Measuring the Resolution a of GEM TPC

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Measuring the Resolution a of GEM - TPC

• Gabe Rosenbaum
• D. Karlen, P. Poffenberger
• University of Victoria

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Overview

• Context of the UVic TPC
• What is a Time Projection Chamber?
• How does a TPC operate?
• Measuring momentum resolution
• Example of results
• Conclusions

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Linear Collider

• Proposed ee- linear collider
• Clean environment to compliment LHC discoveries
• Highest energy lepton collider ever

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Golden Channel

• Clean signal looking at two leptons regardless of
  Higgs decay
• Higgs recoil mass found by measuring lepton
  momentum
• Most challenging with respect to momentum
  resolution

Momentum resolution needed
? ( 1/p) lt 2 x 10-4 (GeV/c)-1
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A Time Projection Chamber

• TPC is a leading candidate for central tracker at
  the e e- linear collider
• Good momentum resolution ) good spatial
  resolution
• Can a TPC achieve the resolution goal?

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What is a Time Projection Chamber?

• Drift region
• Amplification region
• Readout region
• Electric and magnetic fields in axial (z-dir)

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What is a Time Projection Chamber?

• Charged particle ionizes atoms in the gas
• Electrons are drifted toward the amp. region
• Number of electrons increased through avalanche
• Charge collected on pads on readout endplate

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University of Victoria TPC
30cm

• 30 cm drift length
• 25 cm diameter
• 256 channels of 20 MHz flash ADC readout (from
  STAR experiment)

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Why is it special?... GEMs

• Gas Electron Multiplier
• Uses small holes and large potential to multiply
  electrons
• Field lines stay parallel to axis of chamber no
  ExB effect
• Gains of O5000 with two gems

140 ?m
70 ?m
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How is the track reconstructed?
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Readout in the longitudinal direction

• The drift velocity for electrons in the gas is
  known
• z-distance of the ionization track is found by
  using the arrival time of the track

? t1
? t2
y
z
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Readout in the longitudinal direction

• Arrival time after trigger is recorded for each
  pad
• z-coord found with linear fit

2.85 ? s
3.00 ? s
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Readout in the transverse direction

• Four parameter track fit is done.
• x0 (horizontal position)
• ?0 (angle off vertical)
• ? (track width)
• 1/r (inverse of radius of curvature)

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Momentum Resolution

• Momentum measured by radius of curvature of track
• Assuming
• B-field 4T
• L 1m
• N 200

Track
L
B - field
N points measured along track
? (1/p) ¼ 2 10-4 GeV-1) ?x ¼ 130 ? m
?x is the transverse resolution of each point
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Transverse Resolution

• Do a track fit (reference track)

• Do track fit with only one row leaving x0 as only
  free parameter

• Take the difference in the x0 values from the two
  fits are called residuals

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Residuals
Example

• Residuals are histogrammed once including row in
  reference fit and once without
• The geometric mean of the width of the two
  distributions is taken to be the resolution

Resolution (0.082mm x 0.056mm)1/2 0.065mm
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How are we doing?

• Recall, with
• 4 tesla field
• 1m track length
• 200 points
• We require transverse resolution of ¼ 130?m
• Our TPC is achieving resolutions better than 80
  ?m

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Conclusion

• Linear Collider tracker requires excellent
  momentum resolution
• A Time Projection Chamber can reconstruct 3-D
  tracks with good spatial resolution
• The UVic prototype TPC is achieving resolutions
  appropriate to meet design goals of tracker!

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The End
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Additional Slides
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Track Fitting

• Number of electrons assigned to each pad based on
  pulse size

Number of electrons
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The TPC

• 30 cm drift
• Double GEM

• 7mm x 2mm readout pads
• STAR Electronics

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TPC Component Details
Readout Pads

• 7mm x 2mm pads
• 8 rows of 32 pads
• Adjacent rows offset
• Three large pads not used in track fit
• Conducting ground plane

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High Voltage Supply

• Three power supplies
• GEM voltages ratios automatically set
• Drift top and bottom set separately (drain on
  drift bottom)
• Two nano-amp meters read GEM current across GEMS
• Monitor points read 1/1000 of actual voltage

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STAR Readout Electronics

• Each FEE card has 32 pre-amps and 32 ADCs and
  circuitry for shaping
• Readout board controls FEE cards and sends data
  to DAQ on 1.2Gbit/s fiber optic link

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Magnetic Field Tests
TRIUMF 1 tesla solenoid (Vancouver)
DESY 5 tesla super conducting solenoid (Hamburg)
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TPC gases

• Inert gas quencher
• High drift velocity
• Low transverse diffusion in B-field
• Low attachment
• Gas gain at high fields

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Diffusion and Defocusing

• Look at cloud size vs. Drift Distance

s2 (mm2)
s2 (mm2)
drift time (50 ns bins)
drift time (50 ns bins)

• Slope gives diffusion constant (D)
• Y-intercept gives defocusing value (s0)

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Track Fitting

• Four parameter track fit is done.
• x0 (horizontal position)
• phi0 (angle of vertical)
• sigma (track width)
• 1/r (inverse of radius of curvature)

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Choice of Electric Field

• How do we choose E-fields in each section of the
  TPC?

Readout pads
GEMs
Drift Volume 30 cm
Transfer Gap 2.5mm
Induction Gap 5mm
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Choice of Electric Field

• Drift field chosen for low diffusion
• Transfer and induction fields chosen for high
  diffusion

B0.05T
Transverse Diffusion (mm/sqrt(cm)
B1.0T
B5.0T
Electric Field (kV)
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Charge Sharing

• Even at 4.5T charge is still shared over more
  than one pad

data from experiments performed in DESY solenoid
Aug. 2003
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Monte Carlo Simulation

• Excellent agreement between TRIUMF data and
  simulation