23 march 2005international tpc r&d meeting berkeley 1 readout of a tpc using the medipix2 cmos...

23 March 2005 International TPC R&D Meeting Berkeley

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Readout of a TPC using the Medipix2 CMOS pixel sensor

(detection of single electrons on a direct pixel segmented anode)

Jan Timmermans

NIKHEF - Amsterdam


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NIKHEF Auke-Pieter Colijn Arno AartsAlessandro Fornaini Maximilien

ChefdevilleHarry van der Graaf Sipho van der PuttenPeter KluitJan TimmermansJan Visschers

Saclay CEA DAPNIA Paul ColasYannis GiomatarisArnaud Giganon

Univ. Twente/Mesa+ Jurriaan Schmitz

CERN/Medipix Constm Erik HeijneXavie LlopartMichael Campbell


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Goals• Gas multiplication GEM or Micromegas foil(s)• Charge collection with granularity matching

primary ionisation cluster spread• Needs sufficiently low diffusion gas• dE/dx using cluster counting?

( M. Hauschild)

• Proof of principle based on existing Medipix2 readout chip


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Our GEM-equipped TPCWe have constructed a small test TPC equipped with three GEM foils whichcan be read out by means of the MEDIPIX2 CMOS pixel sensor.The GEM foils were obtained from the CERN/Sauli/GEM group;hole-to-hole distance (hexagonal geometry): 140 µm, hole diameter 85 µm,fiducial surface 100 mm x 100 mm, thickness 50 µm.The drift volume (vol. 100x100x100 mm3) is surrounded by square wire loops,spaced 6.3 mm, put at decreasing potential. Three GEM foils are placed 7.4 mmbehind the plane of the bottom wire loop; the distance between GEM foils is 1.6 mm.The anode plane, at ground potential, is 6.6 mm below the third GEM foil.

http://www.cern.ch/Medipix


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DriftSpace

GEM foils

MediPix CMOS pixel sensorBrass spacer blockPrinted circuit boardAluminium base plate

Medipix2: 256 x 256 pixels

55 m x 55 m

area 14 x 14 mm2

each pixel: low-noise preamp,

discriminator, two threshold DAC,

13-bit counter, communication logic

Drift length: 100 mmDistance between GEMs: 1.6/2.6 mmDistance bottom GEM/MEDIPIX: 6.6 mm


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First events, recorded on March 29, 2003!Drift space irradiated with 55Fe quanta

Not immediately understood. Now we do: conversion source ~0.3 mm, defocussing GEM ~0.5 mm, diffusion in driftspace ~1.5 mm

Ar/CH4 90/10


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exposed 0.01 s exposed 2 s exposed 2 s

exposed 0.1 s 90Sr source; exposed 0.01 s

Fiducial field:14 x 14 mm2

Collected ionisationin 14 x 14 x 100 mm3

during exposure time

Feb 9, 2004

Ar/Isobutane 95/5


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MediPix2 pixel sensorBrass spacer blockPrinted circuit boardAluminum base plate

Micromegas

Cathode (drift) plane

55Fe

Baseplate

Drift space: 15 mm

With Paul Colas & Ioannis Giomataris:MediPix2 & Micromegas

Very strong E-field above (CMOS) MediPix!

S1


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MediPix modified by MESA+, Univ. of Twente, The Netherlands

Pixel Pitch: 55 x 55 μm2

Bump Bond pad: 25 μm octagonal75 % surface: passivation SiNNew Pixel Pad: 45 x 45 μm2

Insulating surface was 75 %Reduced to 20 %


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We always knew, but never saw: the conversion of 55Fe quanta in Ar gas

No source, 1sNo source, 1s5555Fe, 1sFe, 1s

5555Fe, 10sFe, 10s

Friday 13 (!) Feb 2004: signals from a 55Fe source (220 e- per photon); 300 m x 500 m clouds as expected

14 mm

The Medipix CMOS chip facesan electric field of 350 V/50 μm

= 7 kV/mm !!

Ar/Isobutane 95/5


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New trial: March 30 - April 2, 2004try to see single electrons from cosmic muons (MIPs)

•New Medipix

•New Micromegas (holds >500 V over 50 m in …..)

•He/Isobutane 80/20 (gas gain 10,000 – 20,000)

•Pixel preamp threshold: ~3000 e-

•Required gain: 5,000 – 10,000

….. it works !


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He/Isobutane80/20Modified MediPix

31 March 2004


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31 March 2004


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He/Isobutane80/20Non ModifiedMediPix

Amaricium Source

1 April 2004


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31 March 2004


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31 March 2004


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Example of a track reconstructed

This track:

•#hits = 24

•#clusters = 11

•length (3d) = 16.8 mm

•1.4 e-/mm; 0.65 cl./mm On average:•1.8 e-/mm; 0.5 cl./mm


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Detection of single electrons using MediPix2/Micromegas assembly as Direct Pixel Segmented Anode

• NIM A540 (2005) 295 (physics/0409048)

• #pixels hit/mm track length = 1.83

• #clusters/mm track length = 0.52

• Single electron efficiency 0.9

• Moire effects: mismatch in pixel and hole pitch: 60/(60-55) = 12 pixels repetition


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Non Modified Modified

Explanation for Moire effect: pitch Micromegas holes: 60 μm pitch MediPix pixels: 55 μm

Periodic position of hole w.r.t. pixel: repeats after 12 pixels!

