A noiseless, kHz frame rate, imaging detector base on MCPs readout with a Medipix2

SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate

Jason McPhate, John Vallerga, Anton Tremsin and Oswald Siegmund

Space Sciences Laboratory, University of California, Berkeley

Bettina Mikulec and Allan ClarkUniversity of Geneva

A noiseless, kHz frame rate, imaging detector base on

MCPs readout with a Medipix2


WFS detector for future AO systems*

• kHz frame rates– Match atmospheric timescales

• Many pixels - eventually 512 x 512– More subapertures and more pixels per subaperture

• Very low readout noise (< 3 e-)– Lower penalty for more pixels per subaperture

• High (~80%) optical QE– Use dimmer guide stars or higher frame rates

*Angel et al., “A Road Map for the Development of Astronomical AO”


Imaging, Photon Counting DetectorsCharge distribution on stripsCharge CloudMCP stackTube Window withphotocathodeγ

Photocathode converts photon to electron

MCP(s) amplify electron by 104 to 108

Rear field accelerates electrons to anode

Patterned anode measures charge centroid,Count stored in digital histogram


Why would you want one?• No readout noise penalty

– Use as many pixels as you wish

• Continuous temporal sampling to ~ nsecs– Choose integration period(s) after the fact or on the

fly

• Other advantages– Large area, curved focal planes– Cosmic ray = 1 count– LN2 not required – Low dark current (0.16 attoamps cm-2)


What’s the Catch?• Global Counting Rates

– 1000 Shack-Hartmann spots per WFS– Kilohertz feedback rates– 1000 counts per spot for sub-pixel centroids

1 Gigahertz counting rate!

• Quantum Efficiency– Historically Optical Photocathodes < ~15%– Silicon devices (CCDs) can get ~90%– Noiseless helps, but not that much

Requires integrating detector

Requires GaAs Photocathode


Our AO detector conceptAn optical imaging tube

using:

0

10

20

30

40

50

60

200 400 600 800 1000

Bialkali (Hamamatsu)

Extended S25 (Hamamatsu)

Extended S25 (Photonis)

GaAs (ITT)

Quantum Efficiency (%)

Wavelength (nm)

• GaAs photocathode• MCPs to amplify to ~104

• Medipix2 ASIC readout


Medipix2 ASIC Readout Pixelated readout for x and gamma ray semiconductor

sensors (Si, GaAs, CdTe etc)

Developed at CERN for Medipix collaboration

55 µm pixel @ 256 x 256 (abutable to 512 x [n x 256]).

Pixel level amp, discriminator, gate & counter.

Counts integrated at pixel No charge transfer!

14mm

16mm

Applications: Mammography, dental radiography, dynamic autoradiography, gamma imaging, neutron imaging, angiography, x-ray diffraction, dynamic defectoscopy, etc.


Readout Architecture35

84 b

it P

ixel

Col

umn

0

3584

bit

Pix

el C

olum

n 25

5

3584

bit

Pix

el C

olum

n 1

256 bit fast shift register

32 bit CMOS output LVDS out

• Pixel values are digital (14 bit)

• Bits are shifted into fast shift register

• Choice of serial or 32 bit parallel output

• Maximum designed bandwidth is 100MHz

• Corresponds to 286µs frame readout in parallel


First test detector

• Demountable detector• Simple lab vacuum (~10-7 Torr)• UV sensitive, no photocathode


Lab Detector Lessons

• Medipix ASIC works well as MCP readout

• Sub-pixel centroiding of Shack-Hartmann like spots was achieved

• Optimized parameters for use in optical tube– Chevron stack of 10 µm pore MCPs (protect cathode from ion feedback)– MCP gain of about 104 (longer tube life and higher counting rates)– MCP to Medipix gap of 300 to 500 µm (Medipix wirebond clearance)– Approximately 1600 V rear field (minimize MCP charge cloud spread)


Vacuum Tube Design

No GaAs capability at UCB

So GaAs photocathode by industrial vendor:

Means using “standard” size tube

Only marginally larger than the Medipix2 device


Thick Film Ceramic Header

• Internal mounting/GND surface for Medipix

• Route ~60 Medipix signals out of vacuum

• Multi-layered to better match Medipix pitch

• Maintain hermetic seal of tube to ≤10-9 Torr

• Provide land pads for external I/F connectors


Vacuum Tube Design


Medipix on a Header


MCP/Medipix Serial I/F Board


Vacuum Tube Design


Parallel Readout Design

• Development by ESRF• 1 to 5 Medipix2 chips• FIFO for each chip• Flat field, deadtime

corrections• Optional centroid

calculation• High speed serial out


Future Work (3 yr. NOAO grant)

• Seal a MCP/Medipix tube with a GaAs photocathode

• Perhaps a multi-alkali photocathode tube (@UCB)

• Finalize and build parallel readout

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

• Test at telescope


Acknowledgements

• Univ. of Barcelona• University of Cagliari• CEA• CERN• University of Freiburg• University of Glasgow• Czech Academy of Sciences• Mid-Sweden University

• University of Napoli• NIKHEF• University of Pisa• University of Auvergne• Medical Research Council• Czech Technical University• ESRF• University of Erlangen-Nurnberg

Thanks to the Medipix Collaboration:

This work was funded by an AODP grant managed by NOAO and funded by NSF

A noiseless, kHz frame rate, imaging detector base on MCPs readout with a Medipix2

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