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 OOTANI
International Center for Elementary Particle Physics
University of Tokyo
For the MEG collaboration
1.
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3.
4.
Introduction
μ→eγSearch Experiment at PSI
Status of R&D of Liquid Xenon Detector
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
μ＋→e＋γ
Ee=
Eγ=
mμ2+me2
2mμ
mμ2－me2
2mμ
e+
~ 52.8MeV
~ 52.8MeV
γ
μ+
e+ and γ
Charged lepton flavor violating (LFV) process
Forbidden in the Standard Model
Sensitive to physics beyond the Standard Model
SUSY-GUT, SUSY+νR , …
Present experimental bound
Br(μ＋→e＋γ) < 1.2 x 10－11 (MEGA experiment)
• Back-to-back
• Coincident
SUSY-GUT
L.J.Hall et al. Nucl. Phys. B267(1986)415
SU(5) SUSY-GUT predicts Br（μ＋→e＋γ） ＝ 10-15 - 10-13
(SO(10) SUSY-GUT: even larger value 10-13 - 10-11)
Neutrino Oscillation and SUSY
“MSW small angle mixing’’ and “Just-so’’ are disfavored
by recent Super Kamiokande results
Signature of μ→eγ could be discovered
at the sensitivity of Br ~ 10-14
μ→eγsearch experiment at PSI
Liquid Xe photon detector
Positron spectrometer with gradient magnetic field
Thin superconducting solenoid
DC muon beam at PSI 108μ/sec
Expected sensitivity Br(μ→eγ) ~ 10-14
Sensitivity and Backgrounds
Single event sensitivity
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
• 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)
eγ
12 – 14 mrad
(FWHM)
teγ
0.15 nsec (FWHM)
Accidental background rate
Baccidental ∝ ΔEe teγ ( ΔEγ )2 ( eγ )2
Backgrounds can be suppressed well below 10-14
Positron Spectrometer
Superconducting solenoid spectrometer with gradient magnetic field

Constant bending radius independent of emission angle
Uniform field

Gradient field
Positrons are quickly swept out
Uniform field
Gradient 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 response
Spatially 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 Scintillator
High light yield (75% of NaI(Tl))
Fast signals
Spatially 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 K
Energy per scintillation photon 24 eV
Radiation length
2.77 cm
Decay 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 PMT
Hamamatsu R6041Q
Dynode structure
Metal channel
Photo cathode
Rb-Cs-Sb
Window
Quartz
Quantum efficiency
10-15 %
PMT size
57 mm dia.
Effective area
46 mm dia.
PMT Length
32 mm
Typical H.V.
1000 V
Current 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
s < 1% for 52.8MeVγ
Position Resolution
Measurements
• Positions are determined by
means weighed by PMT output
• Possible to achieve s < 1mm
for 52.8MeV γ
Timing Resolution Measurements
Possible to achieve σ~ 50psec for 52.8MeV γ
PMT Calibration with Gas Xe
PMT 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 Beam
TERAS 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
G10
PMT
Lucite
Schedule of μ→eγsearch experiment
2000




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 way
Performance of the liquid Xe detector for high energy
γ will be checked with large prototype early 2001
Preparations of other detector components are ongoing
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

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