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The Liquid Xenon Calorimeter of the MEG Experiment

The Liquid Xenon Calorimeter of the MEG Experiment. Fabrizio Cei INFN and Universita’ di Pisa. Incontri di Fisica delle Alte Energie – IFAE 2006 Pavia, 19-21 April 2006. Brief overview of the MEG experiment; The Liquid Xenon scintillation calorimeter;

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The Liquid Xenon Calorimeter of the MEG Experiment

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  1. The Liquid Xenon Calorimeter of the MEG Experiment Fabrizio Cei INFN and Universita’ di Pisa Incontri di Fisica delle Alte Energie – IFAE 2006 Pavia, 19-21 April 2006 Fabrizio Cei

  2. Brief overview of the MEG experiment; The Liquid Xenon scintillation calorimeter; The Calorimeter (Large) Prototype; Measured and expected performances; Status of calorimeter preparation; Conclusions. Outline Fabrizio Cei

  3. Overview of the MEG experiment Fabrizio Cei

  4. The MEG Experiment at PSI Search for Lepton Flavour Violating decaym  eg The Paul Scherrer Institute • The most powerful continuous machine in the world; • Proton energy590 MeV; • Power 1.1 MW; • Nominal operation current1.8 mA. Fabrizio Cei

  5. The MEG Collaboration ~ 40 FTEs Fabrizio Cei

  6. The m eg decay – 1) • Forbidden in the Standard Model of electroweak interactions because of the conservation of lepton family numbers. • With massive neutrinos (we know that mn > 0 !) and mixing, meg is allowed but at a negligible level (relative probability ~ 10-55) Fabrizio Cei

  7. All SM extensionsenhance the rate through mixing in the high energy sector. The m eg decay – 2) Predictions in the range 10-12 10-15 SUSY ≈ 10-12 SM Background negligible  clear evidence for physics beyond the standard model Fabrizio Cei

  8. Historical perspective BR < 0.1 Present limit 1.2 x 10-11 Improvements in physics linked with improvements in technology. Fabrizio Cei

  9. e+ +g e+ +g n n n n e+ + m  eg: Signal and Background Background Signal eg accidental en n egn n ee  g g eZ  eZ g physical egn n qeg = 180° Ee= Eg=52.8MeV Te = Tg Accidental background dominant in the signal region g Fabrizio Cei

  10. m  eg: required performances The sensitivity is limited bythe accidental background BR (m  eg)  10-13 allowed, but needed FWHM Some of them already fulfilled ! Need of a DC beam Fabrizio Cei

  11. qeg = 180° e+ +g Ee= Eg=52.8MeV MEG Experiment Layout • Muon beam stoppedin a150 mmtarget. • Liquid Xenon calorimeter(800 l,  850 PMTs) for photon detection using scintillation light: fast response (~ 20 ns) and high light yield (~ 0.8 NaI). • (Thin wall) solenoidal spectrometer & drift chambersforpositron momentum measurement. • Scintillation counters for positron timing. Easy signal selection for m+ decaying at rest Fabrizio Cei

  12. Positron track One MC event Energy release in LXe Hits on TC Fabrizio Cei

  13. The Liquid Xenon Calorimeter Fabrizio Cei

  14. H.V. Refrigerator Signals Cooling pipe Vacuum for thermal insulation Al Honeycomb Liq. Xe window PMT filler Plastic 1.5m The Calorimeter – 1) Measurement of genergy, direction and timing Liquid Xenon properties Experimental check Fabrizio Cei

  15. The Calorimeter – 2) • Homogeneous 0.8 m3 volume of Liquid Xenon - pulse tube refrigerator - 67 cm < r < 108 cm - |cos(q)| < 0.35; |f| < 60o DW  10 % • Only scintillation light; • Read by 846 PMTs (Hamamatsu): - 2 inches diameter; - Maximum photocathodic coverage on the photon entrance face  43 %; - Immersed in Liquid Xenon; - Low temperature (165 oK); - Quartz window for matching with scintillation light wavelength (178 nm); • Thin entrance wall; • Waveform digitizing @ 2 GHz for pile-up rejection. Fabrizio Cei

  16. PMT R & D history - 1) • The MEG calorimeter will work in an intense g background • environment. Photons will be produced by several sources • (muon radiative decay, bremsstrahlung, positron annihilation, • neutron capture in Xenon and materials surrounding the detector …). • We estimated that the light level due to the background would • correspond to a few mA anodic current for a PMT gain G = 106. • At so high rates one expects undesired behaviours because of • increasing of photocathode resistivity at low temperatures; • changes in amplification when the average anodic current becomes comparable with the PMT base current (50 mA). Fabrizio Cei

  17. PMT R & D history – 2) 1st generation: R6041Q 2nd generation: R9288TB Overlinearity (hidden in the previous plot by Q.E. drop) Photocathode: Rb-Cs-SbK-Cs-Sb Material to reduce surface resistivity: Mn layerAl strips Q.E. @ 165 oK: ~ 5 %~ 15 % ON OFF ON OFF Photon high rate simulated by high frequency (tens of kHz) LED pulsing Fabrizio Cei

  18. PMT R & D history – 3) 3rd generation: R9869Q Doubling the Al strips produces a better stabilization of resistivity at low temperatures. Insertion of Zener diodes in the last two stages of base amplification chain removes overlinearity. Photocathode: K-Cs-Sb Material to reduce surface resistivity: Al strips (doubled) Q.E. @ 165 oK: ~ 15 % Fabrizio Cei

  19. Xenon purity – 1) Energy resolution strongly depends on scintillation light absorption: - reduced number of photoelectrons; - loss of uniformity (combined with Rayleigh scattering). Xenon almost transparent to its own scintillation light, but possible contaminants can be very opaque … Fabrizio Cei

  20. Xenon purity – 2) We developed a purification system to reduce impurities below ppb. Xenon is circulated in liquid phase (100 l/hour by means of a Barber-Nicols cryogenic fluid pump) and water vapor is removed by a purifier cartridge filled with molecular sieves. Purification performances (old system, new one is much faster) Purification system tested and improved by means of the Calorimeter Prototype. Fabrizio Cei

  21. The Calorimeter Prototype(Large Prototype) Fabrizio Cei

  22. The Large Prototype - 1) • Presently the largest Liquid Xenon calorimeter in the world: 40 x 40 x 50 cm3; ~ 70 liters of Liquid Xenon • 228 PMTs (types R6041 & R9288, not the newest ones); • Measurements: - cryogenic and long term operation; - absorption length; - energy, timing and position resolution. • Operating conditions similar to that of final detector. Fabrizio Cei

  23. -source LEDs The Large Prototype – 2) Seen from inside -sources and LEDs for PMTcalibration and monitoring Fabrizio Cei

  24. The Large Prototype - 3) • Home-made Polonium alpha –sources mounted on 50 micron tungsten wires (to be replaced by commercial Am sources, specifically developed by SORAD Ltd.); • ~ 50 Bq for each source; • First application of this type of sources; preprint submitted to NIM. • Already used for Q.E. determination. Fabrizio Cei

  25. Measured Performances of the Calorimeter Fabrizio Cei

  26. Measurement of absorption length • By using alpha sources (on walls and on wires) is possible to give a lower • limit of the Xenon absorption lengthlabsand an estimate of the light yield. • labs > 95 cm (95 % C.L.) • Light Yield ~ 37500 scintillation photons/MeV (0.9 NaI) Fabrizio Cei

  27. Measurement of energy resolution • Charge exchange reaction • p-p  p0n •  • g g • Liquid Hydrogen target to • maximize photon flux; • p0 Frame: g monochromatic spectrum LAB Frame: g flat spectrum; • Back-to-back configuration: Eg = 55, 83 MeV; • Even a modest collimation ( 5o) guarantees a sufficient monochromaticity (DE  0.3 MeV); • Need of an opposite side detector (a NaI array with LYSO preshower). Fabrizio Cei

  28. Experimental setup H2 target LYSO Eff 14 % NaI LP S1 Eff (S1&& LP)  88 % beam Fabrizio Cei

  29. Energy spectra in NaI & LP 83 MeV  55 MeV correlation 129 MeV line from p-p  ng (LXe sensitive to 9 MeV neutrons) Fabrizio Cei

  30. Energy resolution @ 55 MeV • Event selection: • LP && S1 && (NaI + LYSO); • 83 MeV line in NaI + LYSO  65 MeV < ENaI + LYSO < 95 MeV; • No saturated PMTs; • Collimator: r < 4 cm. PMTs Collimator FWHM: DE/E = (4.9  0.4) % Fabrizio Cei

  31. Measurement of timing resolution LXe – LYSO timing difference @ 55 MeV high gain normal gain 103 psec 110 psec sDtLYSOBeam Normal gain 110Ɵ64Ɵ61 = 65 High gain 103Ɵ64Ɵ61 = 53 153 ps FWHM = 125 ps sz  1  2 cm Fabrizio Cei

  32. Status of Calorimeter Preparation Fabrizio Cei

  33. PMTs: 850 PMTs under testing in PSI and in Pisa LXe facility (3  4/day); Cryostatunder construction; delivery at PSI in spring 2006; Gas System: almost ready in pE5 area at PSI. Fabrizio Cei

  34. PMT mounting Fabrizio Cei

  35. Conclusions Fabrizio Cei

  36. The MEG experiment is expected to start engineering runs in 2006; Experimental tests with sub-detectors showed that many of the needed resolutions were already fulfilled; For LXe calorimeter, we obtained an absorption length labs > 100 cm, an energy resolution DE/E < 5 % @ 55 MeV and a timing resolution of ~ 150 ps FWHM; We successfully used alpha sources mounted on wires for calibration and monitoring of the detector (the first application of these sources); The calorimeter building and the PMT testing and calibration are in advanced state. Fabrizio Cei

  37. The MEG web page Please, visit it ! Fabrizio Cei

  38. Backup slides Fabrizio Cei

  39. “Supersymmetric parameter space accessible by LHC” MEG W. Buchmueller, DESY, priv. comm. Comparison with LHC 1999 R&D 2000 2001 2002 2003 2004 2005 2006 Engineering 2007 Data 2008 2009 MEGA • Plans • Data taking from 2007 on to reach 10-13 sensitivity (90% CL) • Obtain a “significant” result before the LHC era • Eventually reach 10-14during LHC era Fabrizio Cei

  40. Experimental bound MEG Connection with n-oscillations –1) Additional contributiontoslepton mixingfromV21, matrix element responsible for solar neutrino deficit. (J. Hisano & N. Nomura, Phys. Rev. D59 (1999) 116005) tan(b) = 30O tan(b) = 0O Largely favoured and confirmed by Kamland After Kamland All solar n experiments combined Fabrizio Cei

  41. Connection with n-oscillations –2) Correlation betweenBR (meg)&sin2(q13) (still unknown !) In these models, BR (meg)is one of the most sensitive tool to measure sin2(q13). 10-11 10-14 Sensitivity of future long-baseline experiments A. Masiero et al., hep-ph/0407325 Fabrizio Cei

  42. MC Simulation • Geometry: full simulation of calorimeter structure, internal and external vessels, PMT holders and honeycomb; • Scintillation light tracking: - decay curve and wavelength spectrum of LXe scintillation; - absorption and Rayleigh scattering in Liquid Xenon; - Fresnel and total reflection on PMT quartz window and PMT holders; - PMT quartz window transmittance; • Outputs: - Energy deposit, position and timing in Liquid Xenon; - Waveform output: hit timing of scintillation photons for each PMT with digitizer binning. (～8 x 104 photoelectrons @ 50 MeV). Fabrizio Cei

  43. The required performances of the detector demand multiple and complementarycalibration and monitoring methods. Alpha sources: Q.E. determination, LXe optical properties, energy scale stability, permanently installed within the calorimeter. But: no good energy reference, useless for timing; g-lines from neutron capture in Ni (Eg = 9 MeV): absolute energy scale, light yield stability, usable very frequently. But: useless for timing & Q.E., low sensitivity to optical properties; Charge exchange reaction: p-p  p0n (followed by p0 decay in two gamma’s): energy scale determination, absolute timing and position calibration, simultaneous calibration of the whole apparatus. But: difficult to use frequently, hardware modifications needed, useless for Q.E., low sensitivity to optical properties; Calibration techniques - 1) Fabrizio Cei

  44. Calibration techniques - 2) • Cockroft-Walton Proton accelerator based on - Resonant cross section at Ep = 440 keV (speak = 6 mbarn, G 15 keV); - Main method: simultaneous calibration of the whole apparatus, useful for absolute energy scale and monitoring, frequently usable. Fabrizio Cei

  45. Alpha sources in LXe Simulated Measured 50 mm thick gold plate clipped around the wire 100 mm thick tungsten wire Fabrizio Cei

  46. -triggerwith5  106 gain; Geometrical cutsto exclude -sources; Energy scale: -source 208Tl (2.59±0.06) MeV 40K (1.42±0.06) MeV Other lines ?? uniform on the front face; few 10 min (withnon-dedicated trigger); nice calibration forlow energy ’s. 40K (1.461 MeV) 208Tl (2.614 MeV) Radioactive Background in LP Never seen before ! Fabrizio Cei

  47. Measurement of position resolution Reconstruction by (localized) weighted average method (40 MeV gamma beam with 1 mm collimator) Fabrizio Cei

  48. Sensitivity Cuts @ 1.4 FWHM Fabrizio Cei

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