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MEG Experiment at PSI

Liquid Xenon Photon Detector

Satoshi MIHARAICEPP, Univ. of Tokyo

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Contents

1. MEG Experiment• Physics Motivation• MEG Detector

2. Liquid Xenon Photon Detector• Liquid Xenon• Detector Components• Performance Studies using Prototypes• Status of the Detector Construction

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μ→eγ

• Lepton Flavor Violation (LFV) is strictly forbidden in SM.

• Neutrino oscillation– LF is not conserved

– Contribute (m∝ /mW)4

• Supersymmetry– Off-diagonal terms in the slepton mass matrix

233

232

231

223

222

221

213

212

211

2~

mmm

mmm

mmm

ml

μ e

W

e

μ e

0~x

e~~

Just below the current limit

Br(μ→eγ) = 1.2 x 10-11

(MEGA, PRL 83(1999)83)

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MEG Experiment at PSI

•Small tan region was being excluded by LEP Higgs searches.

tan

• Proposal submitted and approved in 1999

• Situation at that time• Neutrino oscillation discovery in 1998• 4 possible solutions• SUSY seesaw model

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Current Situation

• KamLAND 766 ton-year data, 2004

• SNO NaCl+D2O data, 2005

• g-2 result

– K.Hagiwara, A.D. Martin, D.Nomura, and T.Teubner

)7.2(10)0.95.24( 10 a

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Signal and Background

• Signal

• E = m/2 = 52.8MeV

• Ee = m/2 = 52.8MeV

• = 180o

• Time coincidence

• Background– Radiative decay

– Accidental overlap

ee

ee

ee????

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Basic Concept

• Intense DC beam– Reduce pile-up

• Photon Detector– Good resolution

• A few % for Energy• A few mm for position• ~100psec for timing

– Fast response– Uniform

• Positron Detector– Reduce BG Michel positron– Minimum amount of

material

PSI

Liquid Xenon Photon Detector

COBRA Spectrometer

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MEG Detector

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COBRA Spectrometer(COnstant Bending Radius)

• Sweep out curling positrons rapidly.• Constant bending radius independent of the

emission angles.

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COBRA Magnet

• Gradient magnetic field, 1.27 T at z=0• Small magnetic field around the photon

detector.

• 0.197X0 around the center

• Cooling by using two GM-type refrigerators No need of liquid He for operation

CERN Courier 44 number 6 21-22 2004

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Drift Chamber

• Position resolutions (~300m) for both r and z.

• Vernier pad readout for z measurement• Low material

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Timing Counter

• Plastic scintillator + Fine-mesh PMTs• SciFi+APD to measure the impact point along the z-direction

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Xenon Detector

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Liquid Xenon Detector

• Why liquid xenon?

• How the detector works?

• Components

• Performance Study using prototypes

• Status of the detector construction

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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

– s = 4.2nsec, =45nsec (for electron, no E)

• Uniform

NaI BGO GSO LSO LXe

Effective Atomic number 50 73 58 65 54

Density (g/cm3) 3.7 7.1 6.7 7.4 3.0

Relative light output (%) 100 15

20-40

45-70

80

Decay time (nsec)

230 300 60 404.2,22,

45

A.Hitachi PRB 27 (1983)5279

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Liquid Xenon and Sci 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~

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MEG Xenon 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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Reconstruction of the event depth

• Using event broadness on the inner face

• Necessary to achieve good timing resolution

3 cm

Liq. Xe

Liq. Xe

14 cm

(a)

(b)

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52.8 MeV

52.8 MeV

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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 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

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) Not used yet in the LP

Rb-Sc-Sb

Mn layer to keep surface resistance at low temp.

K-Sc-Sb

Al strip to fit with the dynode pattern to keep surface resistance at low temp.

K-Sc-Sb

Al strip density is doubled.

4% loss of the effective area.

1st compact version

QE~4-6%

Under high rate background,

PMT output reduced by 10

-20% with a time constant of

order of 10min.

Higher QE ~12-14%

Good performance in high rate BG

Still 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

power ~150W– 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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Pulse Tube Refrigerator

• No mechanically moving part in the cold part– Quiet

– Maintenance free Crucial for the MEG xenon detector

Compressor

Regenerator

Hot endCold end 'Virtual Gas-Piston'

Compression

Regeneration

Expansion

Regeneration

Regenerator

Pulse Tube cryocooler

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Refrigerator R&D

• MEG 1st spin-off

• Technology transferred to a manufacturer, Iwatani Co. 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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Xenon Purifier

• Attenuation of Sci light– Scintillation light emission from an excited molecule

• Xe+Xe*Xe2*2Xe + h

– Attenuation• Rayleigh scattering Ray~30-45cm

• Absorption by impurity

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Possible Contaminants

• Remaining Gas Analysis (RGA) for investigating what causes short absorption length.

• Remaining gas in the chamber was sampled to the analyzing section.

• Vacuum level– LP Chamber 2.0x10-2Pa

– Analyzing section 2.0x10-3Pa

HeH2O CO/N2

O2

CO2Xe

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Water Contamination

• 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.

