Liquid Xenon Carlorimetry at the MEG Experiment

Satoshi MIHARAUniv. 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; e

– Clear evidence of new physics beyond SM SUSY-GUT, SUSY-Seesaw Br ~< 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.

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MEG Detector

e+ measured by COBRA spectrometer by LXe detector

52.8MeV

52.8MeV

5

LXe Detector R&D history

Small Prototype– 2.3 liter active volume

Large Prototype– 70 liter active volume

Final Detector– 800 liter active volume

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LXe and Scintillation light Density 3.0 g/cm3

Triple point 161K, 0.082MPa Normal operation at

– T~167K P~0.12MPa Narrow temperature range between

liquid and solid phases– Stable and reliable temperature

control is necessary Scintillation light emission

mechanism

Solid Liquid

GasTriple point

Temperature [K]P

ress

ure

[M

Pa]

161

0.082

0.1

165

hXeXeXe 2*

hXeXeXeXe

XeXe

XeXeeXe

XeXeXe

2*2

*

***

**2

2

Excitation

Recombination

nm10175~

8

MEG LXe Detector

3 cm

Liq. Xe

Liq. Xe

14 cm

(a)

(b)

05 10 15

2025

3035

0

10

20

30

40

50

0

2000

4000

6000

8000

10000

05

1015 20 25

3035

0

10

20

30

40

50

0

200

400

600

800

1000

1200

1400

1600

1800

52.8 MeV

52.8 MeV

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

1. Evacuation TMP + Cryopump 10-4~5 Pa

2. Pre-cooling 2.0 bar xenon gas at room temp Refrigerator/LN2 cooling

3. Liquefaction Continuously supply Xe gas Pressure control Refrigerator/LN2 cooling

4. Purification Circulation/Purification

5. 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 R&D Photocathode

– Bialkali :K-Cs-Sb, Rb-Cs-Sb Rb-Cs-Sb has less steep increase of sheet res

istance at low temperature K-Cs-Sb has better sensitivity than Rb-Cs-Sb

– Multialkali :+Na Sheet resistance of Multialkali dose not chang

e so much. Difficult to make the photocathod, noisy

Dynode Structure– Compact– Possible to be used in magnetic field up to 1

00G Metal channel Uniformity is not excellent

Ichige et al. NIM A327(1993)144

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1st generation R6041Q 2nd generation R9288TB 3rd generation R9869

228 in the LP (2003 CEX and TERAS)127 in the LP (2004 CEX)

111 In the LP (2004 CEX) Used in the final detector

Rb-Sc-SbMn layer to keep surface resistance at low temp.

K-Sc-SbAl strip to fit with the dynode pattern to keep surface resistance at low temp.

K-Sc-SbAl strip density is doubled.4% loss of the effective area.

1st compact versionQE~4-6%Under high rate background,PMT output reduced by 10-20% with a time constant oforder of 10min.

Higher QE ~12-14%Good performance in high rate BGStill slight reduction of output in very high BG

Higher QE~12-14%Much better performance in very high BG

PMT Development Summary

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PMT Base Circuit

Reference PMT = no Zener

PMT with Zener

Necessary to reduce heat load from the circuit– Heat load in the cryostat ↔ Refrigerator cooling po

wer ~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

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Refrigerator R&D

MEG 1st spin-off Technology transferred to a manufacturer, Iwatani C

o. Ltd Performance obtained at Iwatani

– 189 W @165K– 6.7 kW compressor– 4 Hz operation

Cool i ng power (PC150)

0

50

100

150

200

50 100 150 200

Col d end temperature(K)

Cooling power (W)

Qi wa(W)Qpsi (W)

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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 Ray~30-45cm

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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 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.

Temperature Sensor

Purifier Cartridge

Molecular sieves, 13X 25g water

Freq. InverterOMRON

PT

In ~10 hours, λabs ~ 5m

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Liquid-phase Purification cont’d

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

Wire (50-100 m )

Alpha

40 μm

SORAD/ISOTOPE PRODUCTS

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54.9MeV 82.9MeV

1.3MeV for >170o

0.3MeV for >175o

0

0 decay ’s through CEX proce

ss– -+p 0+n– 55MeV, 83MeV

emission from thermal neutron capture on Ni nuclei– 9MeV

3

7Li(p,)48Be

– E p = 440 keV, 14 keV, peak = 5 mb

– 17.6MeV – obtainable : 106 /s

(isotropic) at 440 KeV resonance (Ip 50 A)

9 MeV Nickel γ-line

NaI

Polyethylene

0.25 cm Nickel plate

3 cm 20 cm

Further Calibration Methods

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Detector Performance

110 - 64 (LYSO) - 61 (Beam) = 65psec

110 psec

= 1.23 ±0.09 %FWHM=4.8 %

5%

1%E

nerg

y R

eso

luti

on

()

[%

]

Energy resolution vs. Energy

Energy distribution @ 55MeV

Timing distribution

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MEG LXe Detector Status

LXe CalorimeterLiquid circulating purifier

Liquid pump (100L/h)

Purifier1000L storage dewar

Cryocooler (100W)

LN2

LN2

Getter+Oxysorb

GXe pump (10-50L/min)

GXe storage tank

Cryocooler (150W)

Heat exchanger

Refrigerator

Xenon storage

1000l liquid xenonstorage tank

purifier

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MEG LXe Detector Status

Cryostat Construction is in progress…

outer

side

top

inner

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Summary

LXe scintillation detector R&D 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