Upgrade of MEG liquid xenon detector with UV-sensitive MPPCs

The MEG II experiment is an upgrade of the MEG experiment, which searches for the lepton flavour violating decay, μ⁺ → e⁺γ. The goal sensitivity in MEG II is 5×10^-14, which is one order of magnitude better than MEG. In order to accomplish this goal, a 900 l liquid xenon γ-ray detector is going to be upgraded in MEG II. The granularity of the xenon scintillation light readout will improve from what we obtained in MEG, by replacing part of the 2-inch photo-multiplier tubes with 12 × 12 mm² Multi-Pixel Photon Counters (MPPCs). Thanks to the higher granularity, the energy resolution and the position resolution for the detection of γ-ray are expected to improve by a factor of 2. A UV-sensitive large area MPPC is successfully developed for this upgrade, in collaboration with Hamamatsu Photonics K. K. They have been successfully tested in liquid xenon, showing an excellent performance such as high photon detection efficiency for xenon scintillation light (>15% at expected operation voltage). For the signal transmission, a multi-layer “co-axial” type board has been developed for the PCBs to mount MPPCs. Similar type of board is also developed for the feed-through. We have carried out a mass test of -600 prototype MPPCs in liquid xenon, using the prototype of PCBs and feed-through. Final model MPPCs (-4000 pcs) are also tested in room temperature, in order to check their performance before installation. According to the results of these measurements, the expected performance of the liquid xenon detector is estimated using Monte-Calro simulation. The results of the tests and the results of the simulation is presented.