Performance of a Large-area GEM Detector with Low Mass for EIC

We present the tracking performance of a large low-mass Triple-GEM detector prototype in a 120 GeV proton beam. The trapezoidal detector covers an azimuthal angle of 30.1 degrees and a radial range of 8-90 cm. This detector has been developed as a prototype for tracking with MPGDs at the future Electron-Ion Collider. In this environment, such a detector would mainly support the pattern recognition for tracking in the forward and backward directions. Because multiple scattering of tracks must be minimized, the material budget for the forward and backward tracking detectors is critical. Consequently, this detector implements drift and readout electrodes on large foils instead of on standard PCBs. All five foils including the three GEM foils are mechanically stretched in a single stack adapting an assembly technique pioneered by CMS for the muon endcap GEM upgrade. This results in total detector material of only 0.6% radiation lengths in the active area. Outer frames custom-made from stiff carbon-fiber composite material are employed to take up the large tension from the stretched foil stack and to provide detector rigidity while keeping the detector mass low. We describe the mitigation of insufficient rigidity of the initial design and of HV instabilities that arise from using conductive carbon fiber material. The signal is read out with one-dimensional radial zigzag strips that reduce the number of electronics channels and cost associated with that while maintaining good spatial resolution. We present performance results including a measurement of the spatial resolution of the detector obtained in a beam test at the Fermilab test beam facility.