Liquid Xenon Carlorimetry at the MEG Experiment

1. 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; m e g • Clear evidence of new physics beyond SM • SUSY-GUT, SUSY-Seesaw Br ~< 10-11 • Present limit 1.2x10-11 by MEGA • Engineering run starts in 2006 and full DAQ will start in 2007 at Paul Scherrer Insitut.

4. MEG Detector 52.8MeV 52.8MeV • e+ measured by COBRA spectrometer • g by LXe detector

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. LXe and Scintillation 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] 0.1 0.082 Gas Excitation Triple point Recombination 161 165 Temperature [K]

7. g 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

8. Operation Procedure • Evacuation • TMP + Cryopump • 10-4~5 Pa • Pre-cooling • 2.0 bar xenon gas at room temp • Refrigerator/LN2 cooling • Liquefaction • Continuously supply Xe gas • Pressure control • Refrigerator/LN2 cooling • Purification • Circulation/Purification • Ready

9. 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

10. 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

11. PMT Development Summary

12. PMT Base Circuit • 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 Reference PMT = no Zener PMT with Zener

13. 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

14. 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 lRay~30-45cm

15. 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 a events

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

17. 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 Liquid-phase Purification cont’d

18. Wire (50-100 mm f) Alpha 40 μm 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 SORAD/ISOTOPE PRODUCTS

19. q Eg Eg p0 54.9MeV 82.9MeV 1.3MeV for q>170o 0.3MeV for q>175o 20 cm 3 cm Eg Further Calibration Methods • p0 decay g’s through CEX process • p-+p  p0+n • 55MeV, 83MeVg • g emission from thermal neutron capture on Ni nuclei • 9MeV • 37Li(p,g)48Be • E p = 440 keV, 14 keV, peak = 5 mb • 17.6MeV g • obtainable :  106/s (isotropic) at 440 KeV resonance (Ip 50 A) 9 MeV Nickelγ-line NaI Polyethylene 0.25 cm Nickel plate

20. Detector Performance Energy distribution @ 55MeV 5% 1% • = 1.23 ±0.09 % FWHM=4.8 % Energy resolution vs. Energy Energy Resolution (s) [%] Timing distribution 110 psec 110 - 64 (LYSO) - 61 (Beam) = 65psec

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

22. MEG LXe Detector Status • Cryostat Construction is in progress… top outer side inner

23. 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