21 April 2004 LCWS 2004 Paris 1

Readout of a TPC using the Medipix2 CMOS pixel sensor

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

NIKHEF: Alessandro Fornaini Univ. Twente/Mesa+:

Harry van der Graaf Jurriaan Schmitz

Jan Timmermans CERN/Medipix Collaboration:

Jan Visschers Erik Heijne

Peter Kluit

Saclay Paul Colas Thanks to: Wim Gotink

(CEA/DAPNIA) Ioannis Giomataris Joop Rovenkamp

Arnaud Giganon Max Chefdeville

21 April 2004 LCWS 2004 Paris 2

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

21 April 2004 LCWS 2004 Paris 3

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.

21 April 2004 LCWS 2004 Paris 4

DriftSpace

GEM foils

MediPix CMOS pixel sensorBrass spacer blockPrinted circuit boardAluminium base plate

Medipix2: 256 x 256 pixels

55 mm x 55 mm

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

21 April 2004 LCWS 2004 Paris 5

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

21 April 2004 LCWS 2004 Paris 6

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

21 April 2004 LCWS 2004 Paris 7

No source; exposed 0.01 s No source; exposed 2 s No source; exposed 2 s

No source; 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

21 April 2004 LCWS 2004 Paris 8

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!

21 April 2004 LCWS 2004 Paris 9

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 %

21 April 2004 LCWS 2004 Paris 10

21 April 2004 LCWS 2004 Paris 11

21 April 2004 LCWS 2004 Paris 12

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

21 April 2004 LCWS 2004 Paris 13

New trial: NIKHEF, March 30 – April 2Essential: try to see single electrons from cosmic muons (MIPs)

Pixel preamp threshold: 3000 e-Required gain: 5000 – 10.000

New MedipixNew Micromegas

Gas: He/Isobutane 80/20Ar/Isobutane 80/20He/CF4 80/20

…… It Works!

21 April 2004 LCWS 2004 Paris 14

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

21 April 2004 LCWS 2004 Paris 15

He/Isobutane80/20Modified MediPix

30 March 2004

21 April 2004 LCWS 2004 Paris 16

He/Isobutane80/20Modified MediPix

30 March 2004

21 April 2004 LCWS 2004 Paris 17

He/Isobutane80/20Modified MediPix

30 March 2004

21 April 2004 LCWS 2004 Paris 18

He/Isobutane80/20Non ModifiedMediPix

Amaricium Source

1 April 2004

21 April 2004 LCWS 2004 Paris 19

He/Isobutane80/20Modified MediPix

30 March 2004

21 April 2004 LCWS 2004 Paris 20

He/Isobutane80/20Modified MediPix

30 March 2004