B. Mikulec* M. Campbell, E. Heijne, X. Llopart, L. Tlustos CERN, Medipix Collaboration * now with...

B. MikulecB. Mikulec**M. Campbell, E. Heijne, X. Llopart, L. TlustosM. Campbell, E. Heijne, X. Llopart, L. Tlustos

CERN, Medipix CollaborationCERN, Medipix Collaboration* now with the University of Geneva, Switzerland* now with the University of Geneva, Switzerland

X-ray ImagingX-ray ImagingUsingUsing

Single Photon ProcessingSingle Photon Processingwithwith

Semiconductor Pixel DetectorsSemiconductor Pixel Detectors

Bettina Mikulec Vertex 2002, 8 Nov. 2002

The Origins...The Origins... High energy physics:

unambiguous reconstruction of particle patterns with micrometer precision

low input noise due to tiny pixel capacitance

WA 97, RD 19 (CERN):

208Pb ions on Pb target7 planes of silicon pixel ladders;1.1 M pixels


Hybrid Pixel DetectorsHybrid Pixel DetectorsElectronics

CMOS technology advances steadily; Moore’s law

Sensors new materials to

increase stopping power and CCE; main problem: inhomogeneities!

1000

10000

100000

1000000

1988 1990 1992 1994 1996 1998 2000 2002

Year

Co

mp

on

ents

/mm

2

OmegaD

Omega3/LHC1

Medipix2

Alice1/LHCb

area penalty forradiation tolerantdesign (x3)


Single Photon ProcessingSingle Photon Processing

Quantum imaging

Example: photon counting

Q has to correspond to a single particle!

if Q > process signal


Quantum Imaging - AdvantagesQuantum Imaging - Advantages Noise suppression

high signal-to-noise ratio; dose reduction low rate imaging applications

Linear and theoretically unlimited dynamic range

Potential for discrimination of strongly Compton scattered photons (for mono-energetic sources) or e.g. fluorescence X-rays

Energy weighting of photons with spectral sources possible

higher dose efficiency; dose reduction


Medical ImagingMedical Imaging

Detector requirements (sensor and electronics) depend on diagnostic X-ray imaging application.

Example: mammography spatial resolution 5-20 lp/mm high contrast resolution (<3%) uniform response patient dose <3 mGy imaging area: 18 x 24 (24 x 30) cm2 compact and easy to handle stable operation no cooling digital cheap

Moore and direct detection

quantum processing

sensors to be improved

high DQE (sensor + q.p.)

to be solved

???


Medipix1 / Medipix2Medipix1 / Medipix2Medipix1

square pixel size of 170 µm 64 x 64 pixels sensitive to positive input

charge

detector leakage current compensation columnwise

one discriminator

15-bit counter per pixel count rate: ~1 MHz/pixel

(35 MHz/mm2)

parallel I/O 1 m SACMOS technology

(1.6M transistors/chip)

Medipix2

square pixel size of 55 µm 256 x 256 pixels sensitive to positive or

negative input charge (free choice of different detector materials)

pixel-by-pixel detector leakage current compensation

window in energy discriminators designed to

be linear over a large range 13-bit counter per pixel count rate: ~1 MHz/pixel

(0.33 GHz/mm2) 3-side buttable serial or parallel I/O 0.25 m technology (33M

transistors/chip)


Medipix1 / Medipix2Medipix1 / Medipix2

IO

Logic

LVDS

Input

LVDS

Output32-bit CMOS Output

256-bit Fast Shift Register

3328

-bit

Pix

el C

olu

mn

-0

13 8-bit DACs

14111 m

3328

-bit

Pix

el C

olu

mn

-1

3328

-bit

Pix

el C

olu

mn

-1

1612

0

mIO

Logic

LVDS

Input

LVDS

Output32-bit CMOS Output

256-bit Fast Shift Register

3328

-bit

Pix

el C

olu

mn

-0

13 8-bit DACs

14111 m

3328

-bit

Pix

el C

olu

mn

-1

3328

-bit

Pix

el C

olu

mn

-1

1612

0

m

12249 m

1390

7 m

the prototype…

the new generation!


Medipix1 ApplicationsMedipix1 ApplicationsExamples: Dental radiography Mammography Angiography Dynamic autoradiography Tomosynthesis Synchrotron applications Electron-microscopy Gamma camera X-ray diffraction Neutron detection Dynamic defectoscopy

General research on photon counting!


ApplicationsApplicationsMammography(INFN Pisa, IFAE Barcelona):

Mo tube 30 kV; Medipix1;part of a mammographicaccreditation phantom

Dynamic Autoradiography:(INFN Napoli):

Medipix1; 14C L-Leucine uptake from the solution into Octopus vulgaris eggs(last slice in time: 80 min)


ApplicationsApplications

Sens-A-Raycommercial dental

CCD system(Regam Medical)

Medipix1

160 Gy 80 Gy 40 Gy

Dental Radiography(Univ. Glasgow, Univ. Freiburg,Mid-Sweden Univ.):


Medipix1 - SNRMedipix1 - SNRPixel-to-pixel non-uniformities:

optimum for counting systems: Poisson limit N

optimum SNR = N / N

determined SNR forMedipix1 taking flood fields(Mo tube) covering the entiredynamic range of the chip:

SNRuncorr(max.) ~30

using a flatfield correction

Medipix1 follows perfectly

the Poisson limit!

