CANDLES for the study of 48 Ca double beta decay T. Kishimoto - PowerPoint PPT Presentation

CANDLES for the study of 48 Ca double beta decay T. Kishimoto Osaka Univ.

Contents • Double beta decay and Majorana Mass • Double beta decay of 48 Ca –ELEGANTS VI • How to sense 1~10 -2 eV region • CANDLES –Concept –CANDLES I, II, III, VI, V • Prospect (Mile Stone)

Neutrino has to be Majorana particle • Dirac mass term • Majorana mass term – Left-handed and right- Relativity: handed particle could have Helicity flip different mass mass – We have only left handed neutrino. – Heavy right-handed neutrino – Lepton number violation Double beta decay has to be measured.

e- ν e Within p n 2 ν mode e- ν e Standard model Double beta n p e- decay V-A h n p e- ν e h 0 ν mode h h n p V-A mass term 1.0 a.u. sum energy spectrum 48 Ca (0+ → 0+) 0.8 0 νβ β 0.6 2 νβ β x 10 -5~6 0.4 0.2 0 0.0 1.0 2.0 3.0 4.0 5.0 Energy (MeV)

Current understanding

Double beta decay of 48 Ca • Largest Q value (4.27 MeV) – next largest 150 Nd (3.3 MeV) – Large phase space factor – Least background ( γ : 2.6 MeV, β : 3.3 MeV) • Natural abundance → 0.187% – Enrichment → (no Gas); Hazama – Small amount ~10g • Next generation ∝ ∝ 2 M 2 1 T / M ν – if background free ∝ ∝ 2 M 4 1 T / M ν – if background limited 76 Ge experiment (already seen backgrounds) • 48 Ca (no backgrounds seen) large Q value •

ELEGANT VI

ELEGANTS VI • CaF 2 (Eu) – Size 45x45x45 mm 3 x 25(23) • ~6.42 g 48 Ca • passive shield – OFHC Cu, Pb ( γ ) – air-tight box + N 2 gas purge (Rn) – LiH + paraffin, Cd sheet, H 3 BO 3 +H 2 O tank (n) • 4 π active shield – CaF 2 (Eu)+CaF 2 (pure) • roll-off ratio (PMT side shield) – Segmentation (single hit) – CsI(Tl) veto detector

Roll-off ratio (4 π active shield) CaF 2 (pure) as light guide active shield against PMT CaF 2 (Eu) is not transparent for U.V. light CaF 2 (Eu) CaF 2 (pure) CaF 2 (pure) n = 1.44 n = 1.47 PMT PMT CaF 2 (Eu) Silicon oil Optical grease n = 1.40 n = 1.47 CaF 2 (pure)

Roll-off ratio LEFT(channel) Arbitary Unit ROLL-O LL-OFF RATIO IO RI RIGH GHT(channel) l) − V V = L R R + V V L R

Oto Cosmo Observatory tunnel constructed for railroad (but not used) 神岡 470m (1.3 km water equivalent) shield 旧国鉄の五新線（ 奈良県五條市～和歌山県 新宮市） 用の鉄道ト ンネルだが、 結局線路は 大阪大学 核物理研究 敷かれず。 センター 大塔コスモ観測所 8 4 5 m 大阪大学理学部 国道1 6 8 号線 大塔村側 西吉野村側 5039.5 m 第三観測室 第二観測室 第一観測室 ELEGANT VI

