1. Study of Double Beta Decay of 48 Ca with CANDLES Y.Hirano, T.Kishimoto, I.Ogawa, R.Hazama, S.Umehara, K.Matsuoka, G.Ito, Y.Tsubota, for the CANDLES Collaboration

2. Double Beta Decay of 48 Ca ■Largest Q value (4.27 MeV) next largest; 150 Nd (3.3 MeV) almost background free (γ: 2.6 MeV, β: 3.3 MeV) large phase space factor ■ Low Natural abundance → 0.187% large detector Enrichment →CANDLES system

3. CANDLES system Undoped CaF 2 (CaF 2 (pure)) Long Attenuation Length (>10m@350nm) Peak Emission at UV Region (280nm) → Wave Length Shifter Liquid Scintillator (LS) 4π active shield Passive shield Wavelength shifter for CaF 2 Photomultiplier large photo-coverage

4. Active Shielding Technique Short Gate （ 200nsec ） Long Gate （ 4μsec ） CaF 2 Liq.Scintil. Ratio=Short / Long Liq ＋ CaF 2 Clear Discrimination ! Different Time Constants CaF 2 (pure) : ~1μsec Liquid Scintillator : a few 10 nsec CaF 2 Liq 208 Tl 40 K Project to Y axis

5. Expected Background in CANDLES ■External Background Strongly Suppressed Because of Large Q ββ of 48 Ca(4.27MeV) 4π Active Shielding System ■Remaining Background 2  Decay Event Improve Energy Resolution Natural Radioactivities in CaF 2 (pure) Crystal Improve Purity of Crystal Rejection by Offline Analyses

6. Backgrounds from Natural Radioactivities in Crystals Th-Chain U-Chain  Sequential Pulse 212 Bi and 208 Tl(T 1/2 =3min)... rejected by Space-Time Correlation Cut CaF2(pure) Decay Constant :1μsec rejected by FADC 4μsec Gate

7. Rejection of Sequential Pulse Typical Pulse Shape (500 MHz FADC) recorded shape fitted shape 99% of sequential pulse events will be rejected …under test Delayed 890ns 68ns PulseHeight(ch) 0 1000 2000 3000 4000 5000 Time(nsec) 0 1000 2000 3000 4000 5000 0 100 200 300 400 500 Prompt

8. CANDLES III ■Constructed at Osaka Univ. (sea level) small version for R&D check the performance of CANDLES ■ CaF 2 modules 10 3 cm 3 × 60 crystal; 191 kg with conversion phase ■ Liquid scintillator f 1m× h 1m acrylic container ■ H 2 O Buffer : passive shield f 2.8m× h 2.6m ■ PMTs 15” PMT (× 8) 13” PMT (×32) Total 40PMTs 33.4% photo-coverage

9. CANDLES III (prototype) 2007/03/25 日本物理学会春の大会＠首都大学東京  2.8 m h  2.6 m Top View Side View

10. PMTs Photomultiplier Tube (13inch) Inside View

11. CaF 2 module half filled filled Index 1.44@586nm (CaF 2 ) Index 1.46@586nm (Mineral Oil) CaF 2 + conversion phase + acrylic case Peak Emission at UV Region (280nm) → Wave Length Shifter

12. Liquid Scintillator Tank 59 CaF 2 modules installed

13. Experimental Condition ■Measurement at Osaka Univ. (sea level) ■CaF 2 modules 10 3 cm 3 x 59 crystal; 189kg 1dirty crystal : U:65 mBq/kg, Th:28 mBq/kg ■PMTs 15” PMT( x 6) 13” PMT( x 29)Total 35 :~30% photo-coverage ■DAQ ADC (40ch) x 2 (Long Gate and Short Gate) 1 module FADC under test ■Live Time 30 Hours

14. Position Reconstruction ∑ i ADC(i) x PMT(i) ∑ i ADC(i) R = Preliminary analysis Position Reconstruction Parameter １ 0cm 20cm σ=~ 5cm (position dep.) Enough to Pos. Resolution to reduction 208 Tl Events Know Where prompt α occur Top View ＞ 2MeV 208 Tl Event 212 Bi 36% Prompt α-ray Delayed β-ray 208 Tl T 1/2 = 3.05min Q b = 5.0MeV 

15. CaF 2 Discrimination (Top View) Dirty Crystal No Crystal (59 crystals installed) 1st(Top)2nd 3rd4th5th(Bottom) x Y

16. CaF 2 Spectra CaF 2 No 214 Bi(U) 220 Rn(Th) ― 606528 (Dirty Crystal) ― 590.0490.149 ― 33 0.0340.020 Contami. [mBq/kg] 40 K(1.4MeV) Compton(1.24MeV) 208 Tl(2.6MeV) Compton(2.38MeV) Cosmic Ray Events?? CaF2No. 33, 59, 60(Dirty Crystal) 1st2nd3rd4th5th No33 No59No60 Q-Value Region(4MeV~4.5MeV) Sequential Pulse Position 2σ region Ratio 2σ region

