Liquid Xenon Carlorimetry at the MEG Experiment

1
Liquid Xenon Carlorimetry at the MEG Experiment

• Satoshi MIHARA
• Univ. of Tokyo

2
Contents

• MEG Experiment
• Liquid Xenon Scintillation Detector
• Liquid Xenon Property
• Operation
• Detector Components
• Calibration
• Performance
• Summary

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MEG Experiment

• Search for Lepton-Flavor violating muon rare
  decay m ? e g
• Clear evidence of new physics beyond SM
• SUSY-GUT, SUSY-Seesaw Br lt 10-11
• Present limit 1.2x10-11 by MEGA
• Engineering run starts in 2006 and full DAQ will
  start in 2007 at Paul Scherrer Insitut.

4
MEG Detector
52.8MeV
52.8MeV

• e measured by COBRA spectrometer
• g by LXe detector

5
LXe Detector RD history

• Small Prototype
• 2.3 liter active volume
• Large Prototype
• 70 liter active volume
• Final Detector
• 800 liter active volume

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Why Liquid Xenon?

• Good resolutions
• Large light output yield
• Wph(1MeV e) 22.4eV
• Pile-up event rejection
• Fast response and short decay time
• ts 4.2nsec, tT45nsec (for electron, no E)
• Uniform

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LXe and Scintillation light

• Density 3.0 g/cm3
• Triple point 161K, 0.082MPa
• Normal operation at
• T167K P0.12MPa
• Narrow temperature range between liquid and solid
  phases
• Stable and reliable temperature control is
  necessary
• Scintillation light emission mechanism

Liquid
Solid
Pressure MPa
0.1
0.082
Gas
Excitation
Triple point
Temperature K
161
165
Recombination
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MEG LXe Detector

• Active volume 800l is surrounded PMTs on all
  faces
• 850PMTs in the liquid
• No segmentation
• Energy
• All PMT outputs
• Position
• PMTs on the inner face
• Timing
• Averaging of signal arrival time of selected PMTs

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Operation Procedure

• Evacuation
• TMP Cryopump
• 10-45 Pa
• Pre-cooling
• 2.0 bar xenon gas at room temp
• Refrigerator/LN2 cooling
• Liquefaction
• Continuously supply Xe gas
• Pressure control
• Refrigerator/LN2 cooling
• Purification
• Circulation/Purification
• Ready

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Detector Components

• Photomultiplier
• Operational in liquid xenon, Compact
• UV light sensitive
• Refrigerator
• Stable temperature control
• Sufficient power to liquefy xenon
• Low noise, maintenance free
• Xenon Purifier
• Purification during detector operation

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Photomultiplier RD
Ichige et al. NIM A327(1993)144

• Photocathode
• Bialkali K-Cs-Sb, Rb-Cs-Sb
• Rb-Cs-Sb has less steep increase of sheet
  resistance at low temperature
• K-Cs-Sb has better sensitivity than Rb-Cs-Sb
• Multialkali Na
• Sheet resistance of Multialkali dose not change
  so much.
• Difficult to make the photocathod, noisy
• Dynode Structure
• Compact
• Possible to be used in magnetic field up to 100G
• Metal channel ? Uniformity is not excellent

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PMT Development Summary
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PMT Base Circuit

• Necessary to reduce heat load from the circuit
• Heat load in the cryostat ? Refrigerator cooling
  power 190W
• Reduce base current
• 800V 55microA ? 44mW/PMT
• 40-50W heat load from 850PMTs
• Zener diodes at last 2 stages for high rate
  background
• Zener diode is very noisy at low temperature ?
  filtering on the base

Reference PMT no Zener
PMT with Zener
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Refrigerator

• Crucial for the MEG xenon detector
• Quiet (less vibration)
• Maintenance free
• Pulse tube refrigerator does not have any
  mechanically moving part in the cold part.

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Refrigerator RD

• MEG 1st spin-off
• Technology transferred to a manufacturer, Iwatani
  Co. Ltd
• Performance obtained at Iwatani
• 189 W _at_165K
• 6.7 kW compressor
• 4 Hz operation

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Purification System

• Usually water can be removed by heating the
  cryostat during evacuation.
• MEG liq. Xenon detector cannot be heated because
  of the PMTs inside.
• Water molecule is usually trapped on cold surface
  in the cryostat. However when the cryostat is
  filled with fluid, water molecules seem to
  dissolve in the fluid.
• Circulation/Purification after filling with fluid
  is necessary.

Rayleigh scattering lRay30-45cm
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Gas-phase Purification

• 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

Cosmic-ray events
a events
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Liquid-phase Purification

• Xenon circulation in liquid phase.
• Impurity (water) is removed by a purifier
  cartridge filled with molecular sieves.
• 100 l/hour circulation.

In 10 hours, ?abs 5m
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Liquid-phase Purification contd

• For the MEG xenon detector

• Another cryostat placed beside the detector for
  independent regeneration of the purifier
  cartridge
• Xenon transferred from the bottom of the detector
  to the cryostat
• Purified and retuned to the detector through
  vacuum insulated pipes

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Calibration

• LED flashed in the liquid
• PMT gain calibration
• Alpha source on wires
• Point-like source as if floating in the active
  volume
• Possible to illuminate all PMTs
• PMT calibration and monitoring/absorption length
  estimation

SORAD/ISOTOPE PRODUCTS
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Further Calibration Methods

• p0 decay gs through CEX process
• p-p ? p0n
• 55MeV, 83MeV g
• g emission from thermal neutron capture on Ni
  nuclei
• 9MeV
• 37Li(p,g)48Be
• E p 440 keV, ??14 keV, ?peak 5 mb
• 17.6MeV g
• obtainable ? 106 ?/s (isotropic) at 440 KeV
  resonance (Ip? 50 ?A)

9 MeV Nickel ?-line
NaI
Polyethylene
0.25 cm Nickel plate
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Detector Performance
Energy distribution _at_ 55MeV
5 1

• 1.23 0.09
• FWHM4.8

Energy resolution vs. Energy
Energy Resolution (s)
Timing distribution
110 psec
110 - 64 (LYSO) - 61 (Beam) 65psec
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Beam Test Setup
H2 targetdegrader
LYSO Eff 14
NaI
LP
S1
Eff(S1xLP)88
beam
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MEG LXe Detector Status
Xenon storage
purifier
Refrigerator
1000l liquid xenon storage tank
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MEG LXe Detector Status
top
outer
side
inner

• Cryostat Construction is in progress

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Summary

• LXe scintillation detector RD for MEG is
  successfully conducted
• PMT for use in liquid xenon
• Pulse tube refrigerator
• Purification system
• Detector performance is proved to be good enough
  for the experiment by using prototype detectors
• Detector construction is in progress and will be
  ready soon