1. MEG Experiment at PSI R&D of Liquid Xenon Photon Detector
R&D work on a Liquid Xenon Detector for the meg Experiment at PSI on behalf of the MEG Collaboration University of Tokyo, Japan Presented by S. Mihara
MEG Experiment at PSI
R&D of Liquid Xenon Photon Detector

2. me g Search as Frontier Physics
meg in…
SM+Neutrino Oscillation
Suppressed as ∝(mn/mW)4
SUSY
Large top Yukawa coupling
Neutrino Oscillation + SUSY
Hisano and Nomura 1998
Current limit
by MEGA
10-10
tanb nm ne
10-11 m e W
10-12 g
Br(meg)
10-13
Solar Neutrino
10-14 g
10-15 ~ m ~ e
MnR(GeV) ~
SK+SNO etc.=Large Mixing Solution m c e

3. MEG Experiment Overview
Detect e+ and g, “back to back” and “in time”
100% duty factor continuous beam of ~ 108m/sec
better than pulsed beam to reduce pile-up events
Two characteristic components
Liquid Xe photon detector
Solenoidal magnetic spectrometer with a graded magnetic field (COBRA)

4. Signal and Background Signal Main background sources ?
Radiative m+ decay
If neutrinos carry small amount of energy, the positron and gamma can mimic the signal.
Accidental overlap
A positron from usual Michel decay with energy of half of mm
Gamma from
Radiative muon decay or
Annihilation in flight of positron
NOT back to back, NOT in time
qeg= 180° m g e
Ee = 52.8 MeV
Eg = 52.8 MeV
menng g n n e e
menn+”g” n ? g

5. Requirement on the Photon Detector
Good resolutions
Energy
Position
Time
Large acceptance with good uniformity
Fast decay time to reduce pile-up events

6. Properties of Xenon
Fast response, Good Energy, and Position resolutions
Wph = 24 eV
(c.f. Wph(NaI) = 17eV)
tfast =4.2nsec tslow=22nsec
Narrow temperature range between liquid and solid phases
Stable temperature control with a pulse-tube refrigerator
Property
Unit
Saturated temperature
T(K)
164.78
Saturated pressure
P(MPa)
0.100
Latent heat (for liquid)
r(J/kg)X103
95.8
Latent heat (for solid)
r'(J/kg)X103
1.2
Specific heat
Cp(J/kgK)X103
0.3484
Density
r(kg/m3)X103
2.947
Thermal conductivity
k(W/mK)
0.108
Viscosity
m(Pa-s)X10-4
5.08
Surface tension
s(N/m)X10-3
18.46
Expansion coefficient
b(1/K)X10-3
2.43
Temperature/Pressure at triple point
Tt(K)/PT(MPa)
161.36/
0.0815

7. Liquid Xenon Photon Detector
Shallow event
Deep event
800 liter LXe viewed
by ~ 800PMTs

8. Absorption of Scintillation Light
Simulation
For Large Prototype
Scintillation light emission from an excited molecule
Xe+Xe*Xe2*2Xe + hn
Water contamination absorbs scintillation light more strongly than oxygen.
labs=7cm
Depth parameter
labs=500cm
Depth parameter
Depth

9. R&D Strategy Small Prototype done Large Prototype in progress
Proof-of-Principle Experiment
2.3liter active volume
Large Prototype in progress
Establish operation technique
70 liter active volume
Final Detector starting
~800 liter

10. Small Prototype
32 2-inch PMTs surround the active volume of 2.34 liter
g-ray sources of Cr,Cs,Mn, and Y
a source for PMT calibration
Operating conditions
Cooling & liquefaction using liquid nitrogen
Pressure controlled
PMT operation of 1.0x106 gain
Proof-of-Principle Experiment
PMT works in liquid xenon?
Light yield estimation is correct?
Simple setup to simulate and easy to understand.
S.Mihara et al. IEEE TNS 49: , 2002

