Study of Large-area GEM Detectors
for a Forward Tracker
at a Future Electron-Ion Collider Experiment
Aiwu Zhang, Vallary Bhopatkar, Marcus Hohlmann
Florida Institute of Technology (FIT)
Kondo Gnanvo, Nilanga Liyanage
University of Virginia (U.Va)
for the EIC RD6-FLYSUB Consortium
Electron Ion Collider Users Meeting
June 24-27, 2014 at Stony Brook University, NY
Contents
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Motivations (will skip)
FIT 1-m size zigzag GEM detector
U.Va 1-m size u-v strip GEM detector
Beam test configuration at Fermilab
Beam test results of the zigzag GEM detector
Beam test results of U.Va’s GEM detector
Summary
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A. Zhang et al., Study of Large-area GEM Detectors as a Forward Tracker at an EIC
Zigzag-strip GEM @ FIT
Zigzag strips, 1.37mrad pitch
1.37 mrad
0.5mm
8
7
6 5 4 3
-sectors
2 1
0.1mm
• The zigzag strips run in radial direction and can measure the azimuthal
direction. Opening angle is 10 degrees, angle pitch 1.37mrad.
• The readout board is designed to fit a 1-m long trapezoidal GEM prototype
(originally for CMS muon upgrade). It is divided to 8 η-sectors with radial
length of each sector ~12cm, and 128 strips/sector.
• For the same GEM prototype with straight strips, 24 APV chips are needed to
fully read out the chamber. In the zigzag case, only 8 APV chips can fully read
out the entire chamber. This means 2/3 electronic channels can be saved.
• We use self-stretch technique so that GEM foils can be changed easily.
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2D u/v strip GEM @ U.Va
Key characteristics:
44 cm
• Largest GEM detector with 2D readout ever build
• Fine strips 2-dimensional flexible small stereo angle u/v
readout so that good spatial resolution can be achieved, and
with low capacitance noise
100
cm
• Low mass (narrow edge and honey comb support) and
small dead area
• Gluing technique is used so that GEM foils can not be changed
2D u/v readout strips
Cross section of low mass triple GEM
12°
Entrance window
Pitch = 550 mm,
Drift region
Top strips = 140 mm,
Transfer region
Transfer region
Bottom strips = 490 mm
Gas outlet
Gas inlet
spacers
Induction region
2D readout board on Honeycomb support
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22 cm
Beam test setup @ FNAL
Trackers
Trackers
zigzag GEM and U.Va GEM
• The RD6-FLYSUB consortium conducted a three-week beam test at Fermilab
(Meson Test area 6, MT6) in Oct 2013, operated 20 GEM detectors.
• The FIT group and U.Va group tested 10 GEMs as a tracking system.
• 4 reference detectors (3/2/2/2mm gaps); the zigzag GEM gaps: 3/1/2/1 mm; Ar/CO2
(70:30) was used to operate all the detectors.
• DAQ: RD51 SRS with SRU to read out 4FECs/64APVs simultaneously.
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Beam test results of the zigzag GEM
– basic performances
Cluster charge distribution
MPV value of charge distribution vs. HV
peak pos.
in sector 5 at 3200V
Stat. errors smaller than marker size
Mean cluster size vs. HV on sector 5
(number of hits in a cluster)
• Cluster charge distribution fits well to
a Landau function.
Stat. errors smaller than marker size
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• Mean cluster size (number of fired
strips in one event) from each cluster
size distribution shows approximately
exponential dependence on HV.
A. Zhang et al., Study of Large-area GEM Detectors as a Forward Tracker at an EIC
Beam test results of the zigzag GEM
– basic performances (cont.)
• Detection efficiency in middle-sector 5.
Fitted with a sigmoid function, plateau
efficiency ~98.4%.
• Different thresholds (N=3,4,5,6 times of
pedestal width σ) were tested, the
efficiency plateau is not affected by
thresholds.
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• On each sector, two points were
measured. The response from
sector to sector varies by ~20%.
• The non-uniformity could be
caused by bending of the drift
board. The CMS-GEM group is
investigating this aspect to avoid
bending after chambers are
assembled.
