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A novel high resolution, A novel high resolution, high frame rate detectorhigh frame rate detector

based on abased on amicrochannel plate read out with the microchannel plate read out with the

Medipix2 counting CMOS pixel chipMedipix2 counting CMOS pixel chip

Bettina Mikulec, Allan ClarkUniversity of Geneva

John Vallerga, Jason McPhate, Anton Tremsin, Oswald Siegmund

Space Science Laboratory, University of California

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Introduction*Introduction*

• Turbulence in the earth’s atmosphere makes stars twinkle

• More importantly, turbulence spreads out the star light making it a blob rather than a point

Even the largest ground-based astronomical telescopes have no better resolution than an 8"

telescope!

Even the largest ground-based astronomical telescopes have no better resolution than an 8"

telescope!

*adapted from AO lectures of Claire Max, Astro289C, UC Santa Cruz

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Adaptive OpticsAdaptive Optics

blur Point focus

Parallel light raysLight rays affected by turbulence

proposal for anew WFS -

Optical Medipix tube

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Example for the enormous improvements using AO(Lick Observatory).

Adaptive OpticsAdaptive Optics

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5Bettina Mikulec

• Determine the distortions with the help of a natural or laser guide star and a lenslet array (one of the methods). Deviations of the spot positions from a perfect grid is a measure for the shape of the incoming wave-front.

Adaptive OpticsAdaptive Optics

Shack-Hartmann wavefront sensor

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Wavefront Sensor RequirementsWavefront Sensor Requirements

• High QE for dimmer guide stars (~80% optical QE)o Many pixels in the order of 512 x 512; future large

telescopes will have about 5000 actuators (controlled via 70 x 70 centroid measurements)

• 1000 photons per spot to get a 3% centroid rms error with respect to the stellar image size.

o 1 kHz frame rate (light integration, readout, calculations, send out 5000 signals and ready for new frame) corresponding to the timescale of the atmospheric turbulences

o Very low readout noise (< 3e-)• Gate the detector in 2-4 s range for operation with laser

guide starsLarge pixel array, high frame rate and no readout noise

not simultaneously achievable with CCDs!

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2 µm pores on 3 µm centers (Burle Industries)

Proposal for a New Wavefront SensorProposal for a New Wavefront Sensor

High-QE GaAs photo-cathode Matched pair of microchannel plates (MCP) with 10 m

pore diameter in chevron configuration Medipix2 counting CMOS pixel chip Noiseless chip readout

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The Medipix2 Photon Counting ChipThe Medipix2 Photon Counting Chip

0.25 m CMOS technology (33M transistors/chip)square pixel size of 55 µm 256 x 256 pixelssensitive to positive or negative input charge (free choice of different detector materials)pixel-by-pixel detector leakage current compensation window in energydiscriminators designed to be linear over a large range14-bit counter per pixel count rate: ~1 MHz/pixel (0.33 GHz/mm2)3-side buttableserial or parallel I/O (min. readout time of full matrix 266 µs)

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Measurement SetupMeasurement Setup

• A Medipix2 photon counting chip • A matched pair of MCPs:

– Photonis MCPs with 33 mm diameter– 10 m hole diameters, L/D = 40/1– low resistivity (~22 MOhms per plate)– gain was varied between 20k and 200k (1430 - 1680 V)

• Vacuum tank pumped down to ~10-6 torr• Hermetic feed-throughs (50-pin connector for Medipix

signals)• A standard UV Hg pen-ray lamp with collimator (~10

counts/s -500M counts/s)

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Feasibility TestsFeasibility Tests

single photonevents

gain 106, rear field 427 V gain 50k, rear field 980 V

It works!It works!

06 April 2004

• Event size function of MCP gain, rear field, MCP-Medipix distance and Medipix threshold

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Flood FieldsFlood Fields

• Take image with collimated UV source at 50ke gain and 1600 V rear field (~5000 counts/pixel). Average single spot area: 2.4 pixels

– Fixed pattern noise from dead spots on the MCPs and MCP multifibres divides out.

take 2 independent uniform illuminations(flood fields at ~500Mcps)

Histogram of ratio consistent with countingstatistics (rms 0.02)

Ratio = flood1 / flood2.

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ResolutionResolution

• The Air Force test pattern was used to demonstrate the imaging properties of the detector, in particular the resolution.

increaseshuttertime

100 s exposure; the spotscorrespond to individualphoton events.

1 s exposure. Group 3-2 visible (~9 lp/mmcorresponding to the Nyquistlimit of 55 m pixels)

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Event CentroidingEvent Centroiding

• Centroiding individual photon events to achieve sub-pixel resolution:– Take many very low count rate images with larger spot area

to avoid overlapping spots. (~100-150 counts/frame; 1000 frames)

– Identify unique spots and reject overlapping events (counts 2), count spots, record their size and calculate the centroids.

Could be useful forlow rate imaging applications!

centroiding

Group 4-2 starts to be resolved(17.95 lp/mm; 55.7 m corresponding to ~28 m pixels).

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UV Photon Counting MovieUV Photon Counting Movie

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

Air Force resolution mask, 100 ms exposures

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Electron DetectionElectron Detection

• First test results with beta sources

QE ~46% for Ni and ~63% for the Tl image;increasing efficiency with e- energies above ~50 keV consistent with literature.

