2004/Dec/12 Low Radioactivity in CANDLES T. Kishimoto Osaka Univ.

2004/Dec/12 Double  decay and CaF 2 crystals Why 48 Ca ELEGANTS VI and CaF 2 (Eu) –Roll off ratio –Radioactivity in CaF 2 (Eu) CANDLES and CaF 2 –External BG rejection by pulse shape –Internal BG rejection by pulse shape –Making low BG CaF 2 crystals

2004/Dec/12 Why 48 Ca (ELEGANTS VI) 48 Ca : Search for 0  decay –highest Q-value (4.27 MeV) largest phase volume least BG (  <2.6 MeV,  <3.3 MeV) Natural abundance 0.187% 19 F : Search for spin coupled Dark Matters Isotope 48 Ca 76 Ge 82 Se 100 Mo 116 Cd 136 Xe 150 Nd Q-value (MeV) G 0v × (year -1 eV -2 )

2004/Dec/12 ELEGANT VI ~3.5kg 19 F ~6.4 g 48 Ca 45mm 3 5x5=25 crystals

2004/Dec/12 The tunnel was constructed for railroad but never used. ELEGANT VI Oto Cosmo Observatory

2004/Dec/12 Roll-off ratio CaF 2 (pure) as light guide active shield against PMT CaF 2 (Eu) is not transparent for U.V. light PMT CaF 2 (Eu) n = 1.44 CaF 2 (pure) n = 1.47 Optical grease n = 1.47 Silicon oil n = 1.40 CaF 2 (Eu) CaF 2 (pure)

2004/Dec/12 Roll-off ratio PH(CaF 2 (Eu))= 3~4 times PH(CaF 2 )

2004/Dec/12 Limit for  decay of 48 Ca Obtained spectrum NPA730 (2004) kg yr cf. Expected BG : 1.3 events (simulation) 0 events in 0  energy window 2  (sim) BG (sim) 0  window BG in Q value region is successive Decay of   or 

2004/Dec/12 Radioactive Backgrounds

2004/Dec/12 U-series hardware (second) trigger –time window : 9 – 499  sec. Ac-series Th-series –time window : 0.05 – 1.0 (0.5) sec. Radioactive Contamination

2004/Dec/12 time window : sec

2004/Dec/12 Contamination inside crystals Average contamination ( #2 － #24 ) ( #2 － #24 ) U – series 1.11×10 -3 Bq/kg Ac – series 3.84×10 -4 Bq/kg Th – series 1.09×10 -4 Bq/kg 1.25x10 -5 = 1 ppt Th 0.1mBq=24.6ppt

2004/Dec/12 How to make Big Detector –CaF 2 (Eu): No (attenuation length ~13 cm) –CaF 2 : Yes (attenuation length > 1 m) CANDLES (CAlcium fluoride for studies of Neutrino and Dark matters by Low Energy Spectrometers) Double beta decay 0.1 eV and further DM annual modulation

2004/Dec/12 10 × 10 × 10 cm 3 CaF 2 crystals (1000 cubes) in liquid scintillator Vessel 3.2 t CaF 2 48 Ca 3.1 kg (natural) 30 kg (further big or enrich) Currently we are making CANDLES III 200g 48 Ca (60 crystals) 1. BG 2. Energy resolution

2004/Dec/12 External BG rejection by Pulse shape (CANDLES I) Signal from Liquid scinti. Signal from CaF 2 1  sec decay time

2004/Dec/12 Signal to Noise Separation Signal from Liquid Scint. Backgrounds Signal from CaF2 total Fast component

2004/Dec/12 Energy resolution (Light output) Bis-MSBDimethyl- POPOP POPOP C 24 H 22 C 26 H 20 N 2 O 2 C 24 H 16 N 2 O 2 absorpt ion 347 nm363 nm362 nm emissio n 426 nm428 nm418 nm Q.E Liquid Scintillator wave length shifter

2004/Dec/12 Two Phase System (1) CaF 2 (Pure) Concept of Method WLS Phase –M.O(100%)+PPO(0.3g/L) –5~10mm in thickness –Large conversion eff. Veto Phase –M.O.(80%)+P.C.(20%)+PPO(1.0g/L) –Large light output CaF 2 Emission (~280nm) Conversion by PPO(350~400nm) PMT propagate

