Upgrade of liquid xenon gamma-ray detector in MEG experiment Daisuke Kaneko, the University of Tokyo, on behalf of the MEG collaboration MEG EXPERIMENT Performance of Upgraded Detector DC μ + beam COBRA magnet with gradient magnetic field stopping target liquid xenon detector drift chamber timing counter MEG is searching for the lepton flavor violating decay, μ → e + γ. μ → e + γ is suppressed in the standard model (~ ), but sizable probability (~10 -12~14 ) is predicted in many promising theories beyond the standard model. Signal is 52.8MeV γ-ray and 52.8MeV e + emitted back-to-back simultaneously. γ → Liquid Xenon Detector e + → Drift Chamber → Timing Counter In order to reject accidental background and thus to achieve high sensitivity, good resolutions are required for γ detector. Annu. Rev. Nucl. Part. Sci : W. J. Marciano, T.Mori, and J. M. Roney We set the current most stringent upper limit of branching ratio, 2.4× at 90% CL. in year Phys. Rev. Lett.,107:171801,2011 We are aiming at ~ 6× as our goal for first stage of MEG experiment. (Year 2013) Further one-order improvement is expected in upgraded experiment. (Run start in Year 2016) MEG upgrade proposal to PSI is approved in Jan arXiv: [physics.ins-det] INTRODUCTION We are searching for the μ → e +γ decay in the MEG experiment at Paul Scherrer Institute in Switzerland with an unprecedented sensitivity. In order to achieve a higher sensitivity, we plan to upgrade the experiment, including an upgrade of the liquid xenon γ-ray detector with MPPC readout. It turned out by a simulation that the energy and position resolution will be significantly improved especially for events where γ-ray converts at a shallow part of the detector. UV-sensitive MPPC is under development to detect liquid xenon scintillation light in VUV range. Design and expected performance of the upgraded LXe detector and R&D status of UV-sensitive MPPC are presented Performance of UV-sensitive MPPC SUMMARY MEG Liquid xenon detector will be upgraded with MPPC The performance of upgraded detector is being confirmed in MC simulation. MPPCs sensitive to LXe scintillation is under development, enough high PDE (~17%) is already obtained PROSPECTS Optimization of MPPC performance is still in progress Further performance improvement is anticipated with recent breakthrough in HPK MPPC technology (HPK IEEE 2012) We are planning a prototype test with hundreds of MPPCs in 2013 Detector construction will start in 2014 Energy resolution will be greatly improved. sensor coverage becomes more uniform especially at shallow position with small sensors. Position resolution will also be improved at shallow part. Detection efficiency improves by 9%, thanks to small thickness of MPPC. Timing resolution is expected to be the same as current detector. MPPC (12mm) Comparison of light distribution of scintillation light, imaging power will be greatly improved Photon Detection Efficiency (PDE) PDE is measured from scintillation photon from α-source. Currently, the most sensitive model has PDE about 17%, which enables us to detect more photons with the upgraded detector than those with the current detector 12x12mm 2 MPPCs on PCB Dark count rate In liquid Xe temp. (165K), thermal dark noise of MPPC is ~10 5 times lower than that in room temperature. This noise level is acceptable for our detector even at larger samples. Temperature dependence of Gain & PDE Gain & PDE depend on temperature, due to the shift of breakdown-voltage. MEG aimed for 2013 goal of upgrad e The effect is small since LXe temperature is highly stabilized. setup inside xenon chamber PMT MPPC mounting board anti- reflection cylinder 241 Am α source Signal transmittance in long cables In actual xenon detector, signal must go through about 10m before readout electronics. Deterioration by different cable length is tested, and it turn out to be moderate. 3x3mm sample gain slope 2%/K PDE slope 5%/K Since MPPC signals contain effect of Cross Talk & After Pulse, PDE is corrected with its probability measured separately. ・ Replace current 2” PMTs on incident face with smaller photosensors ・ Modification in lateral PMT arrangement CG image MPP C PMT γ-ray Present Upgraded Present Upgraded γ γ We evaluated performance of the upgraded xenon detector by Monte Carlo simulation. PCB vacuum feedthrough In order to transmit more channels, we are developing PCB feedthrough. molded with epoxy MMCX connectors Coaxial like pattern is printed. Characteristic impedance 50Ω ・ Vacuum tightness ・ Signal transmittance were confirmed Concept of LXe Detector Upgrade Detector Assembly MPPCs will be mounted on long PCB, and the PCBs will be attached on inner face of xenon cryostat with thin metal wires. quartz protection Detector assembly image γ e+e+ candidates ・ MPPC ・ Smaller, square PMT ・ Flat panel PMT Depth [cm] Position resolution in σ [mm] Red : PMT (present) Blue : MPPC (upgraded) Position Resolution Development of UV-sensitive MPPC UV-sensitive MPPC is under development in collaboration with Hamamatsu. ← In commercial MPPC, most photons are absorbed by protection coating on surface. In addition, contact layer have little sensitivity because E field is weak in the layer. Improve sensitivity to xenon scintillation (λ: 175±5nm) commercially available products don’t have sensitivity to UV-light possible solutions ・ Remove protection layer ・ Reduce thickness of contact layer ・ Anti reflection coating ・ Refractive index of sensor surface better matched to LXe Large-area MPPC currently 3x3mm 2 is the largest commercial model, however it is too small for MEG γ detector. Cross-sectional image of MPPC With 12x12 mm 2 active region, ・ ~4000 channels will be needed to fill inner face. (216 channels, currently) ・ signal waveform become wider due to larger capacitance → high density vacuum feedthrough is required. (see below ”PCE vacuum feedthrough”) → parameter optimization is needed smaller pixel, lower quench resistance Red : present Blue : upgraded Signal γ-ray spectra, left and right correspond to shallow and deep part respectively. Energy Resolution σ up 2.1% ↓ 0.6% σ up 1.0% ↓ 0.5% Slant angle for better uniformity Wider inner face to reduce energy leakage Engineering Design Single Photon Countability 1 photo-electron and 2 photo- electron evens can be resolved. We succeeded to detect UV light with large-area (12x12 mm 2 ) MPPC