The readout of GEM and Micromegas

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The readout of GEM and Micromegas equipped TPCs
by means of the MEDIPIX2 CMOS pixel chip as a
direct anode
P. Colasa, A. Fornainib, Y. Giomatarisa, H. van
der Graafb, E.H.M Heijnec, J. Schmitzd, J.
Timmermansb, J.L. Visschersb
a DAPNIA, CEA Saclay (France) b NIKHEF, Amsterdam
(The Netherlands) c CERN, Geneva (Switzerland) d
Univ. Twente/MESA, (The Netherlands)
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Our goal
The performance of drift chambers equipped with
GEM or Micromegas foils is often limited by the
size of the anode readout pads. Ideally, each
GEM or Micromegas hole is associated with a
single channel including a low-noise preamp, one
or more discriminators and time stamp circuitry.
If the hole pitch is sufficiently small, single
primary electrons, created along the track of a
high-energetic charged particle, could be
detected. This enables the most precise track
position to be measured, as well as the most
accurate value for the density of the primary
electrons along the track (dE/dx).
5 mm X 5 mm pads readout electronics
100 mm X 100 mm pixels
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Advantages
? Simplification of TPCs construction ? Single
electron sensitive - best possible track
position reconstruction - best possible dE/dx
measurement - d-ray recognition and
elimination ? Extremely large number (109) of
readout channels - low occupancy - best
possible track separation
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The Medipix2 chip
Characteristics (more info on
www.cern.ch/Medipix)

• A CMOS chip in standard .25 mm technology
• 256 x 256 pixel cells, total sensitive area 1.4
  x 1.4 cm2
• Pixel size 55 mm x 55 mm
• Positive or negative input charge (e- or holes
  collection)
• 13-bit counter per pixel
• Count rates of 1 MHz/pixel (0.33 GHz/mm2)
• 3-sides buttable

matrix
14111 mm x 16120 mm
periphery
Goals X-ray imaging for many applications
(medical, material analysis, synchrotron
applications, etc.)
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The Medipix2 pixel block diagram

• Main steps input preampl. discr. - counter
• 2 thresholds (high/low), thresh. tuning for each
  individual pixel
• Shutter line low each signal above thresh.
  Increase tha value of the counter
• Shutter line high content of the counter is read
  out

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Medipix2 Interconnectivity
Serial LVDS (Low Voltage Differential
Signal) Reduce noise generation, noise
sensitivity, interconnectivity Better
performance for large arrays
Serial Daisy Chain Token passing protocol
(LVDS) 160 Mhz CLK 1 Mbit data per chip 160 Mhz ?
160/Ntot frames/sec
Medipix2 chip N
Medipix2 chip N1
DATA
LVDS lines
CLK
TOKEN
Common single-ended CMOS bus shutter, mode
control, reset, polarity and analog signals (test
input, DAC output)
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Our GEM/MEDIPIX2 setup the chamber
(not in scale)
Drift Space
10 cm
6.6 mm
GEM foils
MediPix CMOS pixel sensor Brass spacer
block Printed circuit board Aluminium base plate
Gas employed Ar/Methane 90/10
Ar/Isobuthane 95/10 Gain up to 80000
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Our GEM/MEDIPIX2 setup the readout system
(not in scale)
Drifters
Scope
GEM foils
Metal anode strip
PC NI DIO card
MPix2 chip on PCB
Muros2 interface
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GEM/MEDIPIX2 TPC our results
31st March 2003
14 mm
14 mm

• 55Fe source, clusters of charge detected
• N. of clusters depending on the presence/absence
  of a source
• Display ionization collected during fixed
  acquisition time
• (counting chip)

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GEM/MEDIPIX2 TPC our results (2)
February 2004
0.01 sec acquisition, no source
2 sec acquisition, no source

• In addition to clusters, also linear shapes have
  been seen
• NO TRIGGER, we record everything inside a fixed
  time window
• Color scheme black 0 counts, grey 1 count,
  white 2

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GEM/MEDIPIX2 TPC more results
February 2004
0.1 sec, no source
2 sec, no source
0.01 sec, 90Sr

• Number of clusters increases if a source is
  present
• These are chosen images, linear shapes do not
  always appear (since we have no trigger system)
• Lines 1 count. Superpositions 2 counts. Noisy
  areas more

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GEM/MEDIPIX2 TPC our interpretation
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Dean Karlen VRVS meeting Sept 17, 2003
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GEM/MEDIPIX2 TPC our interpretation (2)
August 2002 Jeju, Korea Dean Karlen calculation
of diffusion constants Kirsten Sachs
measurment offset 500 ?m in drift versus
diffusion Sept 2003 VRVS meeting Dean Karlen
Diffusion/defocussing too large at small drift
distance Induced charge effect not ruled out
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GEM/MEDIPIX2 TPC our interpretation (3)
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GEM/MEDIPIX2 TPC our interpretation (4)
Anode strip
Scope
55Fe
Test Pulse (edge)
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GEM/MEDIPIX2 TPC our interpretation (5)

• Possible explanation for charge widening
• Electrons end up in low E-field region
• Electrons are either caught by drift field or
  diffuse to low field regions
• Electrons may migrate to regions associated with
  adjacent GEM holes

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GEM/MEDIPIX2 TPC conclusions

• - We have readout a TPC with triple GEM by means
  of a monolithic pixel chip as
• direct anode
• The distribution of the electrons leaving a GEM
  hole is much wider
• than the hole pitch. This may be explained by
  a large fraction of the
• avalanche not traveling straight to the anode,
  but arriving much later,
• after an effective de-focusing effect.
• Due to the charge spread, the signal per pixel
  is very small, and the
• combination of three GEMs and the MEDIPIX2 can
  not detect single primary
• electrons
• - We expect much better results from a
  Micromegas/Pixel sensor combination

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Our Micromegas/MPIX2 setup the chamber
(not in scale)
Micromegas mesh
Insulating spacers (1/mm)
15 mm drift
50 mm
MediPix CMOS pixel sensor Brass spacer
block Printed circuit board Aluminium base plate
Gas employed Ar/Isobuthane 95/10 Gain from
700 up to 1500
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Our Micromegas/MPIX2 setup the r.o.system
(not in scale)
Drifters
Micromegas
MPix2 chip on PCB
PC NI DIO card
Muros2 interface
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Micromegas/MEDIPIX2 TPC our results
(Friday) 13 February 2004
1 sec, 55Fe
0.1 sec, 55Fe
Black 0 counts, Grey 1 count (detected
quanta!)
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Micromegas/MEDIPIX2 TPC more results
1 sec, 90Sr
60 sec, no source
Structure of the insulating spacers is visible
(shadow in the first image, polarization current
in the second one).
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Micromegas/MEDIPIX2 TPC more results (2)
1 sec, 55Fe, lower discriminator threshold
1 sec, 55Fe
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Micromegas/MEDIPIX2 TPC conclusions

• We have readout a TPC with a Micromegas by means
  of a
• monolithic pixel chip as direct anode
• 55Fe and 90Sr signals clearly observed!
• Next steps detect minimum ionizing particles
  we want to see
• tracks!
• A new pixel CMOS chip will be realized, optimized
  for the readout
• of a gas chamber TimePix. A Micromegas
  structure will be
• created directly on the chip by means of
  post-processing
• technologies.
• - More info www.nikhef.nl/aforna/TPC/main.html

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