Low Radioactivity in CANDLES T Kishimoto Osaka Univ

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

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

Why 48 Ca (ELEGANTS VI) • 48 Ca : Search for 0 nbb decay – highest Q-value (4. 27 Me. V) largest phase volume least BG ( <2. 6 Me. V, <3. 3 Me. V) Isotope 48 Ca Q-value (Me. V) 4. 27 G 0 v× 10 -25 (year-1 e. V-2) 2. 44 76 Ge 82 Se 100 Mo 116 Cd 136 Xe 150 Nd 2. 04 3. 00 3. 03 2. 80 2. 48 3. 37 0. 244 1. 08 1. 75 1. 89 1. 81 8. 00 Natural abundance 0. 187% • 19 F : Search for spin coupled Dark Matters 2004/Dec/12 LRT 2004 @SNOlab

ELEGANT VI 45 mm 3 5 x 5=25 crystals ~3. 5 kg 19 F ~6. 4 g 48 Ca 2004/Dec/12 LRT 2004 @SNOlab

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

Roll-off ratio Ca. F 2(pure) as light guide active shield against PMT Ca. F 2(Eu) is not transparent for U. V. light Ca. F 2(pure) n = 1. 47 PMT Silicon oil n = 1. 40 Ca. F 2(Eu) Ca. F 2(pure) n = 1. 44 PMT Ca. F 2(Eu) Optical grease n = 1. 47 Ca. F 2(pure) 2004/Dec/12 LRT 2004 @SNOlab

Roll-off ratio PH(Ca. F 2(Eu))= 3~4 times PH(Ca. F 2) 2004/Dec/12 LRT 2004 @SNOlab

Limit for 0 nbb decay of 48 Ca • Obtained spectrum • 4. 23 kg yr NPA 730 (2004) 215 Expected BG : 1. 3 events (simulation) 0 events in 0 nbb energy window BG (sim) 0 nbb window 2 nbb (sim) cf. 2004/Dec/12 LRT 2004 @SNOlab BG in Q value region is successive Decay of or g

LRT 2004 Radioactive Backgrounds

Radioactive Contamination • U-series hardware (second) trigger – time window : 9 – 499 msec. • Ac-series • Th-series – time window : 0. 05 – 1. 0 (0. 5) sec. 2004/Dec/12 LRT 2004 @SNOlab

time window : 0. 05 -1. 0 sec 2004/Dec/12 LRT 2004 @SNOlab

Contamination inside crystals Average contamination ( #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 2004/Dec/12 LRT 2004 @SNOlab 1. 25 x 10 -5 = 1 ppt Th 0. 1 m. Bq=24. 6 ppt

• How to make Big Detector – Ca. F 2(Eu): No (attenuation length ~13 cm) – Ca. F 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 e. V and further DM annual modulation 2004/Dec/12 LRT 2004 @SNOlab

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

External BG rejection by Pulse shape (CANDLES I) Signal from Liquid scinti. Signal from Ca. F 2 1 sec decay time 2004/Dec/12 LRT 2004 @SNOlab

Signal from Liquid Scint. Fast component Signal to Noise Separation Backgrounds Signal from Ca. F 2 total 2004/Dec/12 LRT 2004 @SNOlab

Energy resolution (Light output) Liquid Scintillator wave length shifter Bis-MSB Dimethyl. POPOP C 24 H 22 C 26 H 20 N 2 O 2 C 24 H 16 N 2 O 2 2004/Dec/12 LRT 2004 @SNOlab absorpt ion 347 nm 363 nm 362 nm emissio n 426 nm 428 nm 418 nm Q. E. 0. 94 0. 93 --

Two Phase System (1) Concept of Method • WLS Phase – M. O(100%)+PPO(0. 3 g/L) – 5~10 mm in thickness – Large conversion eff. • Veto Phase Ca. F 2(Pure) – M. O. (80%)+P. C. (20%)+PPO(1. 0 g/L) – Large light output Ca. F 2 Emission (~280 nm) Conversion by PPO(350~400 nm) propagate PMT 2004/Dec/12 LRT 2004 @SNOlab

Performance Test (WLS phase) Standard Source Side View Ca. F 2(pure) Counts Liquid Scintillator (WLS phase) CANDLES I with 10 cm cube Ca. F 2(pure) 60 Co 137 Cs (662 ke. V) (1333 ke. V) 9. 14%(FWHM) 5. 91%(FWHM) 2004/Dec/12 LRT 2004 @SNOlab Energy (ke. V)

CANDLES III • Under Construction (Osaka Univ. ) • Ca. F 2(pure) – 103 cm 3 × 60 crystals total weight : 191 kg • Liquid scintillator • f 1000×h 1000 – H 2 O Buffer • acrylic container passive shield 　f 2800×h 2800 – safety regulation • Purification system • PMTs – 15” PMT (× 19) : R 2018 – 13” PMT (× 29) : R 8055 2004/Dec/12 LRT 2004 @SNOlab

Background – reduction / rejection – • Reduction of radioactive impurities inside crystal Raw Materials (Ca. CO 3, HF) Ca. F 2 powder Fused Ca. F 2 • Powder selection • Growing process • Rinse out the powder • Scavenger selection (H 2 O, HNO 3, …) … – Powder selection • U-chain (214 Bi) • Th-chain (220 Rn) R&D study in progress BG measurement at Oto • So Far… 2004/Dec/12 LRT 2004 @SNOlab Ca. F 2 crystal (3. 2 kg x 11 crystals) 51 m. Bq/kg 21 m. Bq/kg Ca. F 2(Eu) 214 Bi 1110 m. Bq/kg 220 Rn 98 m. Bq/kg Enough for CANDELS III ~30 Bq/kg Another one order for CANDLES IV and further

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 LRT 2004 @SNOlab

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

CANDLES-II 45 cm Liquid Scintillator • Prototype Ca. F 2(Pure) 10 cm cube Mount System 15”PMT H 2 O Index 1. 44@586 nm (Ca. F 2) Index 1. 46@586 nm (Mineral Oil) Cosmic-ray Events (High Energy) 2004/Dec/12 LRT 2004 @SNOlab S. Umehara

Mile stone • ELEGANTS VI – running with new BG rejection (2 n) • CANDLES I, II • CANDLES III – 10 cm 3 cube (60 crystals) – ~30 Bq/kg for 0. 5 e. V • CANDLES IV – 10 cm 3 cube (1000 crystals) 3. 2 t – ~5 Bq/kg for 0. 1 e. V • CANDLES V – Several 10 t or enrichment 2004/Dec/12 LRT 2004 @SNOlab