4/Oct/2006 Siena 2006 1

Liquid Xenon Detector for the MEG Experiment

Toshiyuki Iwamoto4/Oct/2006

10th Topical Seminar on Innovative Particle and Radiation Detectors

(IPRD06) Siena

4/Oct/2006 Siena 2006 2

MEG Physics• Lepton flavor violating process

– already violated in neutrino sector

• Forbidden in the Standard Model

• Sensitive to the new physics models, SUSY GUT, or SUSY seesaw etc.

• The present experimental limit – ~ 1.2x10-11 by MEGA experiment

Clear 2-body kinematicsEe = E = 52.8 MeVBack to back (e= 180°)Simultaneous (Te = T)

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SensitivityBackground Radiative decay : e+ e < 10-14

Accidental overlap: decay e random To reject these background, energy, timing and vertex resolution pile-up rejection from waveform analysis

Single event sensitivity ~ 4x10-14

Background event ~ 4x10-14

90% C.L. sensitivity ~ 1x10-13

photon detector requirement E= 5% (FWHM) T= 150ps (FWHM) X= 9mm (FWHM)

MEGA

MEG

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MEG detector

Approved in 1999 at PSIPhysics run in 2007

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MEG Detector590 MeV proton Ring Cyclotron

COBRA magnet w/ graded B field

Photon detector

Drift chamber

Timing counter

H. Nishiguchi, 5th Oct“The MEG positron spectrometer”

Most intense DC muon beam (108/s)

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Photon Detector800 liter liquid xenon, 846 2” PMTs in liquidHigh light yield, fast response and good uniformity (~40000ph/MeV, 4.2, 22, 45ns)

Low temperature (~160K), = 178nm : development of PMT, refrigeratorXenon purity : remove H2O

Performance test of Xe detector 2.3 liter prototype confirmed the principle of Xe 68.6 liter prototype 10, 20, and 40 MeV inverse Compton scattering energy, timing and vertex resolution 55, 83MeV, 129MeV from -p->n0,n reaction energy, and timing resolution

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Improvement PMTQ.E. : ~15%Photocathode : K-Cs-SbMetal channel dynode, 12 stagesSilica window (for UV light)

Compact, and low heat loadTolerance up to 100G magnetic fieldAl strip to keep surface resistance at low temp.Zener diode for high rate background

PurificationGas & Liquid phase purification testedMetal getter (zirconium) by gas ~0.5l/hMolecular sieves by liquid ~100l/h

2.5m

1m

57mm

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Prototype test, 10~40MeV

Vertex distribution

4mm1.6% in

40MeV Energy

68.6l liquid xenon prototype detector

228 PMTs

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Prototype test, 55~129MeV

- (at rest) + p -> 0 + n, 0(28MeV/c) -> + (54.9MeV<E<82.9MeV) Almost monochromatic is available- + p -> n + (129MeV) Energy, timing resolution neutron response

Opening angle ()

Ener

gy (M

eV)

PrototypeLXe

LYSO

NaI

H2 target

170°

175°

ENaI + ELYSO (MeV)

phe

Xeno

n de

tect

or

55MeV

83MeV

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Photon detector performance

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

110 psec

= 1.23 ±0.09 %FWHM=4.8 %

Expected Performance Energy : 5% (FWHM) Timing : 150 psec (FWHM) Vertex : 9mm (FWHM)

5%

1%En

ergy

Res

olut

ion

() [

%]

Energy resolution vs. EnergyEnergy distribution @ 55MeV

Timing distribution

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Detector operation

LXe Calorimeter Liquid 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

250l gas xenontank x 8

1000l liquid xenonstorage tank

purifier

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Calibration method• Gain monitor : LED• Q.E. : 241Am source• Energy

– 55, 83, and 129 MeV from -p0n, 02

– Ni thermal n capture 9MeV– Proton accelerator

• Li(p,)Be 17.6MeV• B(p,)C 16.1MeV

• Timing– Laser– radiative decay– 55 MeV from -p0n reaction (Pb+Scintillat

or)• Vertex

– 55 MeV from -p0n reaction (collimator)

3x3 NaI array

LH2 target

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Detector Construction

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Detector Construction

The performance of all PMTs checked in the liquid xenon before installation

outer

side

top

inner

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Schedule• Cryostat will come to PSI in the end of this

year• PMT installation into holders almost done• Liquid xenon detector assembly will be done

in the beginning of next year, and then liquefaction, purification and stable operation test will be continued.

• physics run starts in 2007• Positron spectrometer run and calibration

run will be done in this year

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Summary• MEG experiment will search for ->e with better

sensitivity than previous experiment • The performance of liquid xenon detector has be

en tested by using prototype, and the real detector is being constructed

• Physics run of MEG experiment will start in 2007.• Positron spectrometer data and calibration data

will be taken in this year.