Under ground laboratory

Radioactive Backgrounds ( A ) ( B ) U 2 3 8 2 3 4 U 2 U 2 U 4 . 4 7 x 2 . 4 5 x 9 9 9 5 1 0 y r 1 0 y r 2 3 4 P a 1 P a 1 P a 4 . 2 0 4 . 7 8 9 9 6 . 7 5h 2 3 0 T h T h 2 3 4 T h 2 3 2 2 2 8 T h Q β = 0 . 1 9 9 0 T h 8 . 0 x 0 T h 1 . 4 1 x 9 9 1 . 9 1y r 2 4 . 1 0d 4 1 0 1 0 y r 1 0 y r 2 2 8 A c Q β 9 A c 4 . 6 9 9 A c 4 . 0 1 2 . 1 8 5 . 4 2 8 8 6 . 1 3h R a 2 2 6 2 2 8 R a 2 2 4 R a Q β 8 R a 1 . 6 0 x 8 R a 0 . 0 3 9 8 8 5 . 7 6y r 3 . 6 6d 3 1 0 y r 7 F r 4 . 7 9 7 F r 8 5 . 6 9 8 2 2 2 R n 2 2 0 R n 6 R n 8 6 R n 3 . 8 2d 8 5 5 . 6s 5 . 4 9 6 . 2 9 5 A t 8 5 A t 8 2 1 8 P o 2 1 4 P o 2 1 0 P o 2 1 6 P o 2 1 2 P o 4 P o 8 4 P o 3 . 0 5m 1 6 4 µ s 1 3 8 . 4d 8 0 . 1 5s 0 . 3 0 µ s 6 4 % 2 1 4 B i 2 1 0 B i Q β Q β 2 1 2 B i Q β 3 B i 6 . 0 0 7 . 6 9 5 . 3 1 3 . 2 6 1 . 1 6 8 3 B i 6 . 7 8 8 . 7 8 1 9 . 7m 5 . 0 1d 8 2 . 2 5 6 0 . 6m 2 1 4 P b 2 1 0 P b 2 0 6 P b Q β Q β 3 6 % 2 1 2 P b Q β 2 0 8 P b 2 P b 0 . 6 4 0 . 0 4 6 8 2 6 . 8m 2 2 . 3y r S t a b l e 2 P b 6 . 0 5 0 . 5 7 8 1 0 . 6 4h S t a b l e 2 0 8 T l Q β 1 T l 8 1 T l 4 . 9 9 8 3 . 0 5 3m 0 H g 8 0 H g 8

Radioactive Contamination • U-series – hardware (second) trigger – time window : 9 – 499 µ sec. = =     →     → 3270 E 214 214 E 7687 210 β α Bi Po Pb µ ( 164 . 3 sec) • Ac-series   =   →   =   → 219 E 6819 215 E 7386 211 α α Rn Po Pb ( 1 . 781 msec) • Th-series – time window : 0.05 – 1.0 (0.5) sec.   =   →   =   → 220 E 6288 216 E 6779 212 α α Rn Po Pb (0. 145 sec)

= =     →     → 6288 6779 220 E 216 E 212 α α Rn Po Pb (0. 145 sec) time window : 0.05-1.0 sec 100 40 100 20000 50 50 20 50 0 0 0 0 0 Average contamination Average contamination 1000 2000 1000 2000 1000 2000 1000 2000 1000 2000 CaF 2 -1 CaF 2 -2 CaF 2 -3 CaF 2 -4 CaF 2 -5 ( #2 － － #24 ) 100 100 ( #2 #24 ) 50 50 100 50 50 U – – series series U 0 × 10 0 1.11 × 3 Bq 0 0 0 10 - -3 Bq/kg /kg 1.11 1000 2000 1000 2000 1000 2000 1000 2000 1000 2000 CaF 2 -6 CaF 2 -7 CaF 2 -8 CaF 2 -9 CaF 2 -10 50 Ac – – series series Ac 50 100 100 100 25 3.84 × × 10 4 Bq -4 10 - 25 3.84 Bq/kg /kg 0 0 0 0 0 Th – – series series Th 1000 2000 1000 2000 1000 2000 1000 2000 1000 2000 CaF 2 -11 CaF 2 -12 CaF 2 -13 CaF 2 -14 CaF 2 -15 × 10 1.09 × 4 Bq 10 - -4 Bq/kg /kg 1.09 50 100 100 100 200 0 0 0 0 0 1000 2000 1000 2000 1000 2000 1000 2000 1000 2000 CaF 2 -16 CaF 2 -17 CaF 2 -18 CaF 2 -19 CaF 2 -20 U: 1.25x10 -5 Bq= 1 ppt 40 100 50 20 20000 20 Th: 0.1 mBq=24.6 ppt 0 0 0 0 0 1000 2000 1000 2000 1000 2000 1000 2000 1000 2000 CaF 2 -21 CaF 2 -22 CaF 2 -23 CaF 2 -24 CaF 2 -25

