Status of 48 Ca double beta decay search and its future prospect in - PowerPoint PPT Presentation

1 Status of 48 Ca double beta decay search and its future prospect in CANDLES 2019 Sep 10 th Konosuke Tetsuno (Osaka University, CANDLES Collaboration)

2 Double beta decay Constraint of ν mass from oscillation experiments Beyond the • Standard model • Standard Model Already observed • Proof of Majorana • in some nuclei neutrino T 1/2 : 10 18 ~ 10 20 yr • T 1/2 : > 10 25 yr • Majorana neutrino … • Lepton number violation • Light neutrino mass (See-Saw mechanism) Matter dominated universe (Leptogenesis) •

3 CANDLES experiment with 48 Ca CANDLES : Double beta decay experiment with 48 Ca For 0 νββ search, Low background level • Good energy resolution • Large target volume • Advantage : Highest Q-value (4.27 MeV) • Second highest : 150 Nd (3.37 MeV) least BG ( γ : 2.6 MeV, β : 3.3 MeV) Disadvantage : Small natural abundance,0.187% • Enrichment technique is under development. - Crown ether resin + chromatography - Crown ether + micro reactor - Laser separation - Multi-channel counter current electrophoresis (MCCCE)

4 CANDLES Ⅲ detector (UG) CANDLES at Kamioka underground laboratory Boron sheet Pb shield CaF 2 scintillator (CaF 2 (pure)) • 305 kg (96 × 3.2 kg) τ ~1µsec PMT’s liquid scintillator (LS) • 4 π active veto 1.8m 3 CaF 2 LS τ ~ a few 10 nsec PMT’s • 4m 62 large PMT’s light pipe • light collection for energy resolution Shield • -Pb shield (10-12cm) light pipe Reduce γ -ray from surrounding rock (1/100~) -Boron sheet (5mm) Reduce n captured by stainless tank(1/2000 ) 3m

5 CANDLES Ⅲ detector (UG)

6 4 π active veto by liquid scintillator • Rejection of external γ -ray background -Pulse shape information by 500 MHz Flash ADC. -Distinguish event type by offline pulse shape analysis taking advantage of different decay time.

7 Background candidates@4.3MeV 2 νββ • Not serious BG in current sensitivity. Will be major BG after 48 Ca enrichment → Improve energy resolution Radioactive impurities in CaF 2 • 212 Bi- 212 Po ( 232 Th chain) • -Double pulse(T 1/2 ~300 ns) , Max E visible : ~5.3MeV -Double pulse and PSD Cut 208 Tl ( 232 Th chain) • -Q β = 5.0MeV, T 1/2 ~3 min -Tag 212 Bi ( α ) and veto events in same crystal for 12min External γ -ray((n, γ ) reaction) • -Remain even after LS active veto -Installed boron sheet (1/2000)

8 95 Crystals 27 Crystals # Event Energy Spectra Live time : 131days 95 Crystals 27 Crystals All data LS Cut Q ββ 212 BiPo + 208 Tl Cut Q ββ Q ββ 4.0-5.0MeV 5.5-6.5MeV 4.0-5.0MeV 5.5-6.5MeV 10 34 6 0 2 1 No event in high purity 27 Crystals is confirmed !

9 Expected BG 27 Crystals Live time (days) 0νββ eff. Sensitivity（Yr） Event in ROI T 1/2 0νββ 48 Ca(yr) Results Exp.Data and BG MC in 27 CaF 2 Q ββ � Data 131 MC Total 2 νββ 0.39±0.06 CaF 2 Contamination (n, γ ) BG Others 0 ~1.2 > 6.2 × 10 22 3.6 × 10 22 • CANDLES is now giving the ELEGANT Ⅳ (previous study) best lifetime limit ! Exposure : 4947 kg ・ d (2yr<) 0 νββ eff. : 0.53 Add purity 14 crystals and • T 1/2 0 νββ : 5.8 × 10 22 yr ( 48 Ca) increase statistics

10 Background Candidates for CaF 2 Tail of 2 νββ spectrum • Improving energy resolution • Scintillator → Bolometer (Goal : 0.5 % [FWHM] energy resolution @ 4.27MeV) 48CaXX internal radioactivities • Th-chain( β - α sequential decays) → Bolometer • Th-chain( 208 Tl) • → Segmentation, Multi-crystal Environmental neutrons • Improving resolution + Multi-crystal • Possible to further reduce the BG by developing Bolometer But… new BG candidate • Q value of 48 Ca : 4267.98(32) keV @ arXiv : 1308.3815 • Q value of 238 U ( α -decay) : 4270 keV • Impossible to avoid → required particle ID → Developing CaF 2 Scintillating Bolometer

11 CaF 2 Scintillating Bolometer The technique (scintillating bolometer) was already established • CRESS- Ⅱ (CaWO 4 ), AMoRE (CaMoO 4 ): Ca crystal • CaF 2 (Eu) scintillating bolometer was also demonstrated. • Ref : NIMA 386 (1997) 453, small size (~0.3 g) of CaF 2 (Eu) • Simultaneous measurement both heat and scintillation enables to identify the particle types ( α / β particle ID) • It is possible to reject alpha decay events of 238 U • Q-value ; 4.27 MeV = Q-value of 48 Ca 0 νββ → Chance to achieve “BG” free measurement”

12 Development for CaF 2 Bolometer Collaborative research with Korean colleague • Yong-Hamb Kim (IBS & KRISS) Minkyu Lee (KRISS) Inwook Kim Do-Hyoung Kwon Hyejin Lee Hye-Lim Kim • Sub-Group of CANDLES (Osaka, Japan) Konosuke Tetsuno Xiaolong Li Saori Umehara Sei Yoshida

13 CaF 2 Scintillating Bolometer Setup Crystal : CaF 2 (pure) Volume : 300g (5cm φ × 5cm) Emission peak : 280nm

14 Resolution and Discrimination • The rising/decay time of signal depend on particles. • define PSD parameter • Heat/Light ratio • Rising time of light signal • Position dependence of scintillation absorption → make worse E-resolution

15 Position dependence We use contaminated CaF 2 crystal for R&D. • ~ 30 mBq of 226 Ra (U-chain) within crystal • Delayed coincidence ( 222 Rn → 218 Po → 214 Pb) • Excellent energy resolution ! Position dependence make • E-resolution worse.

16 Summary • Status and future prospect of CANDLES 48 Ca double beta decay search. • The results obtained by CaF 2 scintillator gives the best lifetime limit. • T 1/2 0 νββ : 48 Ca(yr) > 6.2 × 10 22 • Scintillating bolometer of undoped CaF 2 was firstly demonstrated, and evaluated performance of detector. Δ E( σ ) = 1.8 % @ ~5MeV, not good due to position • dependence. PID ~5 σ separation • Δ E( σ ) = 0.18 % @ ~5MeV w/o position dependence •

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