Development of Liquid Xenon Photon Detector for μ→ eγ Search Experiment at PSI μ→ eγ 崩壊探索実験用液体キセノン検出器の開発

Wataru OOTANIInternational Center for Elementary Particle Physics

University of TokyoFor the MEG collaboration

1. Introduction2. μ→eγSearch Experiment at PSI3. Status of R&D of Liquid Xenon Detector 4. Summary

MEG collaboration （仮名） in Japan東大素セ

浅井祥仁、大谷航、小曽根健嗣、佐伯学行、西口創、真下哲郎、三原智、森俊則、八島純、山下了、吉村浩司

東大理折戸周治

早大理工総研岡田宏之、菊池順、澤田龍、鈴木聡、

寺沢和洋、道家忠義、山下雅樹、吉村剛史

高エ研久野良孝、杉本康博、春山富義、真木昌弘、山本明

名大増田公明

Introduction

Charged lepton flavor violating (LFV) processForbidden in the Standard ModelSensitive to physics beyond the Standard Model

SUSY-GUT, SUSY+νR , …Present experimental bound

Br(μ ＋→ e ＋ γ) < 1.2 x 10 － 11 (MEGA experiment)

μ ＋→ e ＋ γμ+

e+

γEe=mμ

2+me2

2mμ

Eγ=mμ

2 － me2

2mμ

~ 52.8MeV

~ 52.8MeV e+ and γ

• Back-to-back• Coincident

SUSY-GUT

SU(5) SUSY-GUT predicts Br （ μ ＋→ e ＋ γ ） ＝ 10-15 - 10-13

(SO(10) SUSY-GUT: even larger value 10-13 - 10-11)

L.J.Hall et al. Nucl. Phys. B267(1986)415

Neutrino Oscillation and SUSY

Signature of μ→ eγ could be discovered at the sensitivity of Br ~ 10-14

“MSW small angle mixing’’ and “Just-so’’ are disfavored by recent Super Kamiokande results

μ→ eγsearch experiment at PSI

Liquid Xe photon detectorPositron spectrometer with gradient magnetic fieldThin superconducting solenoidDC muon beam at PSI 108μ/sec

Expected sensitivity Br(μ→ eγ) ~ 10-14

Sensitivity and Backgrounds

Nμ=1x108/sec, T =2.2x107sec, Ω/4π=0.09, εγ=0.7,εe=0.95

Br(μ ＋→ e ＋ γ) ~ 0.94 x 10-14

Major backgrounds

Single event sensitivity

• Accidental Coincidence Michel decay(μ ＋→ e ＋ νeνμ) + random γ

• Radiative muon decays (inner bremsstrahlung) μ ＋→ e ＋ νeνμ γ

Expected Detector Performance

ΔEe 0.7% (FWHM)ΔEγ 1.4 – 2.0 % (FWHM)Δeγ 12 – 14 mrad

(FWHM)Δteγ 0.15 nsec

(FWHM)

Backgrounds can be suppressed well below 10-14

Baccidental ∝ ΔEe Δteγ ( ΔEγ )2 ( Δeγ )2

Accidental background rate

Positron Spectrometer Superconducting solenoid spectrometer with gradient

magnetic field Constant bending radius independent of emission angle

Positrons are quickly swept out

Uniform field Gradient field

Gradient fieldUniform field

See also : 西口創他　 μ ＋→ e ＋ γ 崩壊探索実験のための e ＋ スペクトロメータの研究開発　 (25pYE-10)

Liquid Xenon photon detector

Scintillation light is viewed by ~800 PMTs (Mini-Kamiokande type) effective coverage ~ 35%

Good energy resolution Fast responseSpatially uniform response

See also : 八島純他　 μ ＋→ e ＋ γ 崩壊探索実験の 高性能液体 Xe photon detector の R&D (25pYE-9)

Liquid Xenon as ScintillatorHigh light yield (75% of NaI(Tl))Fast signalsSpatially uniform response

