An LED-based calibration system for the new ATLAS Reaction Plane Detector in LHC Run 3*

In 2022, Long Shutdown 2 (LS2) ended, and Run 3 of the CERN Large Hadron Collider (LHC) began. LS2 offered the opportunity of constructing a new Reaction Plane Detector (RPD) for the ATLAS experiment, a novel device aimed at mapping the transverse profile of the shower induced by spectator neutrons in the ATLAS Zero Degree Calorimeter (ZDC) during heavy ion data taking.

The Nuclear Physics Laboratory (NPL) at the University of Illinois at Urbana-Champaign developed and constructed a pair of new radiation-hard RPDs for ATLAS. By profiling the spectator neutron showers in the ZDC, the RPD enables the determination of the reaction plane of Pb+Pb collisions without using any of the impact products.

The ZDC and the RPD are installed in the Target Absorber for Neutrals, one of the most challenging radiation environments in the whole LHC. For this reason, it is crucial to be able to monitor the detector performance during the run, to eventually correct for radiation-induced losses in the performance of the device.

In this contribution, I will discuss the Run 3 RPD LED calibration system, which allows for live real-time monitoring of the RPD PMT gains during data taking. In particular, I will present the results from studying the LED system in a lab mockup emulating LHC working conditions.