1. Upgrade of LXe gamma-ray detector in MEG experiment
Daisuke Kaneko, ICEPP, University of Tokyo, on behalf of the MEG collaboration
Introduction
We are searching for the μ → e +γ decay in the MEG experiment at Paul Scherrer Institute in Switzerland, and improved experimental upper limit on the branching ratio. In order to achieve a higher sensitivity, we plan to upgrade the experiment, including an upgrade of the liquid xenon gamma-ray detector with readout of smaller photosensors such as MPPC. It was turn out by a simulation that the energy and position resolution will be significantly improved especially for events γ-ray convert at a shallow part of the detector. It is needed to develop MPPCs which is operational in liquid xenon detectors, because commercial MPPCs have little sensitivity to vacuum-ultra-violet range photons, from xenon scintillation.
μ → e +γ decay
MEG experiment
liquid xenon
detector
Performed in PSI (Switzerland) using the world’s most intense DC μ-beam.
Signal is 52.8MeV γ-ray and 52.8MeV e+ emitted back-to-back.
γ-ray → Liquid Xenon Detector
positron → Drift Chamber (tracker) → Timing Counter
Main background is accidental coincidence of random γ & Michel e+.
Therefore, good resolutions are needed to achieve a high sensitivity.
○ μ → e +γ is a good probe for new physics,
because charged lepton flavor violations are suppressed in the standard model. B ~ O (10-50)
○ Many theories such as SUSY-GUT, SUSY-seesaw, etc. predict μ → e +γ in the reachable branching ratio. B ~ O (10-12 ~ 15 )
Stopping
target
muon
beam
timing
counter
drift
chamber
cLFV search has a long history, but no signal has been observed yet.
S. Antush et al. JHEP, 11:090, 2006.
Expectation of μ → e +γ branching ratio , from SUSY-seesaw model.
recent θ13 measurement
MEG excluded 90% C.L.
Present limit and future sensitivities
2.4× → ~6× → ~5×10-14
Phys. Rev. Lett., 107:171801, 2011
Current UL (90% C.L.)
Goal of 1st stage of MEG
Aimed in next stage
positron bent by
COBRA magnet
← feature of COBRA magnet
e+ quickly swept out. (left) Radius independent of angle. (right)
○Until 2013
○DAQ for years
MEG
○Construction of new detectors for 2 years
Annu. Ref. Nucl. Part. Sci : W. J. Marciano, T.Mori, and J. M. Roney
Latest physics result → H.Natori’s talk
Positron spectrometer upgrade→ M.Nishimura’s poster
Expected performance
LXe detector upgrade concept
Energy resolution is expected to significantly be improved. →
Position resolutions improve especially for event where γ converts near the inner face. ↓
Detection efficiency improves by about 10%.
Present
Upgraded MC
Replace current 2” PMT with smaller photosensors
for example,
・MPPC
・Smaller, square PMT
・Flat panel PMT
Modification in lateral PMT arrangement
　・Slant angle
　　　for better uniformity
　・Wider inner face
　　　reduce energy leakage γ
MPPC Package
drawing
We plan to mount MPPCs on PCB. And arrange those PCBs onto detector inner face.
Development of UV-sensitive MPPC
It is necessary to develop MPPCs which is operational in liquid xenon, because commercial products don’t have sensitivity to UV light, and 3x3mm2 active area is largest.
UV-sensitive MPPC is under development in collaboration with Hamamatsu.
CG image
PMT
Depth [cm]
Position resolution σ [mm]
Horizontal
Red : PMT (present)
Blue : MPPC (upgraded) MC
MPPC
(12mm)
MPPC
Improve sensitivity to xenon scintillation (λ~175nm)
・Remove protection layer
・Reduce thickness of insensitive layer
・Anti reflection coating
・ Refractive index of sensor surface better matched to LXe
Larger MPPCs are needed to suppress the number of channels
With 12x12 mm2 active region,
(15mm in package dimension) ~4000 channels will be needed to fill inner face.
(currently 216 channels of PMT)
γ-ray
Result of UV-sensitive MPPC Test
photon detection efficiency (PDE)
measurement
Gain is calculated from 1p.e peak. PDE is evaluated with a correction for effect of cross-talk (CT) and after-pulsing (AP).
Testing Facility
charge
charge
1 p.e.
2 p.e.
3 p.e.
pedestal
If CT or AP occur, charge of 1 photon event become larger.
Test bench in LXe
Xenon handling system
xenon
chamber
refrigerator
gas xenon
recovery tank
control panel
getter
Sample MPPCs are mounted on PCB and immersed in liquid xenon. PMT, α source & LEDs for calibration are also attached.
Estimated number of the scintillation photons of 5.5MeV α event from 241Am.
↑ Flashing LED at very low intensity. Probabilities of CT and AP are estimated from deviation from Poisson statistics.
“F” type
“G” type
“H” type
Temperature dependence
3mm x 3mm
MPPC sample
Dark count rate can be reduced at low temperature. Typically, dark count rate (3x3mm2 MPPC) is a few Hz in LXe temp:165K.
Shift of Break-down voltage and quench resistance by temperature are observed.
241Am source
5.5 MeV α
Anti-reflection
cylinder
DAQ with waveform digitizer
We take waveform with　waveform digitizer DRS4, the same system as in MEG experiment.
Currently, the most sensitive MPPC sample (“G” type) has ~10% PDE at nominal operation voltage.
If MPPCs with this PDE are used to detector, almost the same number of photon can be detected compared to current detector.
Plan of LXe upgrade
sample type
2012
2013
2014
・ Development of UV-sensitive MPPC
・PCB & Feed-thru
design, test
・Prototype test
・Detector construction
Conclusion
・Liquid xenon detector will be upgraded with smaller photosensor such as MPPC
・We are developing MPPCs sensitive to LXe scintillation
・PDE of most sensitive sample is currently about 10%
・Prototype test in 2013, detector construction start in 2014
CG image
MPPC
←MPPCs are arranged on gamma incident face
γ-ray
PMT