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 liquid xenon gamma-ray detector under a high-intensity muon beam environment. commissioning phase of with beam, significant degradation Detection Efficiency (PDE) for VUV was observed, while PDE visible much less significant. This wavelength dependence damage level suggests that radiation is localized surface MPPC where incoming photons create electron–hole pairs. The study also found original could be restored by thermal annealing.