Speaker
Description
Fluorescence detector Array of Single-pixel Telescopes (FAST) aims to achieve high-statistics observations of ultra-high-energy cosmic rays by deploying a large number of fluorescence telescopes over a wide area for next-generation cosmic-ray experiments. Total seven FAST prototypes are currently installed and operated at sites of the Telescope Array experiment and the Pierre Auger Observatory. The prototypes consists of mirrors, cameras, and electronics. Variations in the characteristics of these components must be properly taken into account of data analysis. Otherwise, they can cause large uncertainties in the reconstructed energy and arrival direction of cosmic rays. FAST is designed for long-term operation over more than 10 years, making it essential to understand the variations in detector performance due to environmental effects and aging. In this study, we analyzed data from the standard light source YAP collected between 2017 and 2025 with operating the FAST prototypes. We also performed gain measurements of the photomultiplier tubes (PMTs) used in FAST through single photoelectron measurements in a laboratory. In this contribution, we evaluate the long-term variation and temperature dependence of the PMT gain in FAST and report on studies toward establishing a reliable calibration method.
