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MEG Experiment at PSI

This seminar discusses the MEG Experiment, its physics motivation, and the design and performance of the liquid xenon photon detector. Topics include lepton flavor violation, neutrino oscillation, and the use of liquid xenon in the detector.

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MEG Experiment at PSI

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  1. MEG Experiment at PSI Satoshi MIHARA, U Zuerich Seminar 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 Satoshi MIHARA, U Zuerich Seminar

  3. ne nm e μ μ e W g g μ→eγ • 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 Satoshi MIHARA, U Zuerich Seminar Just below the current limit Br(μ→eγ) = 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 Satoshi MIHARA, U Zuerich Seminar • Small tanb region was being excluded by LEP Higgs searches.

  5. 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 Satoshi MIHARA, U Zuerich Seminar

  6. Signal Eg = mm/2 = 52.8MeV Ee = mm/2 = 52.8MeV q = 180o Time coincidence Background Radiative m decay Accidental overlap g m e g Signal and Background g n m n Satoshi MIHARA, U Zuerich Seminar e n n m ? e

  7. 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 Basic Concept Satoshi MIHARA, U Zuerich Seminar

  8. MEG Detector Satoshi MIHARA, U Zuerich Seminar

  9. COBRA COBRA COBRA Spectrometer(COnstant Bending Radius) • Sweep out curling positrons rapidly. • Constant bending radius independent of the emission angles. Satoshi MIHARA, U Zuerich Seminar

  10. 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 Satoshi MIHARA, U Zuerich Seminar

  11. Drift Chamber • Position resolutions (~300mm) for both r and z. • Vernier pad readout for z measurement • Low material Satoshi MIHARA, U Zuerich Seminar

  12. Timing Counter • Plastic scintillator + Fine-mesh PMTs • SciFi+APD to measure the impact point along the z-direction Satoshi MIHARA, U Zuerich Seminar

  13. Xenon Detector Satoshi MIHARA, U Zuerich Seminar

  14. Liquid Xenon Detector • Why liquid xenon? • How the detector works? • Components • Performance Study using prototypes • Status of the detector construction Satoshi MIHARA, U Zuerich Seminar

  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, tT=45nsec (for electron, no E) • Uniform A.Hitachi PRB 27 (1983)5279 Satoshi MIHARA, U Zuerich Seminar

  16. 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 Liquid Solid Pressure [MPa] Satoshi MIHARA, U Zuerich Seminar 0.1 0.082 Gas Excitation Triple point Recombination 161 165 Temperature [K]

  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 Satoshi MIHARA, U Zuerich Seminar

  18. g Reconstruction of the event depth • Using event broadness on the inner face • Necessary to achieve good timing resolution Satoshi MIHARA, U Zuerich Seminar

  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 Satoshi MIHARA, U Zuerich Seminar

  20. Photomultiplier R&D 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 Satoshi MIHARA, U Zuerich Seminar

  21. PMT DevelopmentSummary Satoshi MIHARA, U Zuerich Seminar

  22. 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 Satoshi MIHARA, U Zuerich Seminar 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 Satoshi MIHARA, U Zuerich Seminar

  24. 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 Satoshi MIHARA, U Zuerich Seminar

  25. Xenon Purifier • Attenuation of Sci light • Scintillation light emission from an excited molecule • Xe+Xe*Xe2*2Xe + hn • Attenuation • Rayleigh scattering lRay~30-45cm • Absorption by impurity Satoshi MIHARA, U Zuerich Seminar

  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 Satoshi MIHARA, U Zuerich Seminar 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. Satoshi MIHARA, U Zuerich Seminar

  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 Satoshi MIHARA, U Zuerich Seminar

  29. Gas return To purifier Circulation pump Purification System • Xenon extracted from the chamber ispurified by passing through the getter. • Purified xenon is returned to the chamber and liquefied again. • Circulation speed 5-6cc/minute Satoshi MIHARA, U Zuerich Seminar

  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 Satoshi MIHARA, U Zuerich Seminar 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 Satoshi MIHARA, U Zuerich Seminar Cosmic-ray events a events

  32. Absorption Length • Fit the data with a function : A exp(-x/ labs) • labs >100cm (95% C.L) from comparison with MC. • CR data indicate that labs > 100cm has been achieved after purification. Satoshi MIHARA, U Zuerich Seminar

  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 Satoshi MIHARA, U Zuerich Seminar

  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. Satoshi MIHARA, U Zuerich Seminar

  35. Purifier Cartridge Molecular sieves, 13X 25g water Freq. Inverter OMRON PT PMT’s Temperature Sensor Liquid-phase Purifier Prototype Satoshi MIHARA, U Zuerich Seminar

  36. Liquid-phase Purification Performance In ~10 hours, λabs ~ 5m Satoshi MIHARA, U Zuerich Seminar

  37. Performance Studies • Small Prototype • Test of the detector principle • Large Prototype • Inverse-Compton g beam • p0 gg produced via charge exchange process p-pp0n Satoshi MIHARA, U Zuerich Seminar

  38. TERAS g Beam • Compton Spectrum • (Eg-Ec/2)2+(Ec/2)2 • 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 Satoshi MIHARA, U Zuerich Seminar Collimator size 10MeV 20MeV 40MeV

  39. Energy Spectrum Fitting • Suppose Compton Spectrum around the edge • (E-Ec/2)2+Ec2/4 • Detector Response Function • Gaussian with Exponential tail • f(x) = N*exp{t/s2(t/2-(x-x0)}, x<x0+t • N*exp{-1/2((x-x0)/s)2}, x>x0+t • Convolution • Integration +/- 5s • Principle… Eg Satoshi MIHARA, U Zuerich Seminar Npe Convolution of Compton Spectrum Response Function sE~1.9%

  40. g 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 Satoshi MIHARA, U Zuerich Seminar

  41. Eg 170o q Eg Eg p0 175o q 54.9MeV 82.9MeV 1.3MeV for q>170o 0.3MeV for q>175o Eg CEX beam test • Requiring q>170o • FWHM = 1.3 MeV • Requiring q > 175o • FWHM = 0.3 MeV • Charge Exchange elementary process • p-pp0n • p0(28MeV/c)  g g • 54.9 MeV < E(g) < 82.9 MeV Satoshi MIHARA, U Zuerich Seminar

  42. Beam Test Setup H2 target+degrader LYSO Eff ~14% Satoshi MIHARA, U Zuerich Seminar NaI LP S1 Eff(S1xLP)~88% beam

  43. Energy Resolutions CEX 2004 55 MeV 83 MeV to Xe • = 1.23 ±0.09 % FWHM=4.8 % Satoshi MIHARA, U Zuerich Seminar 55 MeV to Xe Exenon[nph] 83 MeV σ= 1.00±0.08 % FWHM=5.2%

  44. Energy Resolution vs Energy PSI 2003 TERAS 2003 alpha Satoshi MIHARA, U Zuerich Seminar Right  is a nice function of gamma energy

  45. Position Reconstruction • Localized Weight Method Satoshi MIHARA, U Zuerich Seminar • Projection to x and y directions. • Peak point and distribution spread • Position reconstruction using the selected PMT

  46. Examples of Reconstruction Satoshi MIHARA, U Zuerich Seminar (40 MeV gamma beam w/ 1 mm collimator)

  47. 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. • <T> = (TLTR)/2 NaI g S1 Xenon g LYSO tLP - tLYSO Satoshi MIHARA, U Zuerich Seminar TL Left Right TR g

  48. Absolute timing, Xe-LYSO analysis high gain normal gain 103 psec 110 psec 55 MeV Satoshi MIHARA, U Zuerich Seminar Normal gain High gain A few cm in Z

  49. 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 Satoshi MIHARA, U Zuerich Seminar

  50. Summary • MEG at PSI • Search for μ→eγ with better sensitivity than previous experiments • Xenon detector • COBRA spectrometer • PSI m beam • Detector preparation will finish in several months • DAQ in 2006 Satoshi MIHARA, U Zuerich Seminar

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