The pixel readout of TPCs

1. The pixel readout of TPCs Max Chefdeville, NIKHEF, Amsterdam

2. 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. wire Cathode pads GEM Micromegas 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

4. The pixel readout of TPCs • 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

5. The Medipix2 chip • 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 Use the “naked” chip as the detector anode

6. Cathode (drift) plane Drift space: 15 mm Micromegas Baseplate MediPix2 pixel sensor Brass spacer block Printed circuit board Aluminum base plate NIKHEF SACLAY TWENTE Medipix2 & Micromegas

7. He/Isobutane 80/20 δ-ray! Efficiency for detecting single electrons: > 90 %

8. He 20% iC4H10 Ar 20% iC4H10 Larger diffusion Top of track Helium VS Argon mixtures • Argon: larger primary statistic & transverse diffusion Interesting tool to study ionization statistic of photons & charged particles …

9. Integrate amplification and readout structures: InGrid Walls Pillars Wafer post-processing

10. 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. 30 µm Ø pillar 13 % FWHM @ 5.9 keV

11. InGrid on a Medipix2 chip • Goal: post-process full wafers • Post-processing of individual chips @ Twente University, Netherlands InGrid on top of pixel matrix Grid hole centered between 4 pixels pillars pixels First trials promising (using rejected chips)

12. The spark issue Detector very sensitive to gas discharges InGrid mesh (Micromegas OK) Pixel

13. e- multiplication @ high E e- extraction @ low E Un-coated anode Coated anode Discharge protections TwinGrid • destructive discharge • Proposals: • multi-stage amplification; • high R coating of anode. Discharge signals of 2 Micromegas detectors Current attenuated by the high R layer Maybe enough to protect the chip… Both are being applied on Medipix2

14. 40 kV/cm 50 µm 80 kV/cm 50 µm 1 kV/cm 1000 µm 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

15. 6 mm 2 mm 2 mm 1 mm BONN FREIBURG GEMs & MediPix2 @ DESY 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

16. GEMs & MediPix2 @ DESY • Gas mixture: Ar CO2 70/30 ~30 clusters per cm created, ~10 cluster/cm reconstructed Less primary information due to diffusion in the amplification gap • Tracks parallel to the pixel plane: same diffusion along the track

17. High threshold Control logic preamp/shaper Low threshold 5 5 2 2 4 4 1 1 6 6 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 MediPix2 → TimePix

18. Counting modes (simulations) from hit till end of shutter time  time over threshold 17 not detected detected Charge summed 100 MHz clock

19. N e- TOT counts Time over threshold events > 30 ke- per pixels TOT linear above ~ 4 ke- Better calibration to come… Freiburg Bonn • Ar CO2 70/30 • No time information (2D) • Charge information should improve spatial resolution

20. The larger the number of counts, the shorther the drift time “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)

21. 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 CO2 70/30 He CO2 70/30

22. 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

23. NIKHEF Harry van der Graaf Martin Fransen Jan Timmermans Jan Visschers Sipho van der Putten Arno Aarts Saclay CEA DAPNIA David Attie Paul Colas Arnaud Giganon Yannis Giomataris Univ. Twente/Mesa+ Jurriaan Schmitz Victor Blanco Carballo Cora Salm Sander Smits FREIBURG A. Bamberger K. Desch U. Renz M. Titov N. Vlasov A. Zwerger CERN Erik Heijne Xavier Llopart Medipix Consortium