SNRuncorr

Red curve =Poisson limit


Medipix1 - SNRMedipix1 - SNR

First measurementpoint after settingthe detector bias!

First measurementpoint after settingthe detector bias!

SNRuncorr

flat field corrects mainlysensor non-uniformities!

8.5 keV 11.7 keV

12.4 keV

with adj. (35V det. bias)

29.8 18.8

without adj. (35V det. bias)

7


19.2


30.7

differences in the raw SNR, but with flat field correction the Poisson limit is ALWAYS reached BUT: flat field correction dependent on energy spectrum! working in over-depletion reduces charge sharing effects

SNRuncorr


Medipix1 Flat Field StudiesMedipix1 Flat Field Studies

raw image

flat field corrected

17 V detector bias(under-depleted)

35 V detector bias(fully depleted)‘waves’ due to

bulk dopingnon-uniformities

wrong flat field;inverse ‘waves’,BUT: single pixelinhomogeneities

smeared out

fixed pattern noise!

2 kinds of non-uniformities: ‘waves’ and fixed pattern noise


Si Wave PatternsSi Wave Patterns

vary detector bias voltage from under- to over-depletion divide flat field map @Vbias with map @100 V

4 V 8 V12 V16 V24 V32 V48 V64 V80 V


Si Wave PatternsSi Wave PatternsSection of the correction map for different detector bias:

‘waves’ move in under-depletion; stable in over-depletion

amplitude decreases with bias, but waves don’t disappear completely

Remark: images can be corrected for these non-uniformities


Dose OptimizationDose Optimization Dose optimization for specific imaging

tasks:example: accumulation of single X-ray signals during X-ray of an anchovy


Summary Medipix1Summary Medipix1 The Medipix1 prototype chip allows to study the

photon counting approach Comparison to charge integrating systems turned

out to be sometimes difficult due to the larger pixel size of Medipix1

Most of the problems encountered were due to sensor non-uniformities (e.g. locally varying leakage currents) and bump-bonding quality

Medipix1 turned out to be a tool to study the attached sensor; even silicon sensors show non-uniformities

The flat field correction was intensively studied and allows to minimize the pixel-to-pixel variations down to the Poisson limit over the full dynamic range of the chip. The energy dependence of the flat field correction has to be further investigated.

The experience with Medipix1 lead to many improvements implemented in the Medipix2 ASIC.


Medipix2 CharacterizationMedipix2 Characterization

all the reported measurements were done using the electronic calibration (injection capacitor + external voltage pulse).

The 8 fF injection capacitor nominal value has a tolerance of 10%.

The dedicated Muros2 readout system had been used

0

100

200

300

400

500

600

700

800

900

1000

5000 5500 6000 6500 7000 7500 8000 8500 9000 9500 10000

e-

Co

un

ts THL

Elec Noise L Elec Noise H

THH



unadjusted thresholds ~500 e- rms

adjusted thresholds~110 e- rms


Medipix2 CharacterizationMedipix2 Characterization Threshold linearity in the low threshold range:

0

2000

4000

6000

8000

10000

12000

14000

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

THL-FBK (V)

e-



•threshold at 2 ke-

•injection of 1000 pulses of 3 ke-

•matrix unmasked


Summary of the Electrical Summary of the Electrical MeasurementsMeasurements

Electron/Hole Collection

Gain ~12 mV/ke-

Non-linearity <3% to 80 ke-

Peaking time <200 ns

Return to baseline <1s for Qin <50 ke-

Electronic noise nTHL ~100 e- nTHH ~100 e-

Threshold dispersion THL ~500 e- THH ~500 e-

Adjusted threshold dispersion THL ~110 e- THH ~110 e-

Minimum threshold ~1000 e-

Analog power dissipation ~8 W/channel at 2.2 V supply


ConclusionsConclusions Miniaturization of CMOS technology allows for small pixel

sizes and increased functionality. A new single photon processing chip Medipix2 consisting

of a 256 x 256 matrix of 55 m square pixels has been produced and successfully characterized.

The potential of quantum imaging for various applications is still far from being fully explored.

Quantum imaging in the medical domain: rather complete systems are required to convince end users MTF and DQE curves as well as comparative phantom images

are necessary for approval (see e.g. FDA) A lot of progress has been made to achieve large areas;

as yet no satisfactory solution for most medical applications

There is a trend in some applications towards object characterization in addition to simple transmission images need energy information

colour X-ray imaging


‘‘Wishlist’Wishlist’sensors: high absorption efficiency and improved homogeneity

reliable ASIC-to-sensor connections

tiling: large areas without dead space

ASIC: small pixel size with charge sharing solutions (modern

CMOS technologies!) low-noise front-end with appropriate sensor leakage current

compensation; sensitive to electron and hole signals very fast front-end for time-resolved studies a precise threshold above noise a multi-bit ADC/pixel for energy information (optimum

weighting!) large dynamic range …???

cost!



vary detector bias voltage from under- to over-depletion calculate corresponding flat field from flood images (1st row) divide with correction map from 100 V detector bias data (2nd row)

a phantastic tool to study sensor inhomogeneities…





unadjusted thresholds ~400 e- rms

adjusted thresholds~110 e- rms

mean ~1100 e- spread ~160 e- rms

Date post:	19-Dec-2015
Category:	Documents
View:	212 times
Download:	0 times

B. Mikulec* M. Campbell, E. Heijne, X. Llopart, L. Tlustos CERN, Medipix Collaboration * now with...

Documents