1. 1 Study of 48 Ca Double Beta Decay by CANDLES T. Kishimoto Osaka Univ.

2. 2 Why 48 Ca Highest Q value (4.27 MeV) –next largest 150 Nd (3.3 MeV) –Large phase space factor –Little BG （ natural radioactivity  : 2.6 MeV,  : 3.3 MeV ） Natural abundance: 0.187% –Huge amount of CaF 2 crystals –Isotope separation: expensive (no Gas) –Early studies (recent studies use separated isotope) Next generation –M ~ T -1/2 ~ M -2 (no BG) ~ M -4 （ BG limited ） NPA 730 ’04, 215 48 Ca (no BG)

3. Double Beta Decay of 48 Ca Studied by ELEGANT VI 4104 - 4438 keV 0.63 y BG (sim) 2  (sim) 0  window NPA 730 ’04, 215 Not limited by backgrounds 1.76 y Yet no BG But only 6.4g of 48 Ca

4. 4 CANDLES CaF 2 (Pure) 200kg, 300kg, 3t,30t (2%) 48 Ca (200g, 300g, 3kg, 300kg) Liquid Scintillator Wave Length Shifter 4  Active Shield Passive shield Photomultiplier Photomultiplier energy resolution CaF 2 (Pure) Liquid Scintillator (Veto Counter) Buffer Oil Large PMT CAlcium fluoride for studies of Neutrino and Dark matrters by Low Energy Spectrometer

5. 21COE Sep 11, Osaka 5 CANDLES I Rejection of ext. BG Liquid Scintillator CaF 2 ADC(fast) ADC(total) Liquid Scintillator CaF 2 CANDELS I POP detector

6. Background @ Q value region No natural BG @ ~4 MeV –Maximum energy  ~ 2.6 MeV,  ~3.3 MeV,  ~2.5 MeV(quench~0.3) –Successive decay of  ~1  sec decay time 100MHz FADC  T > 30ns(3ch) ; ~5% 500MHz FADC (under preparation)  T > 5ns ; ~1% 212 Bi (  ) 212 Po (  ) Pulse shape

7. 7 Pulse Shape Discrimination PSD (Event by Event)PSD (Event by Event) –FADC (100MHz) –A fast /A slow (Fast and slow component ) Difference in decay time between  and  rays Discrimination between  and  Events Background Reduction ~ 0.3%

8. 8 Development of Low Radioactive CaF 2  delayed coincidence @ Oto Cosmo observatory U~36  Bq/kg 14 (best) Th~20  Bq/kg 6 (best) Number of CaF 2 crystals mBq/kg 95 crystals 3t 600k300k

9. 9 Energy resolution and BG rejection (2 phase) CaF 2 (Pure) CaF 2 : UV region (PMT) Conversion layer –UV→visible (PMT) –UV transparent Veto layer –Liq scint ： absorb UV CaF 2 Emission (~285nm) Conversion by WLS(350~400nm) PMT propagate 9.14%(FWHM) Energy (keV) Counts 137 Cs (662keV)

10. 21COE Sep 11, Osaka 10 CANDLES III (prototype) 10cm 3 x 56 CaF 2   00× h 2600

11. 11 CANDLES III Inside View Photomultiplier Tube(13inch) Tank for Liquid Scintillator (Acrylic Case) 40 PMTs Version Buffer water

12. Test measurement just started @ Osaka (sea level) 2007/03/25 日本物理学会春の大会＠首都大学東京 Crystal with huge BG 1st2nd 3rd 4th 5th no crystals

13. 13 Mile stone ELEGANTS VI –running with new BG rejection (2 ) CANDLES I, II CANDLES III –10cm 3 cube (100 crystals) ~0.5 eV –BG of CaF 2 ~30  Bq/kg (<100  Bq/kg ) CANDLES III(UG) CANDLES IV –10cm 3 cube (1000 crystals) 3.2t –BG of CaF 2 ~10  Bq/kg for 0.2 eV –Kamioka Achieved Kamioka

14. DUSEL town meeting Nov. 2-4, Washington DC 14 CANDLES XMASS SK KamLand New experimental Hall under excavation

15. 15 Future CANDLES V to sense ~10 meV region –~30 ton CaF 2 and 2% enrichment – Further large space Kamland, SNO (Vessel and PMT’s) DUSEL Isotope enrichment –Available: 76 Ge, 100 Mo, 128 Te –exception: 48 Ca, 150 Nd R&D in progress –Crown ether (Resin is made) Current separation constant is a bit low.