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Study of 48 Ca double beta decay with CANDLES

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 48 Ca double beta decay with CANDLES

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  1. Study of 48Ca double beta decay with CANDLES OGAWA Izumi (小川　泉) Osaka Univ.(大阪大学) INPC2007

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. CANDLES III • CANDLES III (U.G.) ‘Sanchika’ • Kamioka • CANDLES III (prototype) • @Osaka University • R&D INPC2007

  12. 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

  13. 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

  14. CANDLES III (prototype) h2.6 m f 2.8 m INPC2007

  15. PMTs • Inside view Photomultiplier Tube (13inch) INPC2007

  16. CaF2 module • CaF2 + conversion phase + acrylic case filled half filled Index 1.44@586nm (CaF2) Index 1.46@586nm (Mineral Oil) INPC2007

  17. LS tank • 58 CaF2 modules installed INPC2007

  18. 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

  19. 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

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