Double Beta Decay of 48Ca with CaF2(Eu) - ELEGANT VI - Saori Umehara umehara@km.phys.sci.osaka-u.ac.jp ELEGANT VI Collaboration Department of Physics, Osaka University T. Kishimoto, I. Ogawa, R. Hazama, H. Miyawaki, S. Yoshida, K. Mukaida, K. Matsuoka, Y. Ishikawa, K. Kishimoto, A. Katsuki, H. Sakai, D. Yokoyama, S. Tomii, Y. Tatewaki, T. Kobayashi, A. Yanagisawa

Double Beta Decay of 48Ca with CaF2(Eu) Outline Double Beta Decay of 48Ca with CaF2(Eu) ELEGANT VI System Detector Design (4p active shield) Backgrounds Background Origins Performance of Background Rejection by FADC Double Beta Decay Measurement Analyses for Event Selection Half Life of 0nbb Summary 14th Sep 2007, Saori Umehara

ELEctron Gamma-ray Neutrino Telescope ELEGANT VI ELEGANT VI ELEctron Gamma-ray Neutrino Telescope Schematic Drawing of ELEGANT VI CaF2(Eu) CaF2 Scintillator (CaF2(Eu)) 23 Crystals(45×45×45cm3:290g) Source of bb Decay : 48Ca (Qbb=4.27MeV) veto counters 46 CaF2(pure) 38 CsI(Tl) →　4p Active Shield Passive Shields for g-ray Cu : 5cm,Pb : 10cm for Neutron LiH+Paraffin :15mm Cd sheet : 0.6mm H3BO3 loaded water CaF2(pure) CsI(Tl) Signal readout from same PMTs LiH Cu Air-tight Box Pb 14th Sep 2007, Saori Umehara

ELEGANT VI Scintillators in ELEGANT VI System CaF2 Module PMT PMT CaF2(pure) . . . UV Light CaF2(Eu) . . . Visible Light CaF2 Module CsI(Tl) Active Shielding Technique in ELEGANT VI system 14th Sep 2007, Saori Umehara

Active Shielding Technique Discrimination between CaF2(Eu) and CaF2(pure) Light Transmission from each Scintillator Visible Light Light from CaF2(Eu) PMT PMT Left PMT VL CaF2(pure) CaF2(Eu) CaF2(pure) Right PMT VR Light from CaF2(pure) Absorption with CaF2(Eu) PMT PMT UV Light Distribution of Roll-off Ratio CaF2(Eu): not transparent to UV light 2000 1500 1000 500 CaF2(Eu) Energy(keV) VR-VL Roll-off Ratio = VR+VL CaF2(pure) CaF2(pure) VL = Charge from Left PMT VR = Charge from Right PMT -1.0 -0.5 0. 0.5 1.0 Roll-off Ratio Clear Discrimination between CaF2(Eu) and CaF2(pure) . . .well act as veto counter 14th Sep 2007, Saori Umehara

Oto Cosmo Observatory ELEGANT VI bb Decay of 48Ca Dark Matter Search with CaF2(Eu) Oto Cosmo Observatory Cosmic Ray 10-2/m2/sec (1/10000 of sea level lab.) Neutron 10-5 /cm2/sec Rn 10 Bq/m3 Japan Oto Cosmo Observatory(1400m.w.e.) EL845m 500m EL435m PICO-LON ELE-VI EL323m 29/1000 14th Sep 2007, Saori Umehara 5036m

Backgrounds Background Origins in ELEGANT VI System External Backgrounds . . . strongly limited by 4p active shield Internal Backgrounds . . . within CaF2(Eu) crystals Background Process U-Chain b a 214Po 238U 214Bi 210Pb 206Pb stable Qb = 3.27MeV Qa = 7.83MeV T1/2 = 164msec Th-Chain b a 212Po 232Th 212Bi 208Pb stable 64% Qb = 2.25MeV Qa = 8.95MeV T1/2 = 0.299msec Pile-up Events b+a because . . . CaF2(Eu) decay constant : ~1ms Emax = 4.7MeV for U-chain 4.3MeV for Th-chain (Qbb = 4.27MeV) Pile-up Events . . . Characteristic Background Origins for ELEGANT VI System 14th Sep 2007, Saori Umehara

Previous Result and Background Study First Result from ELEGANT VI From 1998 to 1999 Simulated Background Energy Spectra Measured Energy Spectrum 100 Experimental data total Qbb of 48Ca Counts 10 212Bi BG (sim) Pile-up Event 1 0nbb window 214Bi 10-1 2nbb (sim) 208Tl 10-2 3000 4000 5000 Energy(keV) Pile-up Events Observed Events : 0 Expected BG : 1.3 Half-Life : >1.4 × 1022 year (90% C.L.) Ref: I. Ogawa et al., NPA730(2004)215 Main Background Origins 94% of expected BG at Qbb region Rejection by FADC 14th Sep 2007, Saori Umehara

Rejection of Pile-up Events T1/2 = 0.299msec Th-Chain decay constant of CaF2(Eu) ：~1msec b a 232Th T1/2 = 1.1 x 1010year 212Bi Qb=2.2MeV 212Po Qa=7.8MeV 64% Prompt Delayed Typical Pulse Shape(100MHz FADC) with long time difference with short time difference 2ms Clearly identified as Pile-up events 60ns Delayed Clearly identified as Pile-up events Prompt Background Rejection by 100MHz FADC 　　DT > 30ns(3ch) 14th Sep 2007, Saori Umehara

