Spectral

1
Micromegas TPC studies at high magnetic fields
using the charge dispersion signal
D. Attié, A. Bellerive, K. Boudjemline, P. Colas,
M. Dixit, A. Giganon, I. Giomataris, V.
Lepeltier, S. Liu, J.-P. Martin, K. Sachs, Y.
Shin and S. Turnbull COSMo (Carleton, Orsay,
Saclay, Montreal) Collaboration
2
Expectations for ILC-TPC

• The Time Projection Chamber (TPC) for the
  International Linear Collider (ILC) will need to
  measure about 200 track points with a transverse
  resolution close to 100 µm.
• Pad width of 2 mm provides good two-track
  separation. With the choice
• of 2 mm x 6 mm pads, the ILC-TPC will have 1.2
  106 channels.
• But is still too wide to give the target
  resolution (s0 pitch/v12).
• Not enough charge sharing,
• even for 1 mm pitch pads
• (in the case of Micromegas
• s avalanche 12 µm)
• Pads narrower than 1 mm are needed.
• This has consequences on
• Electronic cost
• Material budget
• Cooling aspects

3
Solution use a charge dispersion signal

• Disperse the signal sharing the charge between
  several neighbouring pads after amplification,
  using a resistive coating on an insulator
• The charge arrives on the central pad
• and is spread over the others pads which
• see a fraction of the charge after a delay
• Then we fit the signal over the pads which
• gives us the track position

2 x 6 mm2 pads
M.S.Dixit and A. Rankin NIM A566 (2006) 281
4
Micromegas gas amplification system

• Micromegas is a parallel plate gas avalanche
  detector with a small gap
• Micromesh held by pillars 50 ?m above the anode
  plane

Charge particle
Drift electrode
Edrift 100-1kV/cm
Conversion gap gt 3 mm
Amplification gap 50 mm
Micromesh
? Pillars ?
Eamp 50k-100 kV/cm
Pads
5
Charge dispersion in a MPGD with a resistive anode

• 2D Telegraph equation

• Modified MPGD anode with a high resistivity film
  bonded to a readout plane with an insulating
  spacer
• Point charge at r 0 t 0 disperses with time
• The charge density r(r,t) at (r,t) is a solution
  of the 2D Telegraph equation
• The anode charge density is time dependent and
  sampled by readout pads

Q(t)
?(r)
mesh
resistive foil
glue
pads
?(r,t) integral over pads
PCB
M.S.Dixit et.al., NIM A518 (2004) 721
r (mm)
t (ns)
6
Charge dispersion in a MPGD with a resistive anode

• 25 µm mylar with Cermet (Al-Si) of 1 MW/? glued
  onto the pads with 50 µm thick dry adhesive
• The Cermet is a composite material composed of
  ceramic and metallic materials

Al-Si Cermet on mylar
Drift Gap
MESH
Amplification Gap
Micromegas detector resistive anode
7
Micromegas gain

• The resistive foil prevents charge accumulation,
  thus prevents sparks
• Gains higher than obtained with standard anodes
  can be reached

? Without resistive foil ? With resistive foil
(current)
ArIso5
8
The 5 T cosmic-ray test at DESY

• Carleton TPC 10 x 10 cm2 Micromegas (50 mm gap)
  resistive anode
• 128 pads- 126 tracking pads (2 x 6 mm2) in 7
  rows - 2 trigger pads (36 x 6 mm2) in 2 rows
• Drift length 15.7 cm
• Aleph charge preamplifiers 200 MHz FADCs
  digitizer

9
The 5 T cosmic-ray test at DESY

• 4 weeks of data using the 5 T magnet (thanks to
  DESY and T. Behnke et al.)
• 2 gas mixtures used - Ar5 isobutane easy
  gas, for reference - Ar3 CF42 isobutane
• ? so-called T2K gas,
• ? good trade-off for safety, ? high velocity
  (7.2 cm/ms at 200 V/cm),
• ? low longitudinal diffusion,
• ? large wt 20 at 5 T ie DTr 19 mm/vcm.
• Most data taken- at 5 T (to limit the diffusion)
  and - at 0.5 T (low enough field to check the
  effect of diffusion)
• 55Fe source used for gain measurements
• placed inside the chamber

