MEG実験の液体Xe検出器について 東大　ICEPP 　森研究室 M1 金子大輔

What is needed for MEG experiment ? →　Reduce backgrounds as much as possible → High accuracy g-ray detector ! ・ Energy 1.4% @ 53 MeV ・ Timing 0.1 ns ・ Position 4 mm We adopted Liquid Xenon detector

Merits and Difficulty about XENON GOOD BAD Xenon emits much scintillation light (about 75% of NaI(Tl)) Decay time is very short ~ 45 ns Radiation length is short ~ 2.77 cm ↑(Z=54) Uniformity able to purify Xenon is liquid only in 161K ~ 165K Latent heat is small Insulation vessel is needed

Light emission process of xenon There are two ways of scintillation process lpeak= 178nm (vacuum ultra violet ray) no self absorption ! Xe e- ionization Xe Xe+ Xe2+ e- excitation Xe* Xe Xe Xe** Xe t = 45 ns Xe* recombination Xe2* 1S+u 3S+u t = 4.2ns t = 22ns Xe Xe + photon

Our liquid xenon detector >850 litter volume liquid xenon “C” shape to fit magnet 846 photo-multiplier-tubes (PMT) observe scintillation light Pulse Tube Refrigerator control xenon’s temperature To reduce mass of inner face honeycomb window is equipped

Connection around xenon detector storage tanks gas purifier 1000L tank liquid purifier

g Principles of detection and analysis PMTs catch scintillation photons and output signals Each quantity is calculated Energy of g ← sum of signals of all PMTs Time ← mean of timings Position ← light distribution g wave form from a PMT

Detector’s Outside high-voltage connector signal connector COBRA-magnet honeycomb window

Detector’s Inside PMTs work in Low temperature (~160K) magnetic field (<50 Gauss) LEDs for callibration are attached wires & plates with radio-active source (Am)

Photo Multiplier Tube (PMT) Quantum efficiency (Q.E.) ~ 14 % High Voltage ~ 800 V , gain ~ 2×106 2 in. f HAMAMATSU R9288 features Metalchannel-dinode → short body → not to block gray from backward ・ tough to magnetic field quartz window　→transparent to VUV Al strip on window　→ reduce resistance in low temperature Cathode = K-Sc-Sb

Pulse Tube Refrigerator KEK Haruyama et.al stable temperature & pressure control within DT~0.1K , DP~0.001MPa How pulse tube refrigerator work ? Cooling power 200W 165K

Xenon Purification System Water and oxygen mainly absorb scintillation light ↓ MEG detector have two ways of purification systems gas purification liquid purification absorb water more efficiently than gas absorb O2 H2O etc, purifier Finally labs → 300cm getter

Electronics , DAQ DRS on-line computers detector waveform digitizer off-line computers splitter trigger on VME boards every event, waveforms of all PMTs are saved. Domino Ring Sampler (DRS) sample rate 2 GHz trigger board digitalize signals with FADC and FPGA slow control MSCB IN 2007 Runs 9974 Total 29.5 TB

Calibration methods alpha source (241Am) → Q.E. , attenuation length LED → gain , timing C-W proton beam → g energy Charge exchange → g energy etc ↑PMT gain calculation from LED runs ↑position detection from a

gray from nucleic or particle reaction is used for energy scale calibration detector PSI cyclotron Proton beam Cockcroft-Walton (C-W) accelerator target muon beam pion beam Charge exchange reaction P + p- → n + p0 p0 　→ g + g 55 , 83 MeV radiative capture P + p- → n + g 129 MeV Li ( p , g ) Be 17.6 , 14.6 MeV B ( p , g ) C 4.4 , 11.7 , 16.1 MeV

Thanks for your listening !! Status and Prospects 55 MeV g sup = 2.4% FWHM = 6.5% NOW analyzing data improving detector APRIL 2008 ~ Data taking again Thanks for your listening !!