63Ni, 67 keV max.~300 counts/pixel

Gain ~60k,rear field 1600 VMedipix threshold~38 ke-

204Tl, 764 keV max.~100 counts/pixel

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ConclusionsConclusions

• New detector concept proven to work!• Systematic tests varying different detector

parameters underway• No fixed pattern noise yet detectable except

MCP imperfections• Resolution at Nyquist limit and below (for event-

by-event centroiding) demonstrated• Images presented with both UV and electron

sources detector has a great capacity to be used for various wavelengths and particles

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Future PlansFuture Plans

• Tube fabrication at SSL Berkeley and at commercial firm; finalize ceramic chip carrier design

• Specific parallel readout board to be designed in collaboration with ESRF; reduce output bandwidth by using an FPGA; goal: 1 kHz continuous frame rate with 2x2 chip arrangement

• Test prototype tubes at the AO laboratory at CFAO, U.C. Santa Cruz

• Final test at a telescopeSSL received 3-year NOAO grant, 2 more years to go…

• Are there other applications for such a detector?– Beam monitor for hadron therapy, readout for Cherenkov

counter, detector for X-ray microscopes at synchrotron…???

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Backup Slides!

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The Setup at SSL - PhotosThe Setup at SSL - Photos

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Soft X-Ray PhotocathodesSoft X-Ray Photocathodes

0

20

40

60

80

100

0.1 1

CsBr

KI

Energy (keV)

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EUV and FUVEUV and FUV

0

0.1

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0 500 1000 1500 2000

CsI 1985 vs 1999

CsI 1985 30°CsI 1985 20°CsI #3 2/99 20°CsI #3 2/99 30°CsI #2 1/99 20°CsI #2 1/99 30°

Wavelength (Å)

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GaN UV Photocathodes, 1000- GaN UV Photocathodes, 1000- 4000Å4000Å

0.1

1

10

100

150 200 250 300 350 400

NW-BH071#3

NW-BH071#2

NW-JG238#3

NW-JG238#2

Quantum Efficiency (%)

Wavelength (nm)

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hBright st

ars + 0

= 1% sk

y

coverage

Isoplanatic Angle (Isoplanatic Angle (00) & Sky Coverage) & Sky Coverage

TelescopePrimarymirror

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Laser Guide StarsLaser Guide Stars

Can achieve>70% sky

coverage withlaser guide staradaptive optics!

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Laser Guide Star ParallaxLaser Guide Star Parallax

d

L

589.2 nm

• “Star” more of a streak• Shape changes over pupil• Can use pulsed laser to limit

spatial extent• Requires gated detector

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Advantages of Multi-Pixel Sampling of Shack Advantages of Multi-Pixel Sampling of Shack Hartmann SpotsHartmann Spots

Linear response off-nullInsensitive to input widthMore sensitive to readout noise

2 x 22 x 2 5 x 55 x 5

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Deformable MirrorsDeformable Mirrors

• Range from 13 to > 900 actuators (degrees of freedom)

Xinetics~ 50 mm

~ 300mm

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30 m diameter:– California Extremely Large Telescope (CELT) -– Thirty Meter Telescope (TMT)

50 m diameter:– EURO50 on La Palma

100 m diameter:– European Southern Observatory’s “OverWhelmingly

Large Telescope” (OWL)

All propose AO systems with > 5000 actuators

Next Generation of Large Telescopes (proposed)Next Generation of Large Telescopes (proposed)

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collection of secondary electrons (SE) emitted by 0.1 – 0.4 m aluminum (Al-Al2O3-Al) foils

vacuum chamber

HV

secondary emission foil

electron detector

PROFILE/CURRENT MEASUREMENT

hadron beam

e beam

(G. Molinari, CERN)

Electron DetectionElectron Detection

• One example for different application than AO:Beam monitor for hadron therapy

slide from W. Dulinsky, presented atIWORID2004 Glasgow; SUCIMA project.

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Sub-Pixel Spatial LinearitySub-Pixel Spatial Linearity

LampPinhole

Detector

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Average Movement of 700 SpotsAverage Movement of 700 Spots

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

0 5 10 15 20 25

Lamp Position (mm)

Centroid Position (µm)

Delta X

Delta Y

1 pixel

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Position Error (550 Events/Spot) Position Error (550 Events/Spot)

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50

-20 -15 -10 -5 0 5 10 15 20

Centroid difference (microns)

Number of centroids

rms = 2.0 µm

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Spot SizeSpot Size

Spot Area vs Rear Field

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0 200 400 600 800 1000 1200 1400 1600

Rear Field (V)

Spot Area (pixel)

Gain 25k

Gain 50k

Gain 100k

Gain 200k

Gain 400k

Rear Field = 1600V

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2

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Lower Threshold (ke-)

Mean Spot Area (pixel)

G=20k, Area

G=50k, Area

G=100k, Area

G=200k, Area

Spot area versus rear field. Spot area versus Medipix2 low threshold.

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X-ray of FishX-ray of Fish

QuickTime™ and aCinepak decompressor

are needed to see this picture.

(… with silicon detector)