Performance of liquid xenon gamma ray detector for MEG

MEG experiment searches for a rare muon decay ( μ + → e + γ ) at PSI in Switzerland. A physics run to take large amount of data was started at the end of summer in 2008. MEG will have a sensitivity around predicted branching ratio by new theories beyond the standard model. In order to discriminate signal events from backgrounds, precise measurement of energy, emission angle and time is important. The gamma ray detector of the experiment is a scintillation detector utilizing 900 litters of liquid xenon and 846 photo-multiplier tubes. The photon yield of xenon is high and the decay time is fast. Because of these properties, the detector has good energy, position and time resolutions. The construction of the gamma ray detector was completed in 2007. The control of the transfer and purification system of xenon and the operation of the detector worked successfully. In 2008, calibration of the detector was done in one month by using gamma rays from neutral pion decays. During the calibration run, resolutions for energy, position and time were measured and calibration parameters for reconstruction algorithms were obtained. During the physics run, variation of light yield due to purification was observed. The variation was well monitored by using several light sources.