10
Gain dependence on magnetic field
Micromegas gain vs. magnetic field with a 55Fe
source for Ar5C4H10
Gain relative to B 1T
Magnetic field (T)
? Micromegas gain constant to within 0.5 up
to 5 Tesla !
11
Cosmic-ray data taken at DESY
2 mm
6 mm
Sample cosmic ray tracks B 0.5 T
12
Charge dispersion pulses pad response function

• The Pad Response Function (PRF) is a measure of
  signal size as a function of track position
  relative to the pad.
• The pulse shape is variable and non-standard
  because of
• both the rise time pulse amplitude depend on
  track position.
• The PRF amplitude for longer drift distances is
  lower due to Z dependent normalization.

13
Parameterization of the PRFs

• The PRFs are not Gaussian.
• The PRF depends on track position relative to the
  pad PRF PRF(x,z).
• PRFs determined from the data and parameterized
  by a ratio of two symmetric 4th order
  polynomials
• a2, a4, b2 b4 can be written down in terms of
• - FWHM ?(z)
• - base width ?(z) of the PRF.

Amplitude
The parameters depend on TPC gas and operational
details
?
?
x/mm
14
Track fit using the PRF

• For a given track xtrack x0 tan(?) yrow
• Yrow is the y position of the row and x0 ? the
  track fitting parameters
• Determination of x0 ? by fitting the PRF to the
  pad amplitude by minimizing ?2 for the entire
  event
• Definitions of the different stages
• - residual xrow-xtrack
• - bias mean of residual xrow-xtrack
  f(xtrack)
• - resolution geometric mean of the standard
• deviations of track residuals

15
Pad Response Function (PRF)
0 lt z lt 1 cm
1 lt z lt 2 cm
2 lt z lt 3 cm
3 lt z lt 4 cm
4 lt z lt 5 cm
normalized amplitude

• T2K gas
• B 5 T
• 15 z regions / 1 cm step

5 lt z lt 6 cm
6 lt z lt 7 cm
7 lt z lt 8 cm
8 lt z lt 9 cm
9 lt z lt 10 cm
10 lt z lt 11 cm
11 lt z lt 12 cm
12 lt z lt 13 cm
13 lt z lt 14 cm
14 lt z lt 15 cm
xtrack xpad (mm)
4 pads / 4 mm
16
Residuals xrow-xtrack
2 lt z lt 3 cm
0 lt z lt 1 cm
1 lt z lt 2 cm
13 lt z lt 14 cm
14 lt z lt 15 cm
12 lt z lt 13 cm
17
Average residual vs x position

• A bias of up 100 mm is observed attributed to the
  charge spread non-uniformity due to -
  inhomogeneities in the gap size -
  non-uniformity of the foil resistivity

correction
bias after
bias before
row 1
20 mm
row 2
row 3
18
Resolution at 0.5T vs. gain

• B 0.5 T, resolution fit by where
• Resolution ?0 (? at z 0) 50 µm still good at
  low gain (will minimize ion feedback)
• Mean of Neff 27 (value measured before 22)

? s0 1/40 of pad pitch
Gain 4700
Gain 2500
Neff25.22.1
Neff28.82.2
19
Resolution at 5T vs. gas mixtures

• Extrapolate to B 4T with T2K gas for 2x6 mm2
  pads
• DTr 23.3 µm/?cm,
• Neff 27,
• 2 m drift distance,
• ? Resolution of ?Tr ? 80 ?m will be possible !!!

• Analysis - Curved track fit
• - EP lt 2 GeV
• - f lt 0.05 (3)

? ? 50 µm independent of the drift distance
50 mm
20
Conclusions

• Micromegas with a resistive anode has been
  successfully operated in a 5 T magnetic field.
• s 50 ?m over 15 cm (transverse diffusion
  negligible)
• ? extrapolates to 80 ?m at 2 meters.
• The Ar3 CF42 Isobutane gas mixture is
  promising.
• Can be also used with bulk technology for T2K
  experiment (See A. Sarrats poster A22).