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Upgrade of liquid xenon gamma-ray detector in MEG experiment

Daisuke Kaneko, the University of T okyo , on behalf of the MEG collaboration. Upgrade of liquid xenon gamma-ray detector in MEG experiment. INTRODUCTION

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Upgrade of liquid xenon gamma-ray detector in MEG experiment

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  1. Daisuke Kaneko, the University of Tokyo, on behalf of the MEG collaboration Upgrade of liquid xenon gamma-ray detector in MEG experiment INTRODUCTION We are searching for the μ → e+γ decay in the MEG experiment at Paul Scherrer Institute in Switzerlandwith 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 MEG EXPERIMENT Concept of LXe Detector Upgrade MEG is searching for the lepton flavor violating decay, μ → e+ γ. μ → e + γ is suppressed in the standard model (~10-50), 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-backsimultaneously. 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. ・Replace current 2” PMTs on incident face with smaller photosensors ・Modification in lateral PMT arrangement liquid xenon detector stopping target PMT candidates・MPPC ・Smaller, square PMT ・Flat panel PMT Present γ MPPC γ-ray e+ Present DC μ+ beam Upgraded CG image Slant anglefor better uniformity γ timing counter Widerinner face to reduce energy leakage COBRA magnet with gradient magnetic field MPPC (12mm) drift chamber Upgraded We set the current most stringent upper limit of branching ratio, 2.4×10-12at 90% CL. in year 2011. Phys. Rev. Lett.,107:171801,2011 We are aiming at ~6×10-13 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. 2013. arXiv:1301.7225 [physics.ins-det] Comparison of light distribution of scintillation light, imaging power will be greatly improved γ quartz protection Development of UV-sensitive MPPC aimed for 2013 goal of upgrade UV-sensitive MPPC is under development in collaboration with Hamamatsu. MEG Annu. Rev. Nucl. Part. Sci. 2008. 58:315-41 W. J. Marciano, T.Mori, and J. M. Roney 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 ← 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. Performance of Upgraded Detector We evaluated performance of the upgraded xenon detector by Monte Carlo simulation. Energy Resolution • 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. Red : present Blue : upgraded Cross-sectional image of MPPC σup 2.1% ↓ 0.6% σup 1.0% ↓ 0.5% With 12x12 mm2 active region, ・~4000 channels will be needed to fill inner face. (216 channels, currently) ・signal waveform become wider due to larger capacitance Large-area MPPC currently 3x3mm2is the largest commercial model, however it is too small for MEG γ detector. →high density vacuum feedthrough is required. (see below ”PCE vacuum feedthrough”) →parameter optimization is needed smaller pixel, lower quench resistance Signal γ-ray spectra, left and right correspond to shallow and deep part respectively. Position Resolution Signal transmittance in long cables In actual xenon detector, signal must go through about 10m before readout electronics. Deterioration by differentcable length is tested, and it turn out to be moderate. PCB vacuum feedthrough In order to transmit more channels, we are developing PCB feedthrough Engineering Design . Position resolutionin σ [mm] 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. ・Vacuum tightness ・Signal transmittance were confirmed Red : PMT (present) Blue : MPPC (upgraded) Detector assembly image Depth [cm] Coaxial like pattern is printed. Characteristic impedance 50Ω MMCX connectors molded with epoxy • 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 scintillationis 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 presentation @ IEEE 2012) • We are planning a prototype test with hundreds of MPPCs in 2013 • Detector construction will start in 2014 Performanceof UV-sensitive MPPC Temperature dependence of Gain & PDE Gain & PDE depend on temperature, due to the shift of breakdown-voltage. We succeeded to detect UV light with large-area (12x12mm2) MPPC 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 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. The effect is small since LXe temperature is highly stabilized. 12x12mm2 MPPCs on PCB MPPC mounting board Dark count rate In liquid Xetemp. (165K), thermal dark noise of MPPC is ~105 times lower than that in room temperature. This noise level is acceptablefor our detector even at larger samples. anti-reflection cylinder 241Am α source PMT Single Photon Countability 1 photo-electron and 2 photo-electron evens can be resolved. setup inside xenon chamber 3x3mm sample

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