MEG Experiment at PSI

1
MEG Experiment at PSI

• Liquid Xenon Photon Detector
• Satoshi MIHARA
• ICEPP, Univ. of Tokyo

2
Contents

• MEG Experiment
• Physics Motivation
• MEG Detector
• Liquid Xenon Photon Detector
• Liquid Xenon
• Detector Components
• Performance Studies using Prototypes
• Status of the Detector Construction

3
µ?e?

• Lepton Flavor Violation (LFV) is strictly
  forbidden in SM.
• Neutrino oscillation
• LF is not conserved
• Contribute ? (mn/mW)4
• Supersymmetry
• Off-diagonal terms in the slepton mass matrix

Just below the current limit Br(µ?e?) 1.2 x
10-11 (MEGA, PRL 83(1999)83)
4
MEG Experiment at PSI

• Proposal submitted and approved in 1999
• Situation at that time
• Neutrino oscillation discovery in 1998
• 4 possible solutions
• SUSY seesaw model

tanb

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

5
Current Situation

• KamLAND 766 ton-year data, 2004
• SNO NaClD2O data, 2005
• g-2 result
• K.Hagiwara, A.D. Martin, D.Nomura, and T.Teubner

6
Signal and Background

• Signal
• Eg mm/2 52.8MeV
• Ee mm/2 52.8MeV
• q 180o
• Time coincidence

• Background
• Radiative m decay
• Accidental overlap

g
n
m
n
e
n
n
m
?
e
7
Basic Concept

• Intense DC m 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

8
MEG Detector
9
COBRA Spectrometer(COnstant Bending Radius)

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

10
COBRA Magnet

• Gradient magnetic field, 1.27 T at z0
• 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
11
Drift Chamber

• Position resolutions (300mm) for both r and z.
• Vernier pad readout for z measurement
• Low material

12
Timing Counter

• Plastic scintillator Fine-mesh PMTs
• SciFiAPD to measure the impact point along the
  z-direction

13
Xenon Detector
14
Liquid Xenon Detector

• Why liquid xenon?
• How the detector works?
• Components
• Performance Study using prototypes
• Status of the detector construction

15
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

A.Hitachi PRB 27 (1983)5279
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 40 4.2,22,45
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Liquid Xenon and Sci 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
17
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

18
Reconstruction of the event depth

• Using event broadness on the inner face
• Necessary to achieve good timing resolution

19
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

20
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

21
PMT Development Summary
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 QE4-6 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 QE12-14 Much better performance in very high BG
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PMT Base Circuit

• 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

Reference PMT no Zener
PMT with Zener
23
Pulse Tube Refrigerator

• No mechanically moving part in the cold part
• Quiet
• Maintenance free
• Crucial for the MEG xenon detector

24
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

25
Xenon Purifier

• Attenuation of Sci light
• Scintillation light emission from an excited
  molecule
• XeXe?Xe2?2Xe hn
• Attenuation
• Rayleigh scattering lRay30-45cm
• Absorption by impurity

26
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

He
H2O
CO/N2
O2
Xe
CO2
27
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.

28
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
• g from p0 decay

29
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

30
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 bulk diffusion of impurities
31
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
a events
32
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.

33
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

34
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.

35
Liquid-phase Purifier Prototype
36
Liquid-phase Purification Performance
In 10 hours, ?abs 5m
37
Performance Studies

• Small Prototype
• Test of the detector principle
• Large Prototype
• Inverse-Compton g beam
• p0 ? g g produced via charge exchange process
  p-p?p0n

38
TERAS g Beam

• Compton Spectrum
• (Eg-Ec/2)2(Ec/2)2

Collimator size

• Electron beam (TERAS, Tsukuba in Japan)
• Energy 764MeV
• Energy spread 0.48(sigma)
• Divergence lt0.1mrad(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

10MeV
20MeV
40MeV
39
Energy Spectrum Fitting
Suppose Compton Spectrum around the
edge (E-Ec/2)2Ec2/4 Detector Response
Function Gaussian with Exponential tail f(x)
Nexpt/s2(t/2-(x-x0), xltx0t
Nexp-1/2((x-x0)/s)2, xgtx0t Convolution Integra
tion /- 5s

• Principle

Eg
Npe
Convolution of Compton Spectrum Response
Function
sE1.9
40
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

41
TERAS Data
Only 4x4 PMTs on the front face

• Switching off half the front PMTs
• Compton Edge shifts by 6.2
• Resolutions are almost same (1.84 to 1.85 in s)
  before and after switching off.
• Switching off PMTs on the
• side wall(s)
• 1 plane off ? 2.05 in s
• 2 planes off ? 2.22 in s
• 3, 4 planes off ? gt 3 in s

Number of Photoelectrons
42
Switching off PMTs on side walls
D

• Deterioration of the energy resolution when
  switching off PMTs is not mainly caused by loss
  of Npe.
• PMTs on the side walls compensate 1st conversion
  point dependence.

1 plane off
Number of Photoelectrons
3 planes off
D
Number of Photoelectrons
43
Effect of a faulty PMT

• All PMTs on s1.8
• Switching off one PMT on the front wall.
• the nearest PMT ?s2.3
• 2nd nearest PMT ?s1.9
• 3rd nearest PMT ?s1.9
• 300 PMTs on the front face in the final detector
• 4/300 1.3 loss of acceptance

F30 off s2.3
F22 off s1.9
F28 off s1.9
44
CEX beam test

• Requiring qgt170o
• FWHM 1.3 MeV
• Requiring q gt 175o
• FWHM 0.3 MeV

• Charge Exchange elementary process
• p-p?p0n
• p0(28MeV/c) ? g g
• 54.9 MeV lt E(g) lt 82.9 MeV

45
Beam Test Setup
H2 targetdegrader
LYSO Eff 14
NaI
LP
S1
Eff(S1xLP)88
beam
46
Energy Resolutions
CEX 2004
55 MeV
83 MeV to Xe

• 1.23 0.09
• FWHM4.8

55 MeV to Xe
Exenonnph
83 MeV
s 1.000.08 FWHM5.2
47
Energy Resolution vs Energy
PSI 2003 TERAS 2003 alpha
Right ? is a nice function of gamma energy
48
Position Reconstruction

• Localized Weight Method

• Projection to x and y directions.
• Peak point and distribution spread

• Position reconstruction using the selected PMT

49
Examples of Reconstruction
(40 MeV gamma beam w/ 1 mm collimator)
50
Timing/Z Resolution
p-

• Improving Z resolution is essential to improve
  timing resolution.
• Intrinsic timing resolution can be evaluated by
  comparing left and right parts of the detector.
• ltTgt (TL?TR)/2

NaI
g
Xenon
S1
g
LYSO
tLP - tLYSO
TL
Left
Right
TR
g
51
Absolute timing, Xe-LYSO analysis
high gain
normal gain
110 psec
103 psec
55 MeV
s LYSO Beam L-R depth reso.
110 64 61 65 56 33 psec
103 64 61 53 43 31 psec
Normal gain
High gain
A few cm in Z
52
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 pE5 area in PSI

53
Summary

• MEG at PSI
• Search for µ?e? with better sensitivity than
  previous experiments
• Xenon detector
• COBRA spectrometer
• PSI m beam
• Detector preparation will finish in several
  months
• DAQ in 2006

54
Further Information

• Beam
• Drift Chamber system
• Timing Counter
• Electronics
• Software
• Waveform analysis
• Etc.
• Please visit http//meg.psi.ch