17. Background of Q-Value Region No59No60 Bottom CaF 2 No.60 － No.59 Spectrum Contribution only from contaminations inside crystal Sequential Pulse 4.0~4.5MeV Contaminations in crystals CaF 2 No.60 (dirty crystal: av. 46mBq/kg) 8.6 Events/Hour Other Clean Crystals: (av. 42μBq/kg) → 8x10 -3 Events/Hour/ Expected Sequential Pulse Rejection efficiency 99% (by FADC ) Other Background (Cosmic Rays ?) Several Events/Hour (e.g. CaF2No.59: 3events) → move to under ground lab. negligible

18. Summary ■ CANDLES III under the performance check background measurement at surface lab. Several Event Remained in Q-value region ■ Level of Contaminations (U,Th) sufficiently low for Under ground Measurement We expect further reduction by PSD using FADC : ■ Other Background (Cosmic Ray Events…under investigation) Move to Under grand lab. ■ Improvement of trigger system Go.Ito Poster session. TAUP2007 ■ Improvement of energy resolution (install Light Guide ) Y.Tsubota Poster session TAUP2007

19. Mile Stone (Prototype )CANDLES III (at Sea level) – CaF 2 (10cm 3 ) 191 kg CANDLES III (U.G.) –Kamioka underground lab. (summer 2008) –CaF 2 300 kg ~30  Bq/kg for ~0.5 eV CANDLES IV –Several tons; –~3  Bq/kg for ~0.1 eV in 6 yrs CANDLES V –100 t; Kamland or ??? for ~30 meV in 7 yrs –another option; enrichment ( under investigation)

20. Enrichment of 48 Ca

21. Cosmic Ray Spectrum::Ratio (Simulation) Experiment Simulation Ratio Upper Thre. CaF2 Ratio Region Liq.Scinti. Sequential Pulse Rejection×98.8% (by FADC ) PSD efficiency>99% at 4MeV (Energy dep.)

22. Cosmic Ray::Spectrum(simulation) Spectrum : BESS Experiment @ KEK in Japan Angular Distribution : cos2θ Flux : 0.026 /cm 2 /sec [MeV] ―Liq. Cut ―Liq No Cut Cut eff. ~99% Candidates NucleusQ-valueReaction 20 F 7.0MeV 19 F(n,γ) 20 F 16 N 10.4MeV 19 F(n,α) 16 N 19 O 4.6MeV 18 O(n, γ) 19 O, (n,n)

23. CaF 2 Spectra CaF 2 No 214 Bi(U) 220 Rn(Th) 60―6528 (Dirty Crystal) 14―0.0260.013 46―0.0510.356 Contami. [mBq/kg] 40 K(1.4MeV) Compton(1.24MeV) 208 Tl(2.6MeV) Compton(2.38MeV) CaF2No. 14, 46, 60(Dirty Crystal)

24. Calibration and Resolution Position Region 2σ Position Region 0.5σ Compton edge decrease One Photo-electron = 10ch 1461keV = 4930ch → 493p.e → 10% (@1461 40 K) → 6%(Q-value) Peak Channel

25. Development of High Purity CaF 2 Crystals Radioactivities in CaF 2 (pure) Crystal (  -ray measurement) Radioactivities in CaF 2 Powder (HPGe measurement) U-chain( 214 Bi) ~36  Bq/kg … 1/30 of Previous Crystals (14±5  Bq/kg ;Best) Th-chain( 220 Rn) ~29  Bq/kg … 1/3 of Previous Crystals ( 6±1  Bq/kg ;Best) CaF 2 Powder Fused CaF 2 Raw Materials CaCO 3, HF CaF 2 Crystal Powder selection Crystal growing CaF 2 (Eu) in ELEGANT VI U-chain( 214 Bi) : 1100  Bq/kg Th-chain( 220 Rn) : 98  Bq/kg 101 crystals U and Th (ICP-MS)

26. Pulse Shape Discrimination PSD (Event by Event Analysis)PSD (Event by Event Analysis) –FADC (100MHz) –Fit with Two Exponential Function (fixed time constants ) –Ratio ; A fast /A slow (Intensity of Fast and Slow Component) between  and  rays Clear Discrimination between αand γ(β) Events Background Rejection Efficiency > 99.7%