Upgrade of liquid xenon gamma-ray detector in MEG experimentDaisuke 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
stoppingtarget
liquid xenondetector
driftchamber
timingcounter
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-back simultaneously.γ → Liquid Xenon Detectore+ → 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. 2008. 58:315-41 W. J. Marciano, T.Mori, and J. M. Roney
We set the current most stringent upper limit of branching ratio, 2.4×10-12 at 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]
INTRODUCTIONWe 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 presentation @ 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
12x12mm2 MPPCs on PCB
Dark count rateIn liquid Xe temp. (165K), thermal dark noise of MPPC is ~105 times lower than that in room temperature.This noise level is acceptable for our detector even at larger samples.
Temperature dependence ofGain & PDEGain & PDE depend on temperature, due to the shift of breakdown-voltage.
MEG
aimed for 2013
goal of upgrade
The effect is small since LXe temperature is highly stabilized.
setup inside xenon chamber
PMT
MPPCmountin
gboardanti-
reflection
cylinder241Am 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 slope2%/K
PDE slope5%/KSince 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
MPPC
PMT
γ-ray
Present
Upgraded
Present
Upgraded
γ
γ
We evaluated performance of the upgraded xenon detector by Monte Carlo simulation.
PCB vacuum feedthroughIn 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 AssemblyMPPCs will be mountedon long PCB, and thePCBs will be attachedon inner face of xenoncryostat with thin metalwires.
quartzprotection
Detector assembly
image
γ
e+
candidates ・ MPPC ・ Smaller, square PMT ・ Flat panel PMT
Depth [cm]
Posi
tion
res
olut
ion
in σ
[m
m]
Red : PMT (present)Blue : MPPC (upgraded)
Position Resolution
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.60%2%4%6%8%
10%12%14%16%18%20%
3mm sample12mm sample
Over Voltage[V]
PDE
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.60%
5%
10%
15%
20%
25%
30%3mm sample 13mm sample 212mm sample
Over Voltage [V]
AP
+ C
T P
roba
bilit
y
Development of UV-sensitive MPPCUV-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-lightpossible solutions
・ Remove protection layer・ Reduce thickness of contact layer・ Anti reflection coating・ Refractive index of sensor surface better matched to LXe
Large-area MPPCcurrently 3x3mm2 is the largest commercial model, however it is too small for MEG γ detector.
Cross-sectional image of MPPC
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
→ high density vacuum feedthrough is required. (see below ”PCE vacuum feedthrough”)
→ parameter optimization is needed smaller pixel, lower quench resistance
Red : presentBlue : upgraded
Signal γ-ray spectra, left and right correspond to shallow and deep part
respectively.
Energy Resolution
σup2.1%↓
0.6%
σup1.0%↓
0.5%
Slant angle for better uniformityWider inner face to reduce energy leakage
Engineering Design
Single Photon Countability1 photo-electron and 2 photo-electron evens can be resolved.
We succeeded to detect UV light with large-area (12x12mm2) MPPC