Gain Characterization of Photo-Multiplier Tubes used in the ATLAS ZDC Reaction Plane Detector for LHC Run 3

Long Shutdown 2 (LS2) of the CERN LHC offered the opportunity to expand the physics capabilities of existing experiments. Over this period, the Nuclear Physics Laboratory at the University of Illinois at Urbana-Champaign designed and constructed a new Reaction Plane Detector (RPD) for the ATLAS experiment to be integrated within the ATLAS Zero Degree Calorimeter (ZDC) during heavy ion (HI) data taking. The RPD consists of 256 vertical, 0.7\~mm diameter radiation hard fused silica fibers of four different lengths. The Cherenkov light from the fibers is read by 16 Photo-Multiplier Tubes (PMTs). The PMT signals are read out with waveform digitizers on the ATLAS LUCROD readout board. The PMT signals are used to build a 4x4 tile mapping of the shower in the transverse plane. Given limited space for HV cables in the LHC tunnel, the PMTs will be powered in groups of 4 using voltage divider circuits. Based on gain calibrations for each PMT, they are grouped to minimize effects from tube-to-tube variations. The gain calibration will also be used to calibrate the channel-by-channel gain amplifiers on the LUCROD.

This paper presents gain measurements performed on a large set of PMTs using the single-photon method. The methodology, apparatus, and gain matching process are described in detail.