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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 µm2 pixels in combination with Micromegas & GEMs

• Time information (and more) with the new TimePix chip

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

• 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

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

MediPix2 pixel sensorBrass spacer blockPrinted circuit boardAluminum base plate

Micromegas

Cathode (drift) plane

Baseplate

Drift space: 15 mm

Medipix2 & MicromegasNIKHEF

SACLAY

TWENTE

He/Isobutane80/20

δ-ray!

Efficiency fordetecting single

electrons:> 90 %

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 …

Integrate amplification and readout structures:

InGrid

Walls Pillars

Wafer post-processing

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

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)

The spark issue

Detector very sensitive to gas discharges

Pixel InGrid mesh (Micromegas OK)

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

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

GEMs & MediPix2 @ DESYBONN

FREIBURG

6 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

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

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

55μm

prea

mp/

shap

Low

resh

old

Hig

h th

resh

old

Con

trol

logi

MediPix2 → TimePix

Counting modes (simulations)

time over thresholdfrom hit till end of shutter time

100 MHz clock

not detected

detected

Charge summed

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

0 200 400 600TOT-Counts

erg

y [K

TOT counts

N e

Freiburg Bonn

“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

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

2 70

/30

2 70

/30

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