1. Upgrade of LXe gamma-ray detector in MEG experiment
Daisuke Kaneko, ICEPP, Univ. of Tokyo, on behalf of the MEG collaborations
LXe detector upgrade concept
μ → e +γ decay
Replacement current 2” PMT with　 ・MPPC
　　　　・Smaller, square PMT
　　　　・Panel type PMT
Conventional MPPCs don’t have sensitivity to xenon scintillation light (λ~178nm)
Developing with Hamamatsu
Modification in lateral PMT arrangement
・Slant angle
　　　for better uniformity
・Wider inner face
　　　reduce energy leakage
μ → e +γ is a good probe for new physics,
because cLFV is suppressed in SM. B ~ O (10-50)
Many theories such as SUSY-GUT predict μ → e +γ
in the reachable branching ratio. B ~ O (10-12 ~ 15 )
↑ MPPC Package drawing
4000ch of MPPCs will be needed to cover all inner face.
cLFV search has a long history, but no signal has been observed yet.
Expectation of μ → e +γ branching ratio , from SUSY-seesaw model.
CG image
recent θ13 measurement
MPPCs on PCB
Cross-section of PCB
S. Antush et al. JHEP, 11:090, 2006.
Arrangement for detector inner surface
MEG experiment
Latest physics result → H.Natori’s talk
Positron Spectrometer upgrade→ N.Miki’s poster
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 back-ground is accidental event, occasionally coincident γ& e+.
Therefore, good resolution is needed to achieve high sensitivity.
muon
beam
positron bent by
COBRA magnet
Stopping
target
Expected performance
PMT
MPPC
Energy resolution is expected to significantly improved. →
Position resolutions improve
especially at shallow part. ↓
Detection efficiency improve by about 10%.
feature of COBRA magnet,
e+ quickly swept out. (left) Radius independent from angle,(right)
Depth [cm]
position resolution [mm]
Horizontal
Red : PMT
Blue : MPPC
MPPC
(12mm)
Achieved & Aiming Branching Ratio
Current UL (90% C.L.)
Goal of 1st stage of MEG
Aimed in next stage
2.4× → 6× → 6×10-14
Phys. Rev. Lett., 107:171801, 2011
Testing Facility
Result of Recent Element Test
Xenon handling system
xenon
chamber
refrigerator
gas xenon
recovery tank
control panel
getter
PDE(photon detection efficiency)
measurement
Gain is calculated from subtraction of 1p.e & 2p.e. peaks, and corrected by removing effect of optical cross-talk & after-pulse.
We can maintain condition inside of chamber stably.
⊿T ~ 0.5K, ⊿p ~ 0.001MPa
charge
1 p.e.
2 p.e.
3 p.e.
pedestal
charge
If C.T or A.P occur, charge of 1 p.e. event become larger.
↑ Flashing LED at very low intensity. Difference from poisson statistics is effect from C.T. & A.P .
Test bench used in LXe
Sample MPPCs are mounted on board and immersed in liquid xenon. PMT, LEDs for calibration and Pt-100 RTDs also attached.
Currently, the most sensitive MPPC sample have ~10% PDE at nominal operation voltage.
Dark noise rate
Quench resistance
Thermal noise could be reduced at low temperature.
Typically, dark count rate is few Hz in LXe temp:165K.
Shift of quench resistance by temperature is observed.
3x3 mm
“G” type
102kΩ
272kΩ
“H” type
“F” type
DAQ with DRS
We take waveform with waveform-digitizer DRS4, the same system as MEG experiment.
Resistance obtained by fitting I-V curve with MPPC model function.