The Spectator Reaction Plane Detector for CMS*

The Spectator Reaction Plane Detectors (SRPDs) are the latest addition to the CMS Zero Degree Calorimeters (ZDCs), designed to produce a reaction plane by mapping spectator neutrons produced in heavy ion collisions at the CERN LHC. There are two ZDCs each positioned 140-m from the interaction point in the positive and negative z-direction. Each ZDC contains an electromagnetic (EM) section and a hadronic (HAD) section with the SRPDs placed between these elements. Both SRPDs are utilized for reaction plane reconstruction, making it important to inter-calibrate the electronic gains of the corresponding quartz blocks. Correlating the output of the SRPD with the 5 sections of the EM in the x-direction gives a combined x-position resolution of ~ 2 mm. It has been recognized that blocks located around the beam position sustained significant radiation damage during the 2018 PbPb run, requiring calibration factors to account for diminished light transmission to the PMTs. This radiation effect is being studied using an electron linac at the University of Maryland (UMD).