Study on degradation of VUV-sensitivity of MPPC for liquid xenon scintillation detector by radiation damage in MEG II experiment

In the MEG II experiment, Multi-Pixel Photon Counters (MPPC) sensitive to vacuum ultraviolet (VUV) light are used in the liquid xenon gamma-ray detector under a high-intensity muon beam environment. In the commissioning phase of the detector with the beam, a significant degradation in the Photon Detection Efficiency (PDE) for VUV light was observed, while the degradation in the PDE for visible light was much less significant. This wavelength dependence of the damage level suggests that the radiation damage is localized to the surface of the MPPC where incoming VUV photons create electron-hole pairs. The study also found that the original PDE level could be restored by thermal annealing.