correction method

Gain map: detector X2:3

K.M.Smith


Image correction

before

after

K.M.Smith


Hybridized

GaAs

pixel detector

Sens-A-Ray

Si-CCD

K.M.Smith


Source measurements 241 am g photons of 60 kev
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


Pixelcell
Pixelcell

Layout Photo

170 mm

K.M.Smith


First measurement of an organic sample with 109 cd source
First Measurement of an Organic Sample with 109Cd source

g

Fish Tail irradiation

K.M.Smith


Measurements with sources 109 cd photons of 22 and 25 kev
Measurements with sources: 109Cd (photons of 22 and 25 keV)

Tungsten wire,  500 mm 300 mm thick copper mask, 300 mm

K.M.Smith




Source measurements 241 am g photons of 60 kev1
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


Pixel cell
Pixel cell

K.M.Smith






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