2004/Dec/12 Performance Test (WLS phase) Side View 9.14%(FWHM) Energy (keV) Counts Energy (keV) 60 Co (1333keV) 5.91%(FWHM) 137 Cs (662keV) CaF 2 (pure) Liquid Scintillator Standard  Source CANDLES I with 10 cm cube CaF 2 (pure) (WLS phase)

2004/Dec/12 Light Collection – wavelength shifting – Scintillation light from CaF 2 (pure) –Low light emission yield ~1/3 of CaF 2 (Eu) –Peak emission at UV region Mineral oil base : Paraol250 Wavelength shifter

2004/Dec/12 Optimization of Liquid Scintillator (1) (A) Mineral Oil + wavelength shifter (P250) (PPO) –CaF 2 : –Liq. Scint. : Not enough (B) (A) + Pseudocumene –Liq. Scint. : –CaF 2 : Optimize P.C. and PPO concentration

2004/Dec/12 Optimization of Liquid Scintillator (2) Optimization of PPO concentration –Liq. Scint.:Max at M.O. (80 %) + P.C. (20 %) + PPO (1.0 g/l) –CaF 2 :Not recovered (absorbed by P.C.) Two Phase System (W.L.S. phase + Veto phase)

2004/Dec/12 CANDLES III Under Construction (Osaka Univ.) CaF 2 (pure) –10 3 cm 3 × 60 crystals total weight : 191 kg Liquid scintillator  1000× h 1000 acrylic container –H 2 O Bufferpassive shield   00× h 2800 –safety regulation Purification system PMTs –15 ” PMT (×19) : R2018 –13 ” PMT (×29) : R8055

2004/Dec/12 Further BG rejection in CaF 2 (Th chain) Th –Sum energy (Unique BG at the Q value region) Contamination in Crystals (~1/10) –Background study of materials used all steps in the crystal production Reduction by pulse shape measurement (~1/100) ~1/1000 reduction

2004/Dec/12 Background – reduction / rejection – Reduction of radioactive impurities inside crystal So Far… –Powder selection (3.2 kg x 11 crystals) U-chain ( 214 Bi)51  Bq/kg Th-chain ( 220 Rn)21  Bq/kg CaF 2 (Eu) 214 Bi1110  Bq/kg 220 Rn 98  Bq/kg Raw Materials CaF 2 powderFused CaF 2 CaF 2 crystal (CaCO 3, HF) Powder selection Rinse out the powder (H 2 O, HNO 3,…) Growing process Scavenger selection … R&D study in progress BG measurement at Oto Enough for CANDELS III ~30  Bq/kg Another one order for CANDLES IV and further

2004/Dec/12 Rejection by successive decay Flash ADC identifies successive decay No Reduction In ELEGANTS VI Currently 0.04 (20 nsec) Future 0.01 (5 nsec)

2004/Dec/12 Pulse Shape Discrimination Mean Pulse Shape of  pulse and  (  ) pulse (Flash ADC) –Fit Pulse Shape ; A fast ｘ exp(-t/  fast ) + A slow ｘ exp(-t/  slow ) Alpha Particles Gamma  A  A Fast Component111nsec nsec17.3 Slow Component894nsec nsec38.1 Intensity of Fast Component ; Different  Pulse  Pulse Fast Component Second Component (Slow Component) Detector ; CaF 2 (Pure)

2004/Dec/12 Pulse Shape Discrimination(2) PSD by 100MHz FADC –  rays from internal contamination –  rays from 60 Co standard source –fitting pulse shape with two exponential func. (fixed time constants) –Ratio ; A fast /A slow (Intensity Ratio of Fast and Slow Components) –Clear Discrimination between  and  (  ) Events –Background Rejection Efficiency > 99%

2004/Dec/12 H2OH2O 15”PMT Mount System CaF2(Pure) 10cm cube Liquid Scintillator CANDLES-II Prototype 45cm Cosmic-ray Events (High Energy) Index (CaF 2 ) Index (Mineral Oil) S.Umehara

2004/Dec/12 Mile stone ELEGANTS VI –running with new BG rejection (2 ) CANDLES I, II CANDLES III –10cm 3 cube (60 crystals) –~30  Bq/kg for 0.5 eV CANDLES IV –10cm 3 cube (1000 crystals) 3.2t –~5  Bq/kg for 0.1 eV CANDLES V –Several 10t or enrichment