Modified

Non Modified

InGrid: perfect alignment of pixels and grid holes!Small pad: small capacitance!


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Recent work (with new Micromegas and new MediPix):

•Threshold studies (equalisation)

•Implementation external trigger (needed new Medipix software and interface box); it works!

•New data taking: much lower single electron efficiency (not yet understood)

•Kept MediPix chip alive for about 4 weeks!

23 March 200523 March 2005 International TPC R&D Meeting BerkelInternational TPC R&D Meeting Berkeleyey

2525

Integrate GEM/Micromegas and pixel sensor

‘GEM’ ‘Micromegas’

By ‘wafer post processing’

InGrid


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Wafer dia.: 100 mm

30 fields with variety of pillar geometry

First trials with pillars look OK

Had some problems with ‘Micromegas’ grid(holes closed with very thin layer)

First ‘good’ Ingrid in Feb. 2005

Reached 170V before breakdown(after that 80V with 4.6μA current)

New design: -thicker grid layer (few microns) -larger spacing between fields -could be ready in few weeks


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INGRID: some first trials

Various pitches, shapes

By ‘wafer post processing’at MESA+, Univ. of Twente

pads

pillars

grid foils


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with pillars

with walls


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TimePix1•Distribute clock to full 256x256 pixel matrix (100-160MHz)

•Enable counting by first hit after ‘shutter’ opens, until ‘shutter’ closes (common stop)

•Dynamic range 214 x 10 ns = 160 μs

•(for the time being) no zero-suppress to remain fully compatible with Medipix2

•Shaping time ~200 ns, might be reduced for TimePix

•Extra static discharge protection for the front-end will be considered

•Keep same chip-size, pixel-size, readout protocol

•Can be done in 5-6 months (if done by MediPix2 designers)


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Further Tests and Developments

• Investigate possible use of CMS (or Atlas) frontend pixel chip

• Ageing tests for….• GOSSIP: very thin gas/pixel detectors as Vertex

Detector (LHC/ILC) (TIMEPIX2 with ~1 ns resolution)• Chip tiling: large(r) detector surfaces (2x2, 2x4 chips)• Through Si connectivity: avoiding bonding wires• Fast readout technology

(~5 Gb/s)


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•MediPix designed at CERN

•4-chip tiling, serial readout and interface: NIKHEF

•20 keV X-ray source at CERN

•Readout single chips 20 frames/sec

•New exp. software 70 frames/sec

•Next version chip several 103 frames/s


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Backup slides


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Top Drifter 5700 V

Bottom Drifter 3473 V

Top GEM 1 2813 V

Bottom GEM 1 2462 V

Top GEM 2 1876 V

Bottom GEM 2 1524 V

Top GEM 3 938 V

Bottom GEM 3 586 V

In the base plate of the chamber, a hole was cut out for the MEDIPIX2 chip: its pixel surfacewas flush with the (anode plane) base plateplane. The MEDIPIX2 chip contains 256 x 256square pixels with pitch 55 µm x 55 µm givinga total fiducial sensitive area of14.08 mm x 14.08 mm. Each pixel is equippedwith a low-noise charge preamp, discriminator,two threshold DACs, a 13-bit counter andcommunication logic.Since a triggering system had not beenimplemented, we operated the MEDIPIX2sensor by enabling the counters manually,and stop the counting after a pre-set timeinterval (0.1 - 10 s). After that, the counts ofeach pixel are read out.

Drift length: 100 mmDistance between GEMs: 1.6/2.6 mmDistance bottom GEM/MEDIPIX: 6.6 mm


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Eff = e-n/G

n: threshold setting (#e-)G: Gas amplification

Single electron efficiency

0.00

0.20

0.40

0.60

0.80

1.00

0 1000 2000 3000 4000

Threshold setting (number of electrons)

Eff

icie

ncy

(-)

G=500

G=1000

g=2000

g=4000

g=8000

Expon. (G=500)

Expon. (G=1000)

Expon. (g=2000)

Expon. (g=4000)

Expon. (g=8000)

single-electron avalanche distribution

0

0.0005

0.001

0.0015

0.002

0 1000 2000 3000 4000

electrons in avalanche

Pro

b(n

)

G=500

G=1000

G=2000

G=4000

G=8000

Expon. (G=500)

Expon. (G=1000)

Expon. (G=2000)

Expon. (G=4000)

Expon. (G=8000)

Prob(n) = 1/G . e-n/G

23 march 2005international tpc r&d meeting berkeley 1 readout of a tpc using the medipix2 cmos...

Documents

small test tpc

defocussing gem

gem foils whichcan

mm2each pixel

cmos medipix

fe source

passivation sinnew pixel

conversion source