Liquid Xenon Carlorimetry at the MEG Experiment Satoshi

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

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

MEG Experiment p Search for Lepton-Flavor violating muon rare decay; e – Clear evidence of new physics beyond SM p SUSY-GUT, SUSY-Seesaw Br ~< 10 -11 – Present limit 1. 2 x 10 -11 by MEGA p Engineering run starts in 2006 and full DAQ will start in 2007 at Paul Scherrer Insitut. 3

MEG Detector 52. 8 Me. V p e+ measured by COBRA spectrometer p by LXe detector 4

LXe Detector R&D history p Small Prototype p Large Prototype – 2. 3 liter active volume – 70 liter active volume p Final Detector – 800 liter active volume 5

Density 3. 0 g/cm 3 p Triple point 161 K, 0. 082 MPa p Normal operation at p p p – T~167 K P~0. 12 MPa – Stable and reliable temperature control is necessary Narrow temperature range between liquid and solid phases Scintillation light emission mechanism Pressure [MPa] LXe and Scintillation light Liquid Solid 0. 1 0. 082 Excitation Recombination Triple point 161 165 Gas Temperature [K] 7

MEG LXe Detector p p p Active volume ~800 l is surrounded PMTs on all faces ~850 PMTs in the liquid No segmentation Energy – All PMT outputs – PMTs on the inner face – Averaging of signal arrival time of selected PMTs Position Timing 8

Operation Procedure 1. Evacuation TMP + Cryopump p 10 -4~5 Pa p 2. Pre-cooling 2. 0 bar xenon gas at room temp p Refrigerator/LN 2 cooling p 3. Liquefaction Continuously supply Xe gas p Pressure control p Refrigerator/LN 2 cooling p 4. Purification p 5. Circulation/Purification Ready 9

Detector Components p Photomultiplier – Operational in liquid xenon, Compact – UV light sensitive p Refrigerator – Stable temperature control – Sufficient power to liquefy xenon – Low noise, maintenance free p Xenon Purifier – Purification during detector operation 10

Photomultiplier R&D p Photocathode – Bialkali Ichige et al. NIM A 327(1993)144 : K-Cs-Sb, Rb-Cs-Sb p Rb-Cs-Sb has less steep increase of sheet resistance at low temperature p K-Cs-Sb has better sensitivity than Rb-Cs – -Sb Multialkali : +Na p Sheet resistance of Multialkali dose not p p change so much. Difficult to make the photocathod, noisy Dynode Structure – – Compact Possible to be used in magnetic field up to 100 G p Metal channel Uniformity is not excellent 11

PMT Development Summary 1 st generation R 6041 Q 2 nd generation R 9288 TB 3 rd generation R 9869 228 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 10 -20% with a time constant of order of 10 min. 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 12

PMT Base Circuit p Necessary to reduce heat load from the circuit – – – Heat load in the cryostat ↔ Refrigerator cooling power ~190 W Reduce base current p 800 V 55 micro. A 44 m. W/PMT p 40 -50 W heat load from 850 PMTs Zener diodes at last 2 stages for high rate background p Zener diode is very noisy at low temperature filtering on the base Reference PMT = no Zener PMT with Zener 13

Refrigerator R&D MEG 1 st spin-off p Technology transferred to a manufacturer, Iwatani Co. Ltd p Performance obtained at Iwatani p – – – 189 W @165 K 6. 7 k. W compressor 4 Hz operation 15

Purification System Usually water can be removed by heating the cryostat during evacuation. p MEG liq. Xenon detector cannot be heated because of the PMTs inside. p 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. p Circulation/Purification after filling with fluid is necessary. p Rayleigh scattering l. Ray~30 -45 cm 16

Gas-phase Purification Xenon extracted from the chamber is purified by passing through the getter. p Purified xenon is returned to the chamber and liquefied again. p Circulation speed 56 cc/minute p Cosmic-ray events a events 17

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

Liquid-phase Purification cont’d p 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 19

Calibration p LED flashed in the liquid p Alpha source on wires – PMT gain calibration – 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 Wire (50 -100 m f) Alpha 40 μm 20

Further Calibration Methods 54. 9 Me. V p – – 55 Me. V, 83 Me. V p 1. 3 Me. V for q>170 o p 0 decay ’s through CEX process p-+p p 0+n emission from thermal neutron capture on Ni nuclei 82. 9 Me. V 0. 3 Me. V for q>175 o q E E p 0 9 Me. V Nickel γ-line E – 9 Me. V p 3 7 Li(p, ) 8 Be 4 – E p = 440 ke. V, 14 ke. V, peak = 5 mb – 17. 6 Me. V – obtainable : 106 /s (isotropic) at 440 Ke. V resonance (Ip 50 A) Na. I Polyethylene 0. 25 cm Nickel plate 3 cm 20 cm 21

Detector Performance = 1. 23 ± 0. 09 % FWHM=4. 8 % Timing distribution 110 psec Energy Resolution ( ) [%] Energy distribution @ 55 Me. V 5% Energy resolution vs. Energy 1% 110 - 64 (LYSO) - 61 (Beam) = 65 psec 22

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

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

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