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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 beam– 60MeV Electron beam– from 0 decay

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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

Gas return

To purifier

Circulation pump

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Heated Metal Getter Purifier• Metal getter technology based on zirconium

metals form irreversible chemical bonds to remove all oxide, carbide and nitride impurities

• Getter Material (GM) such as Zr– GM + O2  GMO– GM + N2  GMN– GM + CO2 CO + GMO GMC + GMO– GM + CO GMC + GMO– GM + H2O H + GMO GMO + H (bulk)– GM + H2 GM + H (bulk)– GM + Hydrocarbons, CxHx, etc. GMC + H (bulk)– GM + He, Ne, Ar, Kr, Xe (inert gases) No reaction

• These chemical reactions occur on the surface of the metal, and the reaction products then diffuse into the bulk structure.

• Longer life time than catalyst media• Need temperature control of the metal

Heat allows bulkdiffusion of impurities

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Purification Performance• Xenon Detector Large

Prototype• 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 events

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Absorption Length• Fit the data with a function :

A exp(-x/ abs)• abs >100cm (95% C.L)

from comparison with MC.• CR data indicate that abs >

100cm has been achieved after purification.

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Upgrade of the system

• Purification in Gas phase– Evaporate and liquefy

• Slow• Cooling power consumption

• We know that water is the main impurity to be removed.– Purification system

dedicated to remove water– Not in gas phase but in

liquid phase

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Liquid-phase Purification System

• Xenon circulation in liquid phase.• Impurity (water) is removed by a

purifier cartridge filled with molecular sieves.

• 100 l/hour circulation.

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Temperature Sensor

PMT’s

Purifier Cartridge

Molecular sieves, 13X 25g water

Freq. InverterOMRON

PT

Liquid-phase Purifier Prototype

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Liquid-phase Purification Performance

In ~10 hours, λabs ~ 5m

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Performance Studies

• Small Prototype– Test of the detector principle

• Large Prototype– Inverse-Compton beam– 0 produced via charge exchange

process -pn

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TERAS Beam

• Electron beam (TERAS, Tsukuba in Japan)– Energy: 764MeV– Energy spread: 0.48%(sigma)– Divergence: <0.1mrad(sigma)– Beam size: 1.6mm(sigma)

• Laser photon– Energy: 1.17e-6x4 eV (for 40MeV)– Energy spread: 2x10-5 (FWHM)– Divergence: unknown– Beam size: unknown

Compton Spectrum

•(E-Ec/2)2+(Ec/2)2

Collimator size

10MeV

20MeV40MeV

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Energy Spectrum Fitting

• Principle…

E Npe

Convolution of

Compton Spectrum

Response Function

Suppose Compton Spectrum around the edge

(E-Ec/2)2+Ec2/4Detector Response Function

Gaussian with Exponential tailf(x) = N*exp{t/2(t/2-(x-x0)}, x<x0+t N*exp{-1/2((x-x0)/)2}, x>x0+t

ConvolutionIntegration +/- 5

E~1.9%

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Measurement with half the front PMT switched off

• To simulate the convex front geometry of the cryostat – Switch off half of the PMTs in the front face

Use 4x4 PMTs out of 6x6 PMTs

– Switch off PMTs on the side walls

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CEX beam test• Charge Exchange elementary process• -p0n

– 0(28MeV/c) – 54.9 MeV < E() < 82.9 MeV

• Requiring

FWHM = 1.3 MeV

• Requiring > 175o

FWHM = 0.3 MeV

170o

175o

0

54.9MeV 82.9MeV

1.3MeV for >170o

0.3MeV for >175o

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Beam Test Setup

H2 target+degrader

beam

LPNaI

LYSO

Eff ~14%

S1Eff(S1xLP)~88%

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Energy Resolutions

83 MeV to Xe83 MeV to Xe

55 MeV to Xe55 MeV to Xe

Exe

non[

n ph]

= 1.23 ±0.09 %FWHM=4.8 %

55 MeV

σ = 1.00±0.08 % FWHM=5.2%

83 MeV

CEX 2004

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Right is a nice function of gamma energy

PSI 2003TERAS 2003alpha

Energy Resolution vs Energy

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Position Reconstruction

• Localized Weight Method

• Projection to x and y directions.

• Peak point and distribution spread

•Position reconstruction using the selected PMT

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Examples of Reconstruction

(40 MeV gamma beam w/ 1 mm collimator)

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Timing/Z Resolution

• Improving Z resolution is essential to improve timing resolution.

• Intrinsic timing resolution can be evaluated by comparing left and right parts of the detector.

– <T> = (TLTR)/2

XenonNaI S1

LYSO tLP - tLYSO

-

3 cm

Liq. Xe

Liq. Xe

14 cm

(a)

(b)

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52.8 MeV

52.8 MeV

Left

Right

TL

TR

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Absolute timing, Xe-LYSO analysis55 M

eV

high gainnormal gain

110 psec 103 psec

LYSO Beam L-R depth reso.

110 64 61 = 65 = 56 33 psec

103 64 61 = 53 = 43 31 psec

No

rma

l g

ain

Hig

h

ga

in

A few cm in Z

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Status of Xenon Detector Construction

• PMT– 850 PMTs being tested in PSI

and Pisa

• Cryostat– Under construction– Delivery to PSI early in 2006

• Gas system– Getting ready in E5 area in PSI

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Summary

• MEG at PSI– Search for μ→eγ with better sensitivity than

previous experiments• Xenon detector• COBRA spectrometer• PSI beam

– Detector preparation will finish in several months

– DAQ in 2006

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Further Information

• Beam• Drift Chamber system• Timing Counter• Electronics• Software• Waveform analysis• Etc.

Please visit http://meg.psi.ch