1. MEG実験の液体Xe検出器について
東大　ICEPP 　森研究室
M1 金子大輔

2. What is needed for MEG experiment ?
→　Reduce backgrounds as much as possible
→ High accuracy g-ray detector !
・ Energy 53 MeV
・ Timing 0.1 ns
・ Position 4 mm
We adopted Liquid Xenon detector

3. Merits and Difficulty about XENON
GOOD
BAD
Xenon emits much scintillation light (about 75% of NaI(Tl))
Decay time is very short
~ 45 ns
Radiation length is short
~ 2.77 cm ↑(Z=54)
Uniformity
able to purify
Xenon is liquid only in K ~ 165K
Latent heat is small
Insulation vessel is needed

4. Light emission process of xenon
There are two ways of scintillation process
lpeak= 178nm
(vacuum ultra violet ray)
no self absorption ! Xe e-
ionization Xe
Xe+
Xe2+ e-
excitation
Xe* Xe Xe
Xe** Xe
t = 45 ns
Xe*
recombination
Xe2*
1S+u
3S+u
t = 4.2ns
t = 22ns Xe Xe
+ photon

5. Our liquid xenon detector
>850 litter volume liquid xenon
“C” shape to fit magnet
846 photo-multiplier-tubes (PMT) observe scintillation light
Pulse Tube Refrigerator control xenon’s temperature
To reduce mass of inner face honeycomb window is equipped

6. Connection around xenon detector
storage tanks
gas purifier
1000L tank
liquid purifier

7. g Principles of detection and analysis
PMTs catch scintillation photons and output signals
Each quantity is calculated
Energy of g ← sum of signals of all PMTs
Time ← mean of timings
Position ← light distribution g
wave form from a PMT

8. Detector’s Outside high-voltage connector signal connector
COBRA-magnet
honeycomb window

9. Detector’s Inside PMTs work in Low temperature (~160K)
magnetic field (<50 Gauss)
LEDs for callibration are attached
wires & plates with radio-active source (Am)

10. Photo Multiplier Tube (PMT)
Quantum efficiency (Q.E.) ~ 14 %
High Voltage ~ 800 V , gain ~ 2×106
2 in. f
HAMAMATSU R9288
features
Metalchannel-dinode → short body
→ not to block gray from backward ・ tough to magnetic field
quartz window　→transparent to VUV
Al strip on window　→ reduce resistance in low temperature
Cathode = K-Sc-Sb

11. Pulse Tube Refrigerator
KEK Haruyama et.al
stable temperature & pressure control within DT~0.1K , DP~0.001MPa
How pulse tube refrigerator work ?
Cooling power 200W 165K

12. Xenon Purification System
Water and oxygen mainly absorb scintillation light ↓
MEG detector have two ways of purification systems
gas purification
liquid purification
absorb water more efficiently than gas
absorb O2 H2O etc,
purifier
Finally
labs → 300cm
getter

13. Electronics , DAQ DRS on-line computers detector waveform digitizer
off-line computers
splitter
trigger
on VME boards
every event, waveforms of all PMTs are saved.
Domino Ring Sampler (DRS) sample rate 2 GHz
trigger board digitalize signals with FADC and FPGA
slow control
MSCB
IN 2007
Runs 9974
Total TB

14. Calibration methods alpha source (241Am) → Q.E. , attenuation length
LED → gain , timing
C-W proton beam → g energy
Charge exchange → g energy
etc
↑PMT gain calculation from LED runs
↑position detection from a

15. gray from nucleic or particle reaction is used for energy scale calibration
detector
PSI cyclotron
Proton beam
Cockcroft-Walton (C-W) accelerator
target
muon beam
pion beam
Charge exchange reaction
P + p- → n + p0
p0 　→ g + g , 83 MeV
radiative capture
P + p- → n + g MeV
Li ( p , g ) Be
17.6 , 14.6 MeV
B ( p , g ) C
4.4 , 11.7 , 16.1 MeV

16. Thanks for your listening !!
Status and Prospects
55 MeV g
sup = 2.4%
FWHM = 6.5%
NOW
analyzing data
improving detector
APRIL 2008 ~
Data taking again
Thanks for your listening !!