Cryogenic performance and long-term stability of Hamamatsu R8520-506 photomultiplier tubes in liquid argon
Seven Hamamatsu R8520-506 photomultiplier tubes were characterized at room temperature and in liquid argon to evaluate their response and operating stability for liquid-argon light readout. A 405~nm pulsed light source was used to measure gain, peak-to-valley ratio, single-photoelectron charge resolution, and afterpulsing, while dark-count data were acquired with random triggers in the absence of active illumination. Gain and dark-count stability were monitored during a ten-day operating period in liquid argon. At a motherboard input voltage of 900~V, the extracted gains in liquid argon were approximately 27.8\%--51.3\% higher than the corresponding room-temperature values. For all seven tubes, the peak-to-valley ratios were higher and the single-photoelectron charge resolution improved in liquid argon. The dark count rates ranged from approximately 143 to 252~Hz at room temperature and from 166 to 199~Hz in liquid argon, with three tubes showing increases and four showing decreases. The mean number of detected afterpulses per selected primary event was approximately 21.0\%--63.6\% lower in liquid argon. The relative standard deviations across the ten daily means of gain and dark count rate did not exceed approximately 2.1\% and 3.3\%, respectively. The measurements provide a quantitative basis for considering R8520-506 tubes as photosensor candidates for liquid-argon detectors.