Double Beta Decay of 48 Ca Studied by ELEGANT VI 10 3 V e νββ > × 0 22 k T 4 . 5 10 year (68 % C.L.) 1 / 2 0 2 > × / 22 1 . 4 10 year (90 % C.L.) s t 10 2 n u World best value o c BG (sim) < 7 . 2 ~ 44 . 7 eV (90 % C.L.) m 4104 - 4438 keV ν 10 0 νββ window NPA 730 ’04, 215 1 2 νββ (sim) ν > × 0 21 9 . 5 10 years (76% C.L.) Beij ing T 1 / 2 > × 21 1 . 5 10 years (90% C.L.) TGV 10 -1 2000 3000 4000 5000 Not limited by backgrounds Energy (keV) 0.63 y

How to sense m ν =1~10 -2 eV • Big detector – Huge amount of materials • Low radioactive background – Active shield – Passive shield – Low background material – BG rejection by signal processing • High resolution – Backgrounds from 2 νββ decay • CANDLES is our solution

CANDLES CAlcium fluoride for studies of Neutrino and Dark matrters by Low Energy Spectrometer CaF 2 (Pure) CaF 2 (Pure) 200kg, 400kg, 6t,100t Liquid Scintillator (Veto Counter) 48 Ca (Q ββ =4.27MeV) Liquid Scintillator Wave Length Shifter 4 π Active Shield Passive shield Photomultiplier Photomultiplier energy resolution CaF 2 (Pure) Buffer Oil Large PMT

Big detector • CaF 2 crystal – Best optical lens – Long attenuation length • 10m (catalog value for visible light) • >1m (our measurement for scintillation light) • Large volume detector – 10x10x10 cm 3 x 600 (6t) (CANDLES IV) – Increase the number of nuclei ( 48 Ca) 6.4 g (ELE VI) ~6(kg) ～ × 22 2 6 8 . 1 10 atoms 10 atoms

Low Radioactive Background Active shield (Liquid Scintillator) • 4 π active shield decay time of the signal • ～ 1 µ sec CaF 2 • ～ 10 nsec liquid scintillator • passive shield • Large volume with Low radio activity • U/Th ～ 0.1ppt, K ～ 1ppt • purification system (U,Th,K,Rn,…) KamLAND, BOREXINO

CANDLES I POP (Proof of Principle) Detector CaF 2 (pure) crystal in liquid scintillator PMT(5") × 4 liquid Scintillator (with w.l. shifter) viewed by 4 PMTs (5 inch) CaF2(pure) liq. scint. : mineral oil + DPO (3 g/l) + Bis-MSB (0.3 g/l)

Signal discrimination Liquid Scintillator CaF 2 ~1 liquid scintillator ADC(fast)/ADC(total) ~0.04 CaF 2

Signal discrimination ADC(fast)/ADC(total) 57 Co source Co source Signal from Liquid scintillator CaF 2 ADC(total)

Low background material • Backgrounds @ ~4 MeV – Maximum energy • γ ~ 2.6 MeV, β ~3.3 MeV, α (max)~2.5 MeV(quench) – Successive decay of α β γ experimental Data • ~1 µ sec decay time Counts(/20keV) 2 νββ 0 νββ Window 10 2 212 Bi Pulse shape 10 214 Bi Gate width( 4 µ sec ) 1 208 Tl -1 10 E max =5.8MeV(U) β+α -2 10 5.3MeV(Th) 2000 2500 3000 3500 4000 4500 5000 Energy(keV)

Backgrounds U-Chain β α 214 Po 238 U 214 Bi 210 Pb T 1/2 = 164 µ sec Q α = 7.83MeV Q β = 3.27MeV Th-Chain β α 212 Po 208 Pb 232 Th 212 Bi T 1/2 = 0.299 µ sec stable 64% Q β = 2.25MeV Q α = 8.95MeV Th-Chain α β 208 Pb 208 Tl 212 Bi 232 Th stable T 1/2 = 3.05min 36% Q α = 6.09MeV Q β = 4.99MeV Rejection . . . Rejection . . . High Purity CaF 2 (pure) Crystal Rejection of double pulse Pulse Shape Discrimination between α and γ rays Space-Time Correlation Cut . . . For 208 Tl Rejection