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 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 A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/2014 2

Zigzag-strip FIT Zigzag strips, 1.37mrad pitch 0.1mm  -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 A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/2014 3

Pitch = 550  m, Top strips = 140  m, Bottom strips = 490  m 12° 2D u/v readout strips Entrance window Drift region Transfer region Induction region spacers Gas inlet Gas outlet 2D readout board on Honeycomb support Cross section of low mass triple GEM 6/27/2014 A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 4 2D u/v strip U.Va 100 cm 22 cm 44 cm Key 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

Beam test FNAL 1-m GEM w/ zigzag readout 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/CO 2 (70:30) was used to operate all the detectors. DAQ: RD51 SRS with SRU to read out 4FECs/64APVs simultaneously. A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/2014 5

Beam test results – 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 A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/2014 6

Beam test results – 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) were tested, the efficiency plateau is not affected. 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 a Large-area GEM Detector with Zigzag-strip Readout 6/27/2014 7

Beam test results – spatial resolution studies X offset Eta 5 vertex 10° Y offset Aligning trackers to zigzag GEM det. σ=21μrad Inclusive residual for 1 st tracker Resolution in φ for trackers Errors smaller than marker size tracker A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/2014 8

Beam test results – spatial resolution studies (cont.) A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/ Exclusive residual σ=281μrad Inclusive residual σ=223μrad

Beam test results – 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). A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/

Beam test results – 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. A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/ Centroid position distribution from COG method (in middle-sector 5). 2-strip events 3-strip events

Beam test results – cluster position correction (cont) h(η 2 ) distribution h(η 3 ) distribution Correction function for 2-strip events Correction function for 3-strip events A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/

Beam test results – 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 after correction 3-strip before correction 3-strip after correction A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/

Beam test results – 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. A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/

ADC Charges distributionEfficiency vs. HVNb of strips /cluster vs. HV P1 P3 P2 P4 P5 Position scan with 32 GeV hadron beam Spatial resolution in (r,  ) at different location in the chamber 6/27/2014 A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 15 Performances of the U.Va GEM

Gas input Gas out Top Entrance window Bottom gas window 6/27/2014 A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 16 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.

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 performed well in the beam test. A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/

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 plan to work towards entirely domestically sourced GEM foils (see the next talk from Temple U. group). 6/27/2014 A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 18

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. The FLYSUB consortium A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/

Back up – align the zigzag detector X offset Eta 5 vertex 10° Y offset 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. A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/

Back up - references  References on the strip response correction: CERN-Thesis 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 6/27/2014 A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 21

Motivation Conceptual 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. A. Zhang et al., Study of a Large-area GEM Detector with Zigzag-strip Readout 6/27/ Example of zigzag strips 2.5mm