Development of a GEM based cosmic muon tracking system for characterizing Cherenkov detectors

The MOLLER Experiment at the Thomas Jefferson National Accelerator Facility (JLab) aims to measure the parity violating asymmetry (A_pv) in electron-electron (Møller) scattering with unprecedented precision. The flux of Møller-scattered electrons from the liquid hydrogen target is measured by Cherenkov detectors and the longitudinal polarization of the incoming electron beam is rapidly flipped to extract the right-left fractional flux difference and calculate A_pv. At the University of Massachusetts, Amherst, prototype detectors were designed, fabricated, and installed in an apparatus to characterize the Cherenkov detectors using cosmic muons. To investigate the performance of the Cherenkov detectors with different incident angles of muons, tracking detectors for the cosmic muons need to be employed. This study investigates Gas Electron Multiplier (GEM) detectors to implement the cosmic muon tracking system. The study includes a uniformity measurement in gain, energy resolution, and count rate using a Fe-55 source for the single mask triple GEM prototypes. The efficiency of the chambers is also measured using cosmic muons. The chambers are operated with Ar/CO₂ gas mixture in a continuous flow mode. The details of the experimental setup, methodology, and results are presented.