Speaker
Description
The Jiangmen Underground Neutrino Observatory (JUNO) is a 20-kton liquid-scintillator neutrino experiment designed to precisely measure reactor neutrino oscillations and determine the neutrino mass ordering. 20-inch photomultiplier tubes (LPMTs) provide the dominant photon statistics for energy reconstruction, but their charge response can exhibit a time-dependent instrumental non-linearity, motivating continuous monitoring to control energy-scale systematics in precision analyses. JUNO's 3-inch PMT (SPMT) system has a response that is essentially linear in the energy regime of reactor antineutrinos, allowing it to serve as a reference for tracking LPMT performance. In this work, we report the performance of the SPMT system, with a focus on the stability of the gain and the dark count rate. We also present an SPMT-referenced method to measure the instrumental non-linearity of the LPMT system using ultraviolet laser calibration data collected at the detector center over a range of intensities. Deviations from a constant LPMT-to-SPMT mean-charge ratio are used to quantify the LPMT non-linearity. If the LPMT response were perfectly linear, the mean-charge ratio between LPMTs and SPMTs would remain constant as conditions or light intensity change, and any systematic deviation from constancy provides a direct indicator of LPMT non-linearity and response evolution. This measurement supports JUNO by enabling a precise characterization of the detector response required for oscillation measurements with reactor antineutrinos.