Large area VUV-sensitive MPPCs for liquid xenon detector in MEG II experiment

The MEG II experiment is an upgrade of the MEG experiment to search for the charged lepton flavor violating decay of muon, μ⁺ →e⁺γ. The performance of the liquid xenon (LXe) γ-ray detector will be greatly improved with a highly granular scintillation readout realized by replacing 216 photomultiplier tubes (PMTs) on the γ-ray entrance face with 4092 Multi-Pixel Photon Counters (MPPCs). For this purpose, we have developed a new type of large area MPPC which is sensitive to the LXe scintillation light in vacuum ultraviolet (VUV) range, in collaboration with Hamamatsu Photonics K.K. The MPPC has been tested in LXe, and an excellent performance has been confirmed including high photon detection efficiency (> 15%) for LXe scintillation light. This MPPC is a discrete array of four 6 ×6 mm² chips, and a sufficiently short time constant for a single photoelectron waveform is achieved by connecting them in series. A good energy and timing resolution have been confirmed for the LXe scintillation light. For further understanding of the MPPC, we have performed a PDE measurement in gas xenon to know the angular dependence of MPPC PDE. In this paper, results of these tests are reported.