Scintillator ageing of the T2K near detectors from 2010 to 2021

The T2K experiment widely uses plastic scintillator as a target for neutrino interactions

and an active medium for the measurement of charged particles produced in neutrino interactions

at its near detector complex. Over 10 years of operation the measured light yield recorded by the

scintillator based subsystems has been observed to degrade by 0.9–2.2% per year. Extrapolation

of the degradation rate through to 2040 indicates the recorded light yield should remain above the

lower threshold used by the current reconstruction algorithms for all subsystems. This will allow the

near detectors to continue contributing to important physics measurements during the T2K-II and

Hyper-Kamiokande eras. Additionally, work to disentangle the degradation of the plastic scintillator

and wavelength shifting fibres shows that the reduction in light yield can be attributed to the ageing

of the plastic scintillator. The long component of the attenuation length of the wavelength shifting

fibres was observed to degrade by 1.3–5.4% per year, while the short component of the attenuation

length did not show any conclusive degradation.