Data Analysis for bb Study Run Summary for bb Measurement Date Analysis Live Time kg・day Jun1998-Nov1999 without FADC 233 day 1553 kg・day Jan2003-Dec2003 with FADC 183 day 1114 kg・day Jan2004-Sep2006 463 day 2280 kg・day FADC installed Trigger system improved for Dark Matter Search I. Ogawa et al., Nucl. Phys. A730(2004)215 This Talk . . . Data Analyses with FADC Data . . . from 2003 to 2006 Obtained Half-Life for 0nbb Decay of 48Ca Data . . . from 1998 to 2006 14th Sep 2007, Saori Umehara

Event Selection(1) Event Selection(1) . . . Base Analysis No hit with CsI(Tl) Hit with only one CaF2 No hit with CaF2(pure) . . . -0.3 < Roll-off Ratio < 0.3 Position Dependence of Roll-off Ratio Roll-off= +0.3 CaF2(Eu) Roll-off= -0.3 CaF2(pure) (bar = s) 14th Sep 2007, Saori Umehara

Event Selection(2) Pile-up Event Rejection by 100MHz FADC Total energy of pile-up event > 3MeV Time Difference Energy Spectrum of Delayed Events Energy Spectrum of Prompt Events Counts 212Bi Qb 272±10nsec Pile-up Event Consistent 214Bi Qb 212Po 214Po Time(nsec) Energy(keV) Energy(keV) 212Po:T1/2=299nsec 214Po:T1/2=164msec 212Po a:Eee=2.0MeV 214Po a:Eee=1.6MeV 212Bi b:Qb=2.2MeV 214Bi b:Qb=3.3MeV Pile-up Event Pile-up Event Rejection Th-Chain b a Effectively Rejected Rejection Efficiency : 94% for Th(T1/2=0.3msec) 99.9% for U(T1/2=164msec) 232Th 212Bi 212Po 208Pb stable 64% 14th Sep 2007, Saori Umehara

Energy Resolution Energy Resolution of a Peak #10 Measuring Live Time : 463.0 days Source : a rays from contamination in crystals itself (216Po a-ray 6.78MeV) 6.9 % FWHM Energy Resolution 6.9 - 10 % at 1.3 MeV with each CaF2 module →　Corresponding to 4-6 % at Qbb region 14th Sep 2007, Saori Umehara

Preliminary Preliminary Double Beta Decay Energy Spectrum after FADC installed Live Time: 3394 days・kg Energy Spectra after Event Selections (Multi Hit Cut) Energy Spectra after Event Selections (Multi Hit Cut +FADC Cut) Experimental Data Experimental Data 214Bi (Sim) 212Bi(Sim) 212Bi (Sim) Preliminary Preliminary Qbb of 48Ca 208Tl(Sim) 208Tl (Sim) No events in 0nbb Energy Window 14th Sep 2007, Saori Umehara

Double Beta Decay Half-Life for Neutrino-less bb Decay of 48Ca Run Summary Date Analysis Number of Event Expected BG (212Bi,214Bi,208Tl) Detection Efficiency Live Time kg・day Jun 1998 - without FADC 1.30 0.55 1553 Jan 2003 - with FADC 0.27 0.53 1114 Jan 2004 - 0.43 2280 No events in the energy region 0nbb Half-Life of 48Ca : >6 × 1022 year (90% C.L.) Preliminary Previous Result : >1.4 × 1022 year (90% C.L.) Ref: I. Ogawa et al., NPA730(2004)215 14th Sep 2007, Saori Umehara

Future Perspective (Next Project) ELEGANT VI → CANDLES No event . . . not limit detector sensitivity by small natural abundance 0.187% ELEGANT VI CaF2(Eu) CANDLES III CaF2(pure) Mass 6.7kg 300kg Radioactivities in Crystal(Average) 1.2mBq/kg(U) 0.11mBq/kg(Th) 0.039mBq/kg(U) 0.026mBq/kg(Th) Background Rejection Pile-up Event Rejection(94%) Pile-up Event Rejection(98.8%) Pulse Shape Discrimination between a and g rays(99.7%) Sensitivity for Neutrino Mass 0.5eV (~3 ×1024 year) Achieved ×45 1/30 for U 1/4 for Th BG Rej. =1/1000 Achieved with prototype detector CANDLES 14th Sep 2007, Saori Umehara Scale-up Detector from ELEGANT VI

Summary ELEGANT VI System Double Beta Decay Experiment CaF2(Eu) system for bb decay study 4p active shield by CaF2(pure) and CsI(Tl) scintillators Active shield technique CaF2(Eu) . . .not transparent to UV light Main backgrounds around Qbb-value region . . . Pile-up events FADC . . . Installed for background rejection Double Beta Decay Experiment at Oto Cosmo Observatory Live Time : 4947 kg・day No events at Qbb-value region Half-Life : 6 × 1022 year (Preliminary) 14th Sep 2007, Saori Umehara