11. Small Prototype Energy resolution
Results are compared with MC prediction.
Simulation of g int. and energy deposition : EGS4
Simulation of the propagation of scint. Light
EGS cut off energy : 1keV
Rayleigh Scattering Length: 29cm
Wph = 24eV

12. Small Prototype Position and Timing resolutions
PMTs are divided into two groups by the y-z plane
g int. positions are calculated in each group and then compared with each other.
Position resolution is estimated as
　　　　sz1-z2/√2
The time resolution
is estimated by
taking the difference
between two groups.
Resolution improves
as ～1/√Npe

13. Large Prototype 70 liter active volume (120 liter LXe in use)
Development of purification system for xenon
Total system check in a realistic operating condition:
Monitoring/controlling systems
Sensors, liquid N2 flow control, refrigerator operation, etc.
Components such as
Feedthrough,support structure for the PMTs, HV/signal connectors etc.
PMT long term operation at low temperature
Performance test using
10, 20, 40MeV Compton g beam
60MeV Electron beam

14. Purification System
Xenon extracted from the chamber is purified by passing through the getter.
Purified xenon is returned to the chamber and liquefied again.
Circulation speed 5-6cc/minute
Enomoto Micro Pump MX-808ST-S
25 liter/m
Teflon, SUS
Gas return
To purifier
Circulation pump

15. Purification Performance
3 sets of Cosmic-ray trigger counters
241Am alpha sources on the PMT holder
Stable detector operation for more than 1200 hours
Cosmic-ray events
a events

16. Absorption Length Fit the data with a function :
A exp(-x/ labs)
labs >100cm (95% C.L) from comparison with MC.
CR data indicate that labs > 100cm has been achieved after purification.

17. Response to Gamma Beam 10MeV 20MeV 40MeV Electron storage ring,
TERAS, in AIST,
Tsukuba Japan
Electron Energy, Current:
762MeV, 200mA
266nm laser to induce inverse-Compston scattering.
40 MeV (20MeV, and 10MeV) Compton g provided.
The Compton edge is used to evaluate the resolution.
Data taking
Feb (w/o purification)
Apr (w/ purification)
10MeV
20MeV
40MeV

18. w/o xenon purification
Energy Spectrum
s2 :depth parameter:
40MeV Compton gamma data
w/o xenon purification
40MeV Compton gamma data
w/ xenon purification
Depth parameter
Depth parameter
Total Number of Photoelectrons
Total Number of Photoelectrons

19. Energy Resolution
Shallow events have dependence on the depth of the 1st int. point.
Discard these shallow events (~34%) for quick analysis.
Calibration not completed
Very Preliminary: sE < 2%
Simulation
52.8MeV g
Depth parameter
Very Preliminary

20. Position Reconstruction
2-step reconstruction
1st step: Pre-determination of the peak
2nd step: Precise determination with an iteration process
Data 40MeV Compton g
(a)
(b)
(c)
(d)

21. Timing Resolution 104 4x104 Estimated using Electron Beam (60MeV) data
Resolution improves in proportion to 1/sqrt(Npe).
For 52.8 MeV g, s~60 psec + depth resolution.
QE improvement and wave-form analysis will help to achieve better resolution.
(Visit “The DRS chip” by S.Ritt)
s=75.6+/-2.0ps
45 MeV Energy deposit by 60 MeV electron injection
s Timing Resolution (psec)
52.8MeV g
(nsec)
104
4x104
Number of Photoelectron

22. Summary New experiment to search for meg at Paul Scherrer Institut
Two characteristic components (and many others)
Liquid Xenon Photon Detector
Solenoidal magnetic spectrometer with a graded magnetic field (COBRA)
R&D of liquid xenon photon detector using the large prototype
Long term stable operation using a pulse tube refrigerator
Purification of liquid xenon
Very preliminary result from the last g beam test
sE<2% for 40MeV Compton g