Observation of the Plastic Scintillator Ageing During 6 Years of DANSS Operation.

Ageing of the plastic scintillators plays a significant role in the degradation of detectors' performance. Several effects are considered responsible for this process such as mechanical stresses, water penetration and oxidation due to exposure to ultraviolet radiation. The DANSS detector core consists of 2500 polystyrene based scintillation plates with wavelength shifting fiber readout (strips). The detector is located just below an industrial nuclear reactor of Kalininskaya NPP. The room housing the detector is air conditioned and has very low humidity. The temperature of about 20 C was maintained at the detector core. The performance of the strips is permanently monitored with atmospheric muons passing through the detector. Six years of measurements since April 2016 till March 2022 are reported. We observe the average light yield degradation of the strips at the level /year. The decrease of the wavelength shifting fiber attenuation length is /year. [ABSTRACT FROM AUTHOR]