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The pixel readout of TPCs. Max Chefdeville, NIKHEF, Amsterdam. Overview. Motivations 2D readout of small gas volume by means of 55 µm 2 pixels in combination with Micromegas & GEMs Time information (and more) with the new TimePix chip. wire. Cathode pads. GEM. Micromegas. - PowerPoint PPT Presentation
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1
The pixel readout of TPCs
Max Chefdeville, NIKHEF, Amsterdam
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Overview
• Motivations
• 2D readout of small gas volume by means of 55 µm2 pixels in combination with Micromegas & GEMs
• Time information (and more) with the new TimePix chip
3
Motivations for pixel readout of TPCs
• Spatial resolution:– Narrow charge distribution (RMS ~15 μm)– No c.o.g calculation possible– σxy limited by the pad size (pitch/√12)
• Fine granularity– δ-ray recognition/suppression in TPC– Direction of low-energy e- for X-ray polarimetry– Directionality of nuclear recoils in WIMP or
neutrino interactions– Energy & direction of 2 e- from double beta decay– Energy of photo-electrons from axion conversions
wire
Cathode pads
GEMMicromegas
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• High granularity pitch/√12 ~ 15 µm– Potentially better spatial resolution
• Smaller input noise (Cin ~ 15 fF)– Lower gain, less aging, smaller ion backflow– Small charge sensitivity
Single electron detection with an efficiency depending on the gain and the amplification structures
• May be possible to count primary clusters– dE/dx for TPC
• Possible to count primary electrons– Accurate energy measurement of low energy recoils & electrons
The pixel readout of TPCs
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The Medipix2 chip
Use the “naked” chip as the detector anode
• Developed by the Medipix consortium, CERN
• Chip layout:– 1.4 x 1.6 cm2 area– 256 x 256 pixel matrix– 55 x 55 µm2 pixels
• On each pixel:– Preamp. + shaper– 2 discri. (thresholds)– 14 bit counter
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MediPix2 pixel sensorBrass spacer blockPrinted circuit boardAluminum base plate
Micromegas
Cathode (drift) plane
Baseplate
Drift space: 15 mm
Medipix2 & MicromegasNIKHEF
SACLAY
TWENTE
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He/Isobutane80/20
δ-ray!
Efficiency fordetecting single
electrons:> 90 %
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Helium VS Argon mixtures
• Argon: larger primary statistic & transverse diffusion
He 20% iC4H10 Ar 20% iC4H10
Larger diffusion
Top of track
Interesting tool to study ionization statistic of photons & charged particles …
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Integrate amplification and readout structures:
InGrid
Walls Pillars
Wafer post-processing
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InGrid, an integrated Micromegas• Low temperature process:
spin coating, wet etching;
• Perfect alignment between grid holes and pixel pads;
• No dead areas due to pillars;
• Flexible design.
13 % FWHM @ 5.9 keV
30 µm Ø pillar
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InGrid on a Medipix2 chip
• Goal: post-process full wafers
• Post-processing of individual chips @ Twente University, Netherlands
InGrid on top of pixel matrix
pillarspixels
Grid hole centered between 4 pixels
First trials promising (using rejected chips)
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The spark issue
Detector very sensitive to gas discharges
Pixel InGrid mesh (Micromegas OK)
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Discharge protections
• destructive discharge
• Proposals:– multi-stage amplification;– high R coating of anode.
Discharge signals of 2 Micromegas detectors
e- multiplication @ high E
e- extraction @ low E
Current attenuated by the high R layer
Maybe enough to protect the chip… Both are being applied on Medipix2
Un-coated anode
Coated anode
TwinGrid
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Micromegas & GEMs
With GEM, decoupling of amplification and readout:– Smaller charge per pixel, smaller single
electron detection efficiency– Low field above the chip, no gas discharges
involving the chip
80 kV/cm 50 µm
40 kV/cm 50 µm
1 kV/cm 1000 µm
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GEMs & MediPix2 @ DESYBONN
FREIBURG
6 mm
2 mm
2 mm
1 mm
5 GeV/c electron beam
Si telescope
Si-telescope serves as track reconstruction in the drift volume, e. g. for the determination of the drift velocity and the σ0 near top GEM
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GEMs & MediPix2 @ DESY• Gas mixture: Ar CO2 70/30
~30 clusters per cm created, ~10 cluster/cm reconstructedLess primary information due to diffusion in the amplification gap
• Tracks parallel to the pixel plane: same diffusion along the track
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The TimePix chip
• Based on MediPix2 design– Same dimensions and readout protocol– Replace 14 bit hit counter by TDC– Only low threshold– Fired pixels count clock pulses 100 MHz
• Counting modes (can be mixed)– “Time over threshold” Charge info.– “Common stop” Time
info.
• Characteristics:– Dynamic range, 160 µs @ 100 MHz– Integration time, 200 ns
11 22 44
55
66
11 22 44
55
66
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55μm
55μm
prea
mp/
shap
er
Low
th
resh
old
Hig
h th
resh
old
Con
trol
logi
c
MediPix2 → TimePix
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Counting modes (simulations)
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time over thresholdfrom hit till end of shutter time
100 MHz clock
not detected
detected
Charge summed
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Time over threshold events
TOT linear above ~ 4 ke-
Better calibration to come…
> 30 ke- per pixels
• Ar CO2 70/30
• No time information (2D)
• Charge information should improve spatial resolution
0
10
20
30
40
0 200 400 600TOT-Counts
En
erg
y [K
e]
TOT counts
N e
-
Freiburg Bonn
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“Common stop” (Time) mode• Every fired pixel counts till the end of a 12 μs shutter window• Tracks parallel to the pixel plane (same color)
The larger the number of counts, the shorther the drift time
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Mixed Mode operation
• Consecutive pixels have Time and TOT assignment and are here separated via mapping onto a 181x181 matrix
• Benefit from charge & time information
• Interesting for double track separation
Ar
CO
2 70
/30
He
CO
2 70
/30
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Outlook• Pixel readout
– Proof of principle of pixel readout with MediPix2 demonstrated– TimePix works and opens the way to 3D high granularity tracking
• Amplification structures– GEMs: very stable but low detection efficiency, nice tracks with TimePix;– Micromegas: spark issue to be solved soon, provide almost all information
on event spatial structure.
• Future plans– Integrate/place a Micromegas on MediPix2/TimePix
high R protection or not– Study tracking capabilities / gas ionization statistic– Build a small endplate by chip tilling
• Low energy event detection– TimePix + Micromegas is a good candidate
Energy of low recoil by electron counting
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NIKHEF Harry van der GraafMartin FransenJan TimmermansJan VisschersSipho van der PuttenArno Aarts
Saclay CEA DAPNIA David AttiePaul ColasArnaud GiganonYannis Giomataris
Univ. Twente/Mesa+ Jurriaan SchmitzVictor Blanco CarballoCora SalmSander Smits
FREIBURG A. BambergerK. DeschU. RenzM. TitovN. VlasovA. Zwerger
CERN Erik HeijneXavier LlopartMedipix Consortium