R

1
RD work on a Liquid XenonDetector for the m?eg
Experiment at PSIon behalf of the MEG
Collaboration University of Tokyo, Japan
Presented by S. Miharahttp//meg.psi.ch

1. MEG Experiment at PSI
2. RD of Liquid Xenon Photon Detector

2
m?e g Search asFrontier Physics

• Neutrino Oscillation SUSY
• Hisano and Nomura 1998

• m?eg in
• SMNeutrino Oscillation
• Suppressed as ?(mn/mW)4
• SUSY
• Large top Yukawa coupling

Current limit by MEGA
10-10
tanb
nm
ne
10-11
e
m
W
10-12
g
Br(m?eg)
10-13
Solar Neutrino
10-14
g
10-15


m
e
MnR(GeV)
SKSNO etc.Large Mixing Solution

c
m
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)

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Signal and Background
Signal

• 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
g
m
e
Ee 52.8 MeV
Eg 52.8 MeV
m?enng
g
n
n
e
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Requirement onthe Photon Detector

• Good resolutions
• Energy
• Position
• Time
• Large acceptance with good uniformity
• Fast decay time to reduce pile-up events

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Properties of Xenon
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Liquid XenonPhoton Detector
Shallow event
Deep event

• 800 liter LXe viewed
• by 800PMTs

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Absorption of Scintillation Light
Simulation For Large Prototype

• Scintillation light emission from an excited
  molecule
• XeXe?Xe2?2Xe hn
• Water contamination absorbs scintillation light
  more strongly than oxygen.

labs7cm
Depth parameter
labs500cm
Depth parameter
Depth
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RD Strategy

• Small Prototype done
• Proof-of-Principle Experiment
• 2.3liter active volume
• Large Prototype in progress
• Establish operation technique
• 70 liter active volume
• Final Detector starting
• 800 liter

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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 49588-591, 2002
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Small PrototypeEnergy 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

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Small PrototypePosition 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/v2
• The time resolution
• is estimated by
• taking the difference
• between two groups.
• Resolution improves
• as 1/vNpe

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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

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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

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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
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Absorption Length

• Fit the data with a function
• A exp(-x/ labs)
• labs gt100cm (95 C.L) from comparison with MC.
• CR data indicate that labs gt 100cm has been
  achieved after purification.

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Response to Gamma Beam

• 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. 2002 (w/o purification)
• Apr. 2003 (w/ purification)

10MeV
20MeV
40MeV
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Energy Spectrum

• s2 depth parameter

40MeV Compton gamma data w/ xenon purification
40MeV Compton gamma data w/o xenon purification
Depth parameter
Depth parameter
Total Number of Photoelectrons
Total Number of Photoelectrons
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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 lt 2

Simulation 52.8MeV g
Depth parameter
Very Preliminary
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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)
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Timing Resolution

• Estimated using Electron Beam (60MeV) data
• Resolution improves in proportion to 1/sqrt(Npe).
• For 52.8 MeV g, s60 psec depth resolution.
• QE improvement and wave-form analysis will help
  to achieve better resolution.
• (Visit The DRS chip by S.Ritt)

s75.6/-2.0ps
45 MeV Energy deposit by 60 MeV electron injection
s Timing Resolution (psec)
52.8MeV g
(nsec)
104
4x104
Number of Photoelectron
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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)
• RD 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
• sElt2 for 40MeV Compton g