Development of Thin Gap Triple-GEM Detectors for Precise Tracking with Large Acceptance

A number of large-area GEM detectors constructed by the Detector group at the University of Virginia for operation in various nuclear physics experiments at JLab have successfully met critical tracking requirements, demonstrating high efficiency and excellent spatial resolution of 70μm. However, achieving simultaneous high spatial resolution and efficiency for large-area gaseous detectors poses a new challenge for physics programs, such as the Electron-Ion Collider (EIC), which require precision tracking across a wide range of incident track angles. In this case, the spatial resolution of a gaseous detector for a track incident at a large angle significantly degrades as the distance a charged particle traverses in the ionization gas volume increases. Conversely, an ionization region that is too thin may not produce a sufficient number of primary electrons, resulting in reduced detection efficiency. To reduce the impact of the incident angle of the track on the spatial resolution and to maintain high efficiency as well as the stability of the triple-GEM detectors, we have built multiple Thin-Gap triple-GEM prototypes with a drift gap significantly thinner than 3mm gap in typical GEM detectors, and with various configurations of the cathode layer. The performance of prototypes was evaluated in beam studies using the Fermilab Test Beam Facility. A detailed study of the spatial resolution and tracking efficiency versus incident track angle for different drift gap thicknesses and cathode structures of our Thin-Gap triple-GEM prototypes will be presented.