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 the higher sensitivity, we plan to upgrade the experiment, including the upgrade of the liquid xenon gamma-ray detector. It was turn out by simulation that energy and position resolution will be significantly improved especially for events γ-ray convert at shallow part of detector. It is needed to develop MPPCs which operational in liquid xenon detectors, because conventional MPPCs have little sensitivity to vacuum-ultra-violet range photons, from xenon scintillation.
μ → e +γ decay
MEG experiment
muon
beam
positron bent by
COBRA magnet
Stopping
target
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 )
Performed in PSI (Switzerland) in pursuit of 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 event coincident γ& e+.
Therefore, good resolutions are needed to achieve high sensitivity.
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× → 6×10-14
Phys. Rev. Lett., 107:171801, 2011
Current UL (90% C.L.)
Goal of 1st stage of MEG
Aimed in next stage
← feature of COBRA magnet
e+ quickly swept out. (left) Radius independent from angle,(right)
MEG
Latest physics result → H.Natori’s talk
Positron spectrometer upgrade→ M.Nishimura’s poster
Expected performance
LXe detector upgrade concept
Present
Upgraded MC
Energy resolution is expected to significantly be improved. →
Position resolutions improve especially event that γ convert near the inner face. ↓
Detection efficiency improve by about 10%. γ
Replace current 2” PMT with smaller detectors
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
MPPC Package
drawing
We plan to mount MPPCs on PCB. And arrange those PCBs onto detector inner face.
Requirements for MPPC
CG image
Depth [cm]
position resolution [mm]
Horizontal
Red : PMT
Blue : MPPC
It is necessary to develop MPPCs which operational in liquid xenon, because commercial products don’t have sensitivity, and 3x3mm2 active area is largest. MC
MPPC
(12mm)
Testing specifications to detect xenon scintillation (λ~175nm)
　・Remove protection layer
　・Reduce insensitive layer
　・Anti reflection coating
　・Match refractive index to LXe
Larger MPPCs are needed to reduce channels.
With 12x12 mm2 active region,
(15mm in package dimension) ~4000 channels will be needed to fill inner face.
(currently 216 channels of PMT)
Result of UV sensitive MPPC Test
photon detection efficiency (PDE)
measurement
Gain is calculated from 1p.e peak charge.
And PDE is corrected by removing effect of optical cross-talk & after-pulse.
Testing Facility
charge
1 p.e.
2 p.e.
3 p.e.
pedestal
charge
If C.T or A.P occur, charge of 1 photon event become larger.
Xenon handling system
Test bench used in LXe
xenon
chamber
refrigerator
gas xenon
recovery tank
control panel
getter
Sample MPPCs are mounted on board and immersed in liquid xenon. PMT, α source & LEDs for calibration and Pt-100 RTDs also attached.
“F” type
“G” type
“H” type
↑ Flashing LED at very low intensity. Difference from poisson statistics is effect from C.T. & A.P .
Temperature dependence
3mm x 3mm
MPPC sample
Thermal noise 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 DRS
Currently, the most sensitive MPPC sample (“G” type) have ~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.
We take waveform with　waveform-digitizer DRS4, the same system as MEG experiment.
Plan of the LXe γ-ray detector development
sample type
2012
2013
2014
・Element test
・PCB & Feed-thru
design, test
・Proto-type test
・Detector construction
Conclusion
・Liquid xenon detector will be upgraded with smaller detector such as MPPC
・We are developing MPPCs to use in liquid xenon
・PDE of most sensitive sample is currently about 10%
・Prototype test in 2013, detector construction in 2014
CG image
←MPPCs are arranged on gamma incident face