GEM detector performance and efficiency in Proton Charge Radius (PRad) Experiment*

The PRad experiment (E12-11-106\footnote{Spokespersons: A.Gasparian(Contact), H. Gao, M. Khandaker, D. Dutta}) was performed in 2016 at Jefferson Lab in Hall B. It aims to investigate the proton charge radius puzzle through electron proton elastic scattering process. The experiment used a non-magnetic spectrometer method, and reached a very small ep scattering angle and thus an unprecedented small four-momentum transfer squared region, $Q^2$ from $2\times10^{-4}$ to $0.06 (GeV/c)^2$. PRad experiment was designed to measure the proton charge radius within a sub-percent precision. Gas Electron Multiplier (GEM) detectors have contributed to reach the experimental goal. A pair of large area GEM detectors, and a large acceptance, high resolution calorimeter(HyCal) were utilized in the experiment to detect the scattered electrons. The precision requirements of the experiment demands a highly accurate understanding of efficiency and stability of GEM detectors. In this talk, we will present the preliminary results on the performance and efficiency of GEM detectors.