High Luminosity Electromagnetic Calorimeter for the GEp-SBS Experiment

A new electron calorimeter has been commissioned for the GEp experiment in Hall A running from February to June 2025. One of the toughest challenges for the high Q² form factor measurements is the immensity of the background relative to the elastic scattering rate. This new high resolution electron calorimeter was designed specifically for the GEp experiment with a primarily purpose of background suppression by allowing a threshold as high as 80-85% of the elastic peak with 95% efficiency. This calorimeter uses1656 lead glass crystals (42.5mm x 42.5mm x 340mm) each attached to a 6 inch long, cylindrical, glass light guide attached to a PMT for readout. To maintain crystal clarity in a high radiation environment, a novel scheme has been utilized for continuous annealing of the crystals. The front end of the crystals is heated to 220 degrees Celsius and the readout end of the crystal is maintained at 180 degrees Celsius. The light guides and PMTs are actively cooled to keep them below 50 degrees Celsius. I will show calibration and commissioning data illustrating the overall performance of the detector and demonstrating its ability to exclude backgrounds as designed, also showing the success of the annealing process in maintaining the detector light output.