Measurement of Gain Evolution of PMTs for HL-ZDC and LUCID 3

The ATLAS High Luminosity Zero Degree Calorimeter (HL-ZDC) and the Luminosity Cherenkov Integrating Detector (LUCID) 3 are forward detectors which are critical for the goals of the ATLAS p+p, p+A and A+A programs in Run 3 and beyond. Both detectors heavily rely on photomultiplier tubes (PMT) for the accomplishment of their main physics objectives: enable a sub-percent precision luminosity measurement for LUCID 3 and achieve spectator neutron detection with high performance for the HL-ZDC.

The two projects jointly developed a test setup to characterize the response of the PMTs used in the experiment. It features two PMTs of different types, an LED and a bismuth source to dynamically monitor the evolution of the gain of the PMTs between different periods of continuous light solicitation. The LED intensity is tuned to mimic the average response to the light illuminating the detectors during a typical run of the Large Hadron Collider. In this contribution, I will discuss in detail the design, test and improvement of this experimental setup which was established after multiple iterations of data taking. I will show the results obtained and summarize the main conclusions for the HL-ZDC and LUCID 3 upgrade projects.