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 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
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