Measurement of the Neutron Electromagnetic Form Factor at High Q² and GEM Detector Performance

The neutron electromagnetic form factors, G_Eⁿ and G_Mⁿ, give important insights into the neutron structure. In previous experiments the proton electromagnetic form factors, G_E^p, was measured up to Q² = 8.5 GeV², and G_M^p was measured up to Q² = 30 GeV², while G_Eⁿ has only been measured up to Q² = 3.4 GeV². The Super BigBite Spectrometer (SBS) program at JLab will measure the neutron form factor ratio, G_Eⁿ/G_Mⁿ, at Q² values of 1.5, 3.8, 6.8, and 10.2 GeV² by colliding a polarized electron beam with a polarized ³He target, used here as an effective polarized neutron target, and measuring the transverse asymmetry of the cross section. Scattered electrons are measured in the BigBite spectrometer equipped with Gas Electron Multiplier (GEM) detector based tracker, while scattered neutrons are magnetically separated from scattered protons and detected in a segmented hadron calorimeter. GEM tracking detectors measure the scattered electrons trajectory with 70 μm resolution and can operate at rates up to 500 kH/cm². A new high voltage design was implemented prior to this experiment to improve the GEM efficiency and preliminary performace will be presented.