Performance in test beam of a large-area and light-weight GEM detector with 2D stereo-angle (U-V) strip readout

A large-area and light-weight gas electron multiplier (GEM) detector was built at the University of Virginia as a prototype for the detector R&D program of the future Electron Ion Collider. The prototype has a trapezoidal geometry designed as a generic sector module in a disk layer configuration of a forward tracker in collider detectors. It is based on light-weight material and narrow support frames in order to minimize multiple scattering and dead-to-sensitive area ratio. The chamber has a novel type of two dimensional (2D) stereo-angle readout board with U-V strips that provides (r,φ) position information in the cylindrical coordinate system of a collider environment. The prototype was tested at the Fermilab Test Beam Facility in October 2013 and the analysis of the test beam data demonstrates an excellent response uniformity of the large area chamber with an efficiency higher than 95%. An angular resolution of 60μrad in the azimuthal direction and a position resolution better than 550 μm in the radial direction were achieved with the U-V strip readout board. The results are discussed in this paper.