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 (: 2.6 MeV, : 3.3 MeV) large phase space factor Low Natural abundance 0.187% large detector Enrichment CANDLES system CANDLES system Undoped CaF 2 (CaF 2 (pure)) Long Attenuation Length Peak Emission at UV Region (280nm) Wave Length Shifter Liquid Scintillator (LS) 4 active shield Passive shield Wavelength shifter for CaF 2 Photomultiplier large photo-coverage Active Shielding Technique Short Gate 200nsec Long Gate 4sec CaF 2 Liq.Scintil. Ratio=Short / Long Liq CaF 2 Clear Discrimination ! Different Time Constants CaF 2 (pure) : ~1sec Liquid Scintillator : a few 10 nsec CaF 2 Liq 208 Tl 40 K Project to Y axis Expected Background in CANDLES External Background Strongly Suppressed Because of Large Q of 48 Ca(4.27MeV) 4 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 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 :1sec rejected by FADC 4sec Gate 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) Time(nsec) Prompt 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 CANDLES III (prototype) 2007/03/25 2.8 m h 2.6 m Top View Side View PMTs Photomultiplier Tube (13inch) Inside View CaF 2 module half filled filled Index (CaF 2 ) Index (Mineral Oil) CaF 2 + conversion phase + acrylic case Peak Emission at UV Region (280nm) Wave Length Shifter Liquid Scintillator Tank 59 CaF 2 modules installed 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 Position Reconstruction i ADC(i) x PMT(i) i ADC(i) R = Preliminary analysis Position Reconstruction Parameter 0cm 20cm =~ 5cm (position dep.) Enough to Pos. Resolution to reduction 208 Tl Events Know Where prompt occur Top View 2MeV 208 Tl Event 212 Bi 36% Prompt -ray Delayed -ray 208 Tl T 1/2 = 3.05min Q b = 5.0MeV CaF 2 Discrimination (Top View) Dirty Crystal No Crystal (59 crystals installed) 1st(Top)2nd 3rd4th5th(Bottom) x Y CaF 2 Spectra CaF 2 No 214 Bi(U) 220 Rn(Th) (Dirty Crystal) 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 2 region Ratio 2 region 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. 42Bq/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 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 Eventsunder 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 Mile Stone (Prototype )CANDLES III (at Sea level) CaF 2 (10cm 3 ) 191 kg CANDLES III (U.G.) Kamioka underground lab. (summer 2008) CaF 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) Enrichment of 48 Ca Cosmic Ray Spectrum::Ratio (Simulation) Experiment Simulation Ratio Upper Thre. CaF2 Ratio Region Liq.Scinti. Sequential Pulse Rejection98.8% (by FADC ) PSD efficiency>99% at 4MeV (Energy dep.) Cosmic Ray::Spectrum(simulation) Spectrum : BESS KEK in Japan Angular Distribution : cos2 Flux : /cm 2 /sec [MeV] Liq. Cut Liq No Cut Cut eff. ~99% Candidates NucleusQ-valueReaction 20 F 7.0MeV 19 F(n,) 20 F 16 N 10.4MeV 19 F(n,) 16 N 19 O 4.6MeV 18 O(n, ) 19 O, (n,n) CaF 2 Spectra CaF 2 No 214 Bi(U) 220 Rn(Th) 606528 (Dirty Crystal) 14 Contami. [mBq/kg] 40 K(1.4MeV) Compton(1.24MeV) 208 Tl(2.6MeV) Compton(2.38MeV) CaF2No. 14, 46, 60(Dirty Crystal) Calibration and Resolution Position Region 2 Position Region 0.5 Compton edge decrease One Photo-electron = 10ch 1461keV = 4930ch 493p.e 10% 40 K) 6%(Q-value) Peak Channel 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 (145 Bq/kg ;Best) Th-chain( 220 Rn) ~29 Bq/kg 1/3 of Previous Crystals ( 61 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) 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 and () Events Background Rejection Efficiency > 99.7%