Performance Evaluation of MPPC Used for MEG II Gamma Ray Detector

MEG II is the upgrade project from MEG experiment, which aims to find charged lepton flavor violation (cLFV) mu -> e + gamma with the world highest sensitivity. With the upgrade, the sensitivity is expected to be 4 x 10(-14), which is ten times better than that of MEG. For the upgrade of the gamma ray detector, 216 photo-multipliers on the gamma ray incident wall are replaced with 4092 vacuum ultraviolet (VUV) sensitive Multi Pixel Photon Counters (MPPCs) which are developed uniquely for this experiment in cooperation with Hamamatsu Photonics K. K.. We carried new measurements to understand the characteristics of the MPPC in detail for precise operation of the detector. First, PDE measurement was done with the improved setup whose wall is coated with a special anti-reflective material. Second, PDE was measured as the function against incident angle of photons, which is important for the reconstruction of gamma ray position in high precision.