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

Status of 48Ca double beta decay search and its future prospect in CANDLES

2019 Sep 10th

Konosuke Tetsuno (Osaka University, CANDLES Collaboration)

1

2

Double beta decay

• Standard model
• Already observed

in some nuclei

• T1/2 : 1018 ~ 1020 yr
• Beyond the

Standard Model

• Proof of Majorana

neutrino

• T1/2 : > 1025 yr

Constraint of ν mass from

• scillation experiments

Majorana neutrino…

• Lepton number violation
• Light neutrino mass (See-Saw mechanism)
• Matter dominated universe (Leptogenesis)

3

CANDLES experiment with 48Ca

• Low background level
• Good energy resolution
• Large target volume
• Advantage : Highest Q-value (4.27 MeV)

Second highest : 150Nd (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)

CANDLES : Double beta decay experiment with 48Ca

For 0νββ search,

4

CANDLESⅢ detector (UG)

• CaF2 scintillator (CaF2 (pure))

305 kg (96 × 3.2 kg) τ ~1µsec

• liquid scintillator (LS)

4π active veto 1.8m3

τ ~ a few 10 nsec

• PMT’s

62 large PMT’s

• light pipe

light collection for energy resolution

• Shield
• Pb shield (10-12cm)

Reduce γ-ray from surrounding rock (1/100~)

• Boron sheet (5mm)

Reduce n captured by stainless tank(1/2000 ) CANDLES at Kamioka underground laboratory

CaF2 LS PMT’s Boron sheet Pb shield

4m 3m

light pipe

5

CANDLESⅢ detector (UG)

6

4π 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

• 2νββ

Not serious BG in current sensitivity. Will be major BG after 48Ca enrichment → Improve energy resolution

• Radioactive impurities in CaF2
• 212Bi-212Po (232Th chain)
• Double pulse(T1/2 ~300 ns) , Max Evisible : ~5.3MeV
• Double pulse and PSD Cut
• 208Tl (232Th chain)
• Qβ = 5.0MeV, T1/2 ~3 min
• Tag 212Bi (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

Energy Spectra

95 Crystals 27 Crystals Live time : 131days

95 Crystals 27 Crystals

Qββ

4.0-5.0MeV 5.5-6.5MeV

Qββ

4.0-5.0MeV 5.5-6.5MeV

# Event 10 34 6 2 1

No event in high purity 27 Crystals is confirmed !

All data LS Cut

212BiPo + 208Tl Cut

Qββ

9

Results

27 Crystals

Live time (days)

131

0νββ eff.

0.39±0.06

Event in ROI

Expected BG

~1.2

T1/2 0νββ 48Ca(yr)

> 6.2 × 1022

Sensitivity（Yr）

3.6 × 1022

Qββ

• CANDLES is now giving the

best lifetime limit !

• Add purity 14 crystals and

increase statistics

ELEGANT Ⅳ (previous study) Exposure : 4947 kg・d (2yr<) 0νββ eff. : 0.53 T 1/2 0νββ : 5.8×1022 yr (48Ca)

Data MC Total 2νββ CaF2 Contamination (n, γ) BG Others

Exp.Data and BG MC in 27 CaF2

• Tail of 2νββ spectrum
• Improving energy resolution

Scintillator → Bolometer

• 48CaXX internal radioactