Liquid Xenon Carlorimetry at the MEG Experiment Satoshi MIHARA Univ. of Tokyo

2 Contents  MEG Experiment  Liquid Xenon Scintillation Detector –Liquid Xenon Property –Operation –Detector Components –Calibration –Performance  Summary

3 MEG Experiment  Search for Lepton-Flavor violating muon rare decay;   e  –Clear evidence of new physics beyond SM  SUSY-GUT, SUSY-Seesaw Br ~< –Present limit 1.2x by MEGA  Engineering run starts in 2006 and full DAQ will start in 2007 at Paul Scherrer Insitut.

4 MEG Detector  e + measured by COBRA spectrometer   by LXe detector 52.8MeV

5 LXe Detector R&D history  Small Prototype –2.3 liter active volume  Large Prototype –70 liter active volume  Final Detector –800 liter active volume

6 Why Liquid Xenon?  Good resolutions –Large light output yield –W ph (1MeV e) = 22.4eV  Pile-up event rejection –Fast response and short decay time –  s = 4.2nsec,   =45nsec (for electron, no E)  Uniform NaI BGOGSOLSOLXe Effective Atomic number Density (g/cm3) Relative light output (%) Decay time (nsec) ,22, 45

7 LXe and Scintillation light  Density 3.0 g/cm 3  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 Gas Triple point Temperature [K] Pressure [MPa] Excitation Recombination

8 MEG LXe 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

9 Operation Procedure 1. Evacuation  TMP + Cryopump  10 -4~5 Pa 2. Pre-cooling  2.0 bar xenon gas at room temp  Refrigerator/LN 2 cooling 3. Liquefaction  Continuously supply Xe gas  Pressure control  Refrigerator/LN 2 cooling 4. Purification  Circulation/Purification 5. Ready

10 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

11 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

12 1 st generation R6041Q2 nd generation R9288TB3 rd generation R in the LP (2003 CEX and TERAS) 127 in the LP (2004 CEX) 111 In the LP (2004 CEX)Used in the final detector 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. 1 st compact version QE~4-6% Under high rate background, PMT output reduced by % 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

13 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 ~190W –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

14 Refrigerator  Crucial for the MEG xenon detector –Quiet (less vibration) –Maintenance free  Pulse tube refrigerator does not have any mechanically moving part in the cold part.

15 Refrigerator R&D  MEG 1 st spin-off  Technology transferred to a manufacturer, Iwatani Co. Ltd  Performance obtained at Iwatani –189 –6.7 kW compressor –4 Hz operation

16 Purification System  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 is necessary. Rayleigh scattering Ray ~30-45cm

17 Gas-phase Purification  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 Cosmic-ray events  events

18 Liquid-phase Purification  Xenon circulation in liquid phase.  Impurity (water) is removed by a purifier cartridge filled with molecular sieves.  100 l/hour circulation. Temperature Sensor Purifier Cartridge Molecular sieves, 13X 25g water Freq. Inverter OMRON PT In ~10 hours, λabs ~ 5m

19 Liquid-phase Purification cont’d  For the MEG xenon detector –Another cryostat placed beside the detector for independent regeneration of the purifier cartridge –Xenon transferred from the bottom of the detector to the cryostat –Purified and retuned to the detector through vacuum insulated pipes

20 Calibration  LED flashed in the liquid –PMT gain calibration  Alpha source on wires –Point-like source as if floating in the active volume –Possible to illuminate all PMTs –PMT calibration and monitoring/absorption length estimation Wire (  m  ) Alpha 40 μm SORAD/ISOTOPE PRODUCTS

21  54.9MeV82.9MeV 1.3MeV for  >170 o 0.3MeV for  >175 o 00      0 decay  ’s through CEX process –  - +p   0 +n –55MeV, 83MeV    emission from thermal neutron capture on Ni nuclei –9MeV  3 7 Li(p,  ) 4 8 Be –E p = 440 keV,  14 keV,  peak = 5 mb –17.6MeV  –obtainable :  10 6  /s (isotropic) at 440 KeV resonance (I p  50  A) 9 MeV Nickel γ-line NaI Polyethylene 0.25 cm Nickel plate 3 cm 20 cm Further Calibration Methods

22 Detector Performance (LYSO) - 61 (Beam) = 65psec 110 psec  = 1.23 ±0.09 % FWHM=4.8 % 5% 1% Energy Resolution (  ) [%] Energy resolution vs. Energy Energy 55MeV Timing distribution

23 Beam Test Setup H 2 target+degrader beam LP NaI LYSO Eff ~14% S1 Eff(S1xLP)~88%

24 MEG LXe Detector Status Refrigerator Xenon storage 1000l liquid xenon storage tank purifier

25 MEG LXe Detector Status  Cryostat Construction is in progress… outer side top inner

26 Summary  LXe scintillation detector R&D for MEG is successfully conducted –PMT for use in liquid xenon –Pulse tube refrigerator –Purification system  Detector performance is proved to be good enough for the experiment by using prototype detectors  Detector construction is in progress and will be ready soon