Excitation Xe+Xe*→ Xe2*→2Xe+hν(175nm)

Recombination

Xe++Xe→ Xe2+

Xe2++e→ Xe**+Xe

Xe**→ Xe*

Xe+Xe*→ Xe2*→ 2Xe+hν(175nm)

Properties of Liquid Xenon Scintillator

Mass number 131.29

Density 3.0 g/cm3

Boiling and melting points 165 K, 161 KEnergy per scintillation photon

24 eV

Radiation length 2.77 cmDecay time 4.2 nsec (fast

component)22 nsec (slow component)45 nsec (recombination)

Scintillation light wave length

175 nm

Refractive index 1.57

R&D of PMTHamamatsu R6041Q

Dynode structure Metal channel

Photo cathode Rb-Cs-SbWindow QuartzQuantum efficiency

10-15 %

PMT size 57 mm dia.Effective area 46 mm dia.PMT Length 32 mmTypical H.V. 1000 VCurrent amplification

9x106

TTS 0.3 ns typ.

Dynode structure

R&D of PMT

R6041Q can be stably operated at liquid Xe temperature (165K)

First Prototype of Liquid Xenon Detector

32 x PMTs (R6041Q)Active Xe volume

116 x 116 x 174 mm3

PMT Frame of First Prototype

Energy Resolution Measurements

Possible to achieve < 1% for

52.8MeVγ

Position Resolution Measurements

• Positions are determined by means weighed by PMT output

• Possible to achieve < 1mm for 52.8MeV γ

Timing Resolution Measurements

Possible to achieve σ~ 50psec for 52.8MeV γ

PMT Calibration with Gas XePMT calibration with

scintillation light (175nm) from gas Xe

Quantum efficiency Gain Position dependence on photo-cathode plane

Movable α- source with collimator spread of light spot ~ 2mm

Operating temperature 190K – 300K

Position Dependence on Photo-cathode Plane

300K 190K

Liquid Xenon Level Meter Capacitance level meter Level resolution < 3mm

Large Prototype of Liquid Xenon Detector

Prototype of larger size 1/4 - 1/3 size of final detector 264 PMTs

Measurement of Resolutions for high energy γ Energy, timing, position,… Light attenuation length, light absorption length

Establishment of calibration technique and cryogenics, , purification system, …

Tests with large prototype will start at the beginning of 2001

…

Tests with High Energy Photon BeamTERAS electron storage ring of Electrotechnical Laboratory (ETL)Inverse Compton gamma rays with an energy up to 40MeV

ETL electron LINAC facility TERAS

See also : 豊川弘之他　蓄積リング TERAS における汎用 LCS ラインの建設　 (23aYC-2)

Support structure of PMTs on the front wall of the large prototype

PMTG10

Lucite

Schedule of μ→ eγsearch experiment2000

Construction of large prototype of liquid Xe detector R&D of positron tracker and timing counter Beam test of positron tracker at PSI (18/10/200~) Design work of superconducting solenoid

2001 Test with large prototype at ETL Construction of superconducting solenoid (winding,

cryostat,…) Beam line studies at PSI Design work of final version of liquid Xe detector

2002 Fabrication and assembly of detector component Tests of each detector component

2003 Engineering/physics run

Summary

New experiment to search for μ ＋→ e ＋ γwith a sensitivity of 10-14 at PSI is in preparation R&D works of Liquid Xe photon detector with good energy and timing resolutions are under wayPerformance of the liquid Xe detector for high energy γ will be checked with large prototype early 2001 Preparations of other detector components are on-going Engineering/physics run will be started in 2003

Mechanical Analysis of Superconducting Solenoid

Stress distribution in the coil (cross-sectional view)

Max 180MPa

Center of the solenoid →

Stray Magnetic Field in Liquid Xe detector Region

Iron yoke

Active shields (compensation coil)PMT with fine mesh dynode structure