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Study of 48 Ca double beta decay with CANDLES. OGAWA Izumi ( 小川 泉 ) Osaka Univ. ( 大阪大学 ). Double beta decay of 48 Ca. Largest Q value (4.27 MeV) next largest; 150 Nd (3.3 MeV) large phase space factor almost background free ( g : 2.6 MeV, b : 3.3 MeV)
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Study of 48Ca double beta decay with CANDLES OGAWA Izumi (小川 泉) Osaka Univ.(大阪大学) INPC2007
Double beta decay of 48Ca • Largest Q value (4.27 MeV) • next largest; 150Nd (3.3 MeV) • large phase space factor • almost background free (g: 2.6 MeV, b: 3.3 MeV) • Low Natural abundance → 0.187% • large detector • enrichment • Next generation detector • Mdet2if background free • Mdet4 if background limited INPC2007
Liquid Scintillator (Veto Counter) CaF2(Pure) Buffer Oil Large PMT CANDLES • undoped CaF2 (CaF2(pure)) • 48Ca (Qbb=4.27 MeV) • 300 kg 3 t 100 t • Liquid Scintillator (LS) • 4p active shield • Passive shield • wavelength shifter for CaF2 • Photomultiplier • large photo-caoverage CAlcium fluoride for studies of Neutrino and Dark matters by Low Energy Spectrometer INPC2007
CANDLES I - POP (Proof of Principle) Detector • CaF2(pure) crystal in liquid scintillator (with w.l. shifter) viewed by 4 PMTs (5 inch) • LS : mineral oil + DPO (3 g/l) + Bis-MSB (0.3 g/l) 4p active shield INPC2007
Performance Test (4p active shield) 2 ADCs with different gate width 40K events from LS events from CaF2 Clear separation between CaF2 and LS INPC2007
Energy resolution 137Cs (662 keV) 9.14%(FWHM) Standard g Source CaF2(pure) Counts Side View wavelength shifter Energy (keV) 60Co (1333 keV) CANDLES I with 10 cm3 CaF2(pure) • Energy Resolution: • 9.1 %(FWHM) at 662keV • 3.4 % (FWHM)at 4.27MeV • Req. for CANDLES III ; 4.0 % 5.91%(FWHM) Energy (keV) INPC2007
Radioactive BG inside CaF2 • Natural Radioactive BGs ~Qbb • Maximum energy • g~ 2.6 MeV, b~3.3 MeV, a(max)~2.7 MeVe.e. (quench;fa~ 0.3) • Successive decays of a, b, gin decay chain • ~1 msec decay time CaF2 Pulse shape Emax=5.8MeV(U) 5.3MeV(Th) b +a INPC2007
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) INPC2007
Rejection of Double Pulse Typical Pulse Shape (500 MHz FADC) pulse height (ch) pulse height (ch) 8.8ns 810ns Delayed Prompt time(2 nsec) time(2 nsec) recorded shape fitted shape 99% of double pulse events will be rejected INPC2007
Pulse Shape Discrimination • Pulse Shape discrimination • Shape Indicator (PRC 67(2003) 014310) Difference in decay shape betweenaandgrays a-event g-event 1s mean value: no energy dependence (>1 MeV) ~99.7% reduction at Q-value INPC2007
CANDLES III • CANDLES III (U.G.) ‘Sanchika’ • Kamioka • CANDLES III (prototype) • @Osaka University • R&D INPC2007
CaF2(pure) 103 cm3 × 96 crystals; 305 kg Liquid scintillator two phase system Purification system H2O Buffer passive shield PMTs 17” PMT (×14) : R7250 13” PMT (×56) : R8055 mirror type reflector photon trans. simulation energy res. ~3.5 % @ Qbb Kamioka underground lab. CANDLES III(U.G.) INPC2007
CANDLES III (prototype) • 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 • f1000×h1000 acrylic container • H2O Buffer : passive shield • f2800×h2600 • PMTs • 15” PMT (× 8) : R2018 • 13” PMT (×32) : R8055 1 dirty crystal U:65 mBq/kg, Th:28 mBq/kg 33.4% photo-coverage INPC2007
CANDLES III (prototype) h2.6 m f 2.8 m INPC2007
PMTs • Inside view Photomultiplier Tube (13inch) INPC2007
CaF2 module • CaF2 + conversion phase + acrylic case filled half filled Index 1.44@586nm (CaF2) Index 1.46@586nm (Mineral Oil) INPC2007
LS tank • 58 CaF2 modules installed INPC2007
Study of 48Ca double beta decay Largest Q value CANDLES system (CaF2 in L.S.) CANDLES III (prototype) 58 CaF2 modules installed R&D using prototype detector at Osaka University. CANDLES III (U.G.) Design study expected BG: 0.14 events/year Construction at Kamioka will start in spring 2008 Conclusion INPC2007
Mile stone • CANDLES I, II, POP • CANDLES III (prototype) • CaF2 (10cm3) 191 kg • CANDLES III (U.G.) • Kamioka underground lab. (spring 2008) • CaF2 300 kg ~30 Bq/kg for ~0.5 eV • CANDLES IV • 3 t; Kamioka • ~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 INPC2007