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Description
This work presents the characterization and gain calibration of large-area hemispherical photomultiplier tubes (PMTs) utilized in the FAST telescope for ultra-high-energy cosmic ray detection. Specifically, the Hamamatsu R5912-03 and the Hamamatsu R14688 models were thoroughly tested inside a climate chamber across a temperature range of -20°C to +40°C. The gain calibration was performed using a multi-photon measurement method. Beyond standard voltage and temperature dependence fitting, a next phase of the study focused on the spatial uniformity of the photocathode response. This uniformity is significantly influenced by the Earth's magnetic field.
Furthermore, comparing controlled laboratory benchmarks with actual on-site deployments at the Pierre Auger Observatory revealed systematic shifts in performance. These variations are driven by local magnetic field differences and ambient environmental cycles at the observation sites. The best positionn of PMTs were set to minimize the impact of the magnetic field.
Accounting for these environmental and magnetic field dependencies proved essential for maintaining the long-term data accuracy of the FAST telescope array.