Study of Large-area GEM Detectorsfor a Forward Tracker

at a Future Electron-Ion Collider Experiment

Aiwu Zhang, Vallary Bhopatkar, Marcus HohlmannFlorida Institute of Technology (FIT)

Kondo Gnanvo, Nilanga LiyanageUniversity of Virginia (U.Va)

for the EIC RD6-FLYSUB Consortium

Electron Ion Collider Users MeetingJune 24-27, 2014 at Stony Brook University, NY

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Contents

• 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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Zigzag-strip GEM @ FITZigzag strips, 1.37mrad pitch

0.1mm

12345678-sectors

1.37 mrad

• 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.

0.5mm

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Pitch = 550 mm,

Top strips = 140 mm,

Bottom strips = 490 mm

12°

2D u/v readout strips

Entrance window

Drift region

Transfer regionTransfer region

Induction region

spacers

Gas inlet

Gas outlet

2D readout board on Honeycomb support

Cross section of low mass triple GEM

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2D u/v strip GEM @ U.Va

100

cm

22 cm

44 cmKey characteristics:

• Largest GEM detector with 2D readout ever build

• Low mass (narrow edge and honey comb support) and small dead area

• Fine strips 2-dimensional flexible small stereo angle u/v readout so that good spatial resolution can be achieved, and with low capacitance noise

• Gluing technique is used so that GEM foils can not be changed

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Beam test setup @ FNAL

zigzag GEM and U.Va GEM

Trackers

Trackers

• 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. 6/27/2014

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Beam test results of the zigzag GEM– basic performances

Cluster charge distribution

in sector 5 at 3200V

MPV value of charge distribution vs. HV

Stat. errors smaller than marker size

peak pos.

• Cluster charge distribution fits well to a Landau function.

• Mean cluster size (number of fired strips in one event) from each cluster size distribution shows approximately exponential dependence on HV.

Mean cluster size vs. HV on sector 5(number of hits in a cluster)

Stat. errors smaller than marker size

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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.

• 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.

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Beam test results of the zigzag GEM – spatial resolution studies

• 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: . 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.

X offset

Eta5

vertex10°

Y of

fset

Aligning trackers to zigzag GEM det.

σ=21μrad

Inclusive residual for 1st trackerResolution in φ for trackers

Errors smaller than marker size

tracker

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Beam test results of the zigzag GEM– spatial resolution studies (cont.)

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Exclusive residualσ=281μrad

Inclusive residualσ=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 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).

• Resolution of the zigzag-GEM on different sectors at 3200V (without cluster size cut).

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Beam test results of the zigzag GEM– cluster position correction

• 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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• Centroid position distribution from COG method (in middle-sector 5).

2-strip events 3-strip events

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Beam test results of the zigzag GEM– cluster position correction (cont)

• The idea is to build strip response functions for different cluster sizes (η-algorithm).• , is defined as the centroid position (in strip units) minus the center of strip with

maximum charge.• The position correction functions can be calculated: .

h(η2)distribution

h(η3)distribution

Correction function for 2-strip events

Correction function for 3-strip events

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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

2-strip aftercorrection

3-strip before correction 3-strip after correction

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Beam test results of the zigzag GEM– spatial resolution after correction

• After position correction, we observe that resolution gets improved at higher voltages (to ~170μrad).

Resolution vs. HV in middle-sector 5 after positions are corrected (with 2, 3, 4-strip events)

• The results give us a clue that strip response correction is affected by gas gain and incident angle of particles.

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ADC Charges distribution Efficiency vs. HV Nb of strips /cluster vs. HV

P1P3 P2

P4

P5

Position scan with 32 GeV hadron beam

p1 p2 p3 p4 p5354045505560657075

380400420440460480500520540

Beam spot location on the chamber

spat

ial r

esol

utio

n (m

rad)

spat

ial r

esol

utio

n (m

m)

Spatial resolution in (r, ) at different location in the chamber

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Performances of the U.Va GEM

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Gas input

Gas out

Top Entrance window

Bottom gas window

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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.

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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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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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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.

Thanks!

The FLYSUB consortium

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Back up – align the zigzag detector

X offset

Eta5

vertex10°

Y of

fset

Aligning trackers to zigzag GEM det.

tracker

At a fixed Y offset, check residual sigma and chi-2

Residual sigma vs. X offset

Chi2 vs. X offset

At a fixed X offset, check residual mean and chi-2

Residual mean vs. Y offset

Chi2 vs. Y offset

After checked (X,Y) groups in reasonable ranges, an intersecting point can be found from the scattering plot.

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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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MotivationConceptual design of EIC detector

Forward/backward GEM trackers

• 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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Example of zigzag strips

2.5mm