  In the MEG II experiment, the liquid xenon gamma-ray detector uses
Multi-Pixel Photon Counters (MPPC) which are sensitive to vacuum ultraviolet
(VUV) light 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 found, while the degradation in
the PDE for visible light was much less significant. This implies that the
radiation damage is localized to the surface of the MPPC where incoming VUV
photons create electron-hole pairs. It was also found that the PDE can recover
to the original level by thermal annealing.
