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Study of Double Beta Decay of 48 Ca with CANDLES

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. Double Beta Decay of 48 Ca. ■Largest Q value (4.27 MeV) next largest; 150 Nd (3.3 MeV) almost background free

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Study of Double Beta Decay of 48 Ca with CANDLES

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  1. Study of Double Beta Decay of 48Ca 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 48Ca ■Largest Q value (4.27 MeV) next largest; 150Nd (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. Liquid Scintillator (Veto Counter) CaF2(Pure) Buffer Oil Large PMT CANDLES system Undoped CaF2 (CaF2(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 CaF2 Photomultiplier large photo-coverage

  4. Active Shielding Technique Different Time Constants CaF2(pure) : ~1μsec Liquid Scintillator : a few 10 nsec Clear Discrimination ! CaF2 Liq 40K Ratio=Short / Long Liq.Scintil. 208Tl Project to Y axis Liq＋CaF2 CaF2 Short Gate（200nsec ） Long Gate（4μsec）　

  5. Expected Background in CANDLES ■External Background Strongly Suppressed Because of Large Qββ of 48Ca(4.27MeV) 4π Active Shielding System ■Remaining Background 2nbb Decay Event Improve Energy Resolution Natural Radioactivities in CaF2(pure) Crystal Improve Purity of CrystalRejection by Offline Analyses

  6. b+a Backgrounds from Natural Radioactivitiesin Crystals U-Chain Sequential Pulse 4μsec Gate CaF2(pure) Decay Constant :1μsec rejected by FADC Th-Chain 212Bi and 208Tl(T1/2=3min) . . . rejected by Space-Time Correlation Cut

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

  8. CANDLES III ■Constructed at Osaka Univ. (sea level) small version for R&D check the performance of CANDLES ■ CaF2 modules 103 cm3 × 60 crystal; 191 kg with conversion phase ■ Liquid scintillator f1m×h1m acrylic container ■ H2O Buffer : passive shield f2.8m×h2.6m ■ PMTs 15” PMT (× 8) 13” PMT (×32) Total 40PMTs 33.4% photo-coverage

  9. CANDLES III (prototype) Top View Side View h2.6 m f 2.8 m 2007/03/25 日本物理学会春の大会＠首都大学東京

  10. PMTs Inside View Photomultiplier Tube (13inch)

  11. CaF2 module CaF2 + conversion phase + acrylic case Peak Emission at UV Region (280nm) → Wave Length Shifter half filled filled Index 1.44@586nm (CaF2) Index 1.46@586nm (Mineral Oil)

  12. Liquid Scintillator Tank 59 CaF2 modules installed

  13. Experimental Condition ■Measurement at Osaka Univ. (sea level) ■CaF2 modules 103 cm3 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. ∑i ADC(i) x PMT(i) R = ∑i ADC(i) Position Reconstruction Preliminary analysis Position Reconstruction Parameter Top View ＞2MeV 208Tl Event Qb = 5.0MeV a b 212Bi 208Tl T1/2 =3.05min 36% Prompt α-ray Delayed β-ray σ=~ 5cm (position dep.) Enough to Pos. Resolution to reduction 208Tl Events Know Where prompt α occur ＞2MeV 20cm １0cm

  15. CaF2 Discrimination (Top View) 1st(Top) 2nd No Crystal (59 crystals installed) Dirty Crystal Y 3rd 4th 5th(Bottom) x

  16. CaF2 Spectra CaF2No. 33, 59, 60(Dirty Crystal) 1st 2nd 3rd 4th 5th Contami. [mBq/kg] No33 No59 No60 CaF2No 214Bi(U) 220Rn(Th) ― 60 65 28 (Dirty Crystal) ― 59 0.049 0.149 ― 33 0.034 0.020 Sequential Pulse 40K(1.4MeV) Compton(1.24MeV) Cosmic Ray Events?? 208Tl(2.6MeV) Compton(2.38MeV) Position 2σ region Ratio 2σ region Q-Value Region(4MeV~4.5MeV)

  17. Background of Q-Value Region Bottom CaF2No.60 － No.59 Spectrum No59 No60 4.0~4.5MeV Contaminations in crystals CaF2No.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 Sequential Pulse Contribution only from contaminations inside crystal

  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) • CaF2 (10cm3) 191 kg • CANDLES III (U.G.) • Kamioka underground lab. (summer 2008) • CaF2 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 48Ca

  21. Cosmic Ray Spectrum::Ratio (Simulation) Ratio Simulation Experiment Upper Thre. Liq.Scinti. CaF2 Ratio Region CaF2 Ratio Region 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 /cm2/sec ―Liq. Cut ―Liq No Cut [MeV] Cut eff. ~99% Candidates Nucleus Q-value Reaction 20F7.0MeV 19F(n,γ)20F 16N10.4MeV 19F(n,α)16N 19O4.6MeV 18O(n, γ)19O, (n,n) [MeV]

  23. CaF2 Spectra CaF2No. 14, 46, 60(Dirty Crystal) Contami. [mBq/kg] CaF2No 214Bi(U) 220Rn(Th) 60 ― 65 28 (Dirty Crystal) 14 ― 0.026 0.013 46 ― 0.051 0.356 40K(1.4MeV) Compton(1.24MeV) 208Tl(2.6MeV) Compton(2.38MeV)

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

  25. Development of High Purity CaF2 Crystals • CaF2(Eu) in ELEGANT VI • U-chain(214Bi) :1100 mBq/kg • Th-chain(220Rn) : 98 mBq/kg U and Th (ICP-MS) CaF2 Powder Fused CaF2 CaF2 Crystal Raw Materials CaCO3, HF Radioactivities in CaF2 Powder (HPGe measurement) Radioactivities in CaF2(pure) Crystal (a-ray measurement) Powder selection Crystal growing 101 crystals • U-chain(214Bi) ~36 mBq/kg …1/30 of Previous Crystals (14±5 mBq/kg ;Best) • Th-chain(220Rn) ~29 mBq/kg …1/3 of Previous Crystals ( 6±1 mBq/kg ;Best)

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

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