A. Zhang et al., Study of Large-area GEM Detectors as a Forward Tracker at an EIC
Beam test results of the zigzag GEM
– spatial resolution studies
Resolution in φ for trackers
Aligning trackers to zigzag GEM det.
σ=21μrad
vertex
Y offset
Inclusive residual for 1st tracker
10°
Eta
5
tracker
X offset
Errors smaller than marker size
• The zigzag strips measure the azimuthal coordinate φ. Angle pitch between two
strips is 1.73mrad. So we study its spatial resolution in polar coordinates.
• Spatial resolution is calculated from the geometric mean of exclusive and
inclusive residual widths: 𝜎 = 𝜎𝑒𝑥 × 𝜎𝑖𝑛 . Exclusive (Inclusive) means the
probed detector is excluded (included) when fitting the tracks.
• The trackers are aligned first and their spatial resolutions in (x, y) are found to
be around 70μm, which is the typical resolution of a standard triple-GEM. Their
resolutions in φ coordinate are then calculated to be 30-40μrad.
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Beam test results of the zigzag GEM
– spatial resolution studies (cont.)
Exclusive
residual
σ=281μrad
Inclusive
residual
σ=223μrad
• Residual distributions of the zigzag GEM in middle-sector 5 at 3350V
• Hit positions are calculated with Center of Gravity (COG) method, and all
cluster size >0 events are used.
• Resolution is 𝝈 = 𝟐𝟖𝟏 ∗ 𝟐𝟐𝟑 = 𝟐𝟓𝟎𝝁𝒓𝒂𝒅 for this case.
• Note that the resolution in number of strips is about ~18%
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Beam test results of the zigzag GEM
– spatial resolution studies (cont.)
• Resolution of the zigzag-GEM vs. HV in
middle-sector 5.
• At highest tested voltage, resolution is
~240μrad.
• If only use 2-strip events, resolution is
smaller (especially at lower voltages).
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• Resolution of the zigzag-GEM on
different sectors at 3200V
(without cluster size cut).
A. Zhang et al., Study of Large-area GEM Detectors as a Forward Tracker at an EIC
Beam test results of the zigzag GEM
– cluster position correction
• Centroid position distribution from COG method (in middle-sector 5).
2-strip events
3-strip events
• By further checking the centroid position distributions of fixed cluster size
events, we observe that these distributions have apparent bumps around each
strip.
• This brings us to study the non-linear strip response of charge distribution on
position reconstruction, and hence make these distributions flat.
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Beam test results of the zigzag GEM
– cluster position correction (cont)
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•
•
The idea is to build strip response functions for different cluster sizes (η-algorithm).
𝜼𝑵 = 𝒔𝒄𝒆𝒏𝒕𝒓𝒐𝒊𝒅 − 𝒔𝒎𝒂𝒙 , is defined as the centroid position (in strip units) minus the center
of strip with maximum charge.
The position correction functions can be calculated: 𝒔′ 𝒄𝒆𝒏𝒕𝒓𝒐𝒊𝒅 = 𝒔𝒎𝒂𝒙 − 𝟎. 𝟓 +
h(η2)
distribution
Correction function
for 2-strip events
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h(η3)
distribution
Correction function
for 3-strip events
A. Zhang et al., Study of Large-area GEM Detectors as a Forward Tracker at an EIC
𝜼′
−𝟎.𝟓 𝒉
𝟎.𝟓
−𝟎.𝟓 𝒉
𝜼 𝒅𝜼
𝜼 𝒅𝜼
.
Beam test results of the zigzag GEM
– cluster position correction (cont.)
• After correction functions are figured out, the centroid position of an event can be
corrected. Only clusters with 2,3 and 4 strips are because of better statistics (they
make up ~90% of all clusters on the efficiency plateau).
2-strip before
correction
3-strip before correction
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2-strip after
correction
3-strip after correction
A. Zhang et al., Study of Large-area GEM Detectors as a Forward Tracker at an EIC
Beam test results of the zigzag GEM
– spatial resolution after correction
Resolution vs. HV in middle-sector 5 after positions are corrected (with 2, 3, 4-strip events)
• After position correction, we observe that resolution gets improved at higher
voltages (to ~170μrad).
• The results give us a clue that strip response correction is affected by gas gain
and incident angle of particles.
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Performances of the U.Va GEM
Spatial resolution in (r, ) at different
location in the chamber
P4
P1
P2
P5
spatial resolution (mrad)
P3
resolution in phi
resolution in r
540
520
500
480
460
440
420
400
380
75
70
65
60
55
50
45
40
35
p1
p2
p3
p4
spatial resolution (mm)
Position scan with 32 GeV hadron beam
p5
Beam spot location on the chamber
ADC Charges distribution
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Efficiency vs. HV
Nb of strips /cluster vs. HV
A. Zhang et al., Study of Large-area GEM Detectors as a Forward Tracker at an EIC
4 new ideas from U.Va towards a lighter,
better resolution GEM detector
• Ultra low mass chamber to minimize multiple scattering and background
• “Re-openable” chamber – without gluing GEM foils
• “mini-drift” GEM tracker to improve spatial resolution at large angle tracks
• All readout electronics arranged at the outer edge of the chamber, to further
reduce dead area and get better radiation hardness.
Gas out
Gas input
Top Entrance window
Bottom gas window
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A. Zhang et al., Study of Large-area GEM Detectors as a Forward Tracker at an EIC
Summary on the zigzag GEM
• The zigzag-GEM detector worked well in the beam test at FNAL.
• The 98% detection efficiency is good. The gain uniformity needs
to be further investigated.
• Corrections for non-linear strip responses bring the resolution
from ~240μrad down to ~ 170μrad on the eff. Plateau, which
could be transferred to 170μm at R=1m. The zigzag structures
can probably still be optimized by interleaving zigs and zags
more to improve resolution performance even further.
• We conclude that a readout with zigzag strips is a viable option
for cost efficient construction for a forward tracker with GEMs.
• The U.Va u/v strip GEM detector also performance well in the
beam test.
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A. Zhang et al., Study of Large-area GEM Detectors as a Forward Tracker at an EIC
Summary on the dedicated EIC
forward tracker with GEMs
• Both FIT and U.Va groups have experience on building and
operating large-area GEM detectors.
• U.Va group has experience on low-materials for drift and
readout; FIT group constructs GEMs without gluing foils, and
are pursuing a optimized cost effective zigzag readout
structure.
• We are joining forces with Temple U. in designing and
constructing a dedicated GEM prototype for the EIC
forward tracker, which goes to even higher eta regions in
the forward region.
• We plane to work out entirely domestically sourced GEM
foils (see the next talk from Temple U. group).
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The FLYSUB
consortium
We would like to acknowledge BNL
for the support of this work through the EIC RD-6 collaboration
and the staff of the FNAL test beam facility for all their help.
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Back up – align the zigzag detector
Aligning trackers to zigzag GEM det.
vertex
Y offset
10°
Eta
5
tracker
X offset
Residual sigma vs.
X offset
Chi2 vs. Y offset
Residual mean
vs. Y offset
At a fixed X offset, check residual mean and chi-2
Chi2 vs. X offset
At a fixed Y offset, check
residual sigma and chi-2
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After checked (X,Y) groups in reasonable ranges, an
intersecting point can be found from the scattering plot.
A. Zhang et al., Study of Large-area GEM Detectors as a Forward Tracker at an EIC
Back up - references
 References on the strip response correction:
• CERN-Thesis-2013-284 by Marco Villa.
• G. Landi, NIMA 485 (2002) 698; NIMA 497 (2003) 511
 Reference about inclusive and exclusive residual study
• R. K. Carnegie, NIMA 538 (2005) 327
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Motivation
Conceptual design
of EIC detector
Example of zigzag strips
Forward/backward
GEM trackers
2.5mm
• The RD6-FLYSUB consortium is jointly working on tracking and particle ID,
based on the Gaseous Electron Multiplier (GEM) technique, for a future EIC.
• The zigzag-strip readout structure is proposed and under study by Florida Tech
to make the forward tracker much less costly.
• Each zigzag strip occupies more space than a straight strip so that the total
readout channels can be reduced and hence reduce the cost significantly, while
good spatial resolution can be conserved because of charge sharing on these
zigs and zags.
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