1. Liquid Xenon Detector for the MEG Experiment
Toshiyuki Iwamoto
4/Oct/2006
10th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD06) Siena
4/Oct/2006
Siena 2006

2. MEG Physics Lepton flavor violating process
already violated in neutrino sector
Forbidden in the Standard Model
Sensitive to the new physics models, SUSY GUT, or SUSY seesaw etc.
The present experimental limit
~ 1.2x10-11 by MEGA experiment
Clear 2-body kinematics
Ee = Eg = 52.8 MeV
Back to back (qeg = 180°)
Simultaneous (Te = Tg)
4/Oct/2006
Siena 2006

3. Sensitivity Background Radiative m decay : m+  e+ nm ne g < 10-14
Accidental overlap:
m decay m  e n n + random g
To reject these background,
energy, timing and vertex resolution
pile-up rejection from waveform analysis
Single event sensitivity ~ 4x10-14
Background event ~ 4x10-14
90% C.L. sensitivity ~ 1x10-13
photon detector requirement
DEg = 5% (FWHM)
DTg = 150ps (FWHM)
DXg = 9mm (FWHM)
MEGA
MEG
4/Oct/2006
Siena 2006

4. MEG detector Approved in 1999 at PSI Physics run in 2007 4/Oct/2006
Siena 2006

5. MEG Detector 590 MeV proton Ring Cyclotron Drift chamber COBRA magnet
w/ graded B field
Most intense DC muon beam (108/s)
H. Nishiguchi, 5th Oct
“The MEG positron spectrometer”
Timing counter
Photon detector
4/Oct/2006
Siena 2006

6. Photon Detector 800 liter liquid xenon, 846 2” PMTs in liquid
High light yield, fast response and good uniformity
(~40000ph/MeV, t=4.2, 22, 45ns)
Low temperature (~160K), l = 178nm
: development of PMT, refrigerator
Xenon purity : remove H2O
Performance test of Xe detector
2.3 liter prototype
confirmed the principle of Xe
68.6 liter prototype
10, 20, and 40 MeV inverse Compton scattering g
energy, timing and vertex resolution
55, 83MeV, 129MeV g from p-p->np0,ng reaction
energy, and timing resolution
4/Oct/2006
Siena 2006

7. Improvement Purification Gas & Liquid phase purification tested
PMT
Q.E. : ~15%
Photocathode : K-Cs-Sb
Metal channel dynode, 12 stages
Silica window (for UV light)
Compact, and low heat load
Tolerance up to 100G magnetic field
Al strip to keep surface resistance at low temp.
Zener diode for high rate background
Purification
Gas & Liquid phase purification tested
Metal getter (zirconium) by gas ~0.5l/h
Molecular sieves by liquid ~100l/h 1m
57mm
2.5m
4/Oct/2006
Siena 2006

8. Prototype test, 10~40MeV g 68.6l liquid xenon prototype detector
228 PMTs
40MeV g Energy
1.6% in s
s ~ 4mm
Vertex distribution
4/Oct/2006
Siena 2006

9. Prototype test, 55~129MeV g H2 target Prototype LXe LYSO g g NaI p-
Opening angle (q)
Energy (MeV)
83MeV
H2 target
Prototype
LXe
LYSO g g
175°
170°
NaI
55MeV p- m
phe Xenon detector
p- (at rest) + p -> p0 + n, p0(28MeV/c) -> g + g
(54.9MeV<Eg<82.9MeV)
Almost monochromatic g is available
p- + p -> n + g (129MeV)
Energy, timing resolution
neutron response
4/Oct/2006
Siena 2006
ENaI + ELYSO (MeV)

10. Photon detector performance
Energy 55MeV 5% 1%
Energy resolution vs. Energy
= 1.23 ±0.09 %
FWHM=4.8 %
Energy Resolution (s) [%]
Timing distribution
110 psec
Expected Performance
Energy : 5% (FWHM)
Timing : 150 psec (FWHM)
Vertex : 9mm (FWHM)
(LYSO) (Beam) = 65psec
4/Oct/2006
Siena 2006

11. Detector operation 250l gas xenon tank x 8 purifier Refrigerator
1000l liquid xenon
storage tank
4/Oct/2006
Siena 2006

12. Calibration method Gain monitor : LED Q.E. : 241Am a source 3x3 Energy
55, 83, and 129 MeV g from p-pp0n, p02g
Ni thermal n capture 9MeV
Proton accelerator
Li(p,)Be 17.6MeV
B(p,)C 16.1MeV
Timing
Laser
radiative m decay
55 MeV g from p-pp0n reaction (Pb+Scintillator)
Vertex
55 MeV g from p-pp0n reaction (collimator)
3x3
NaI array
LH2 target
4/Oct/2006
Siena 2006

13. Detector Construction
4/Oct/2006
Siena 2006

14. Detector Construction
top
outer
side
inner
The performance of all PMTs checked in the liquid xenon before installation
4/Oct/2006
Siena 2006

15. Schedule Cryostat will come to PSI in the end of this year
PMT installation into holders almost done
Liquid xenon detector assembly will be done in the beginning of next year, and then liquefaction, purification and stable operation test will be continued.
physics run starts in 2007
Positron spectrometer run and calibration run will be done in this year
4/Oct/2006
Siena 2006

16. Summary
MEG experiment will search for m->eg with better sensitivity than previous experiment
The performance of liquid xenon detector has been tested by using prototype, and the real detector is being constructed
Physics run of MEG experiment will start in 2007.
Positron spectrometer data and calibration data will be taken in this year.
4/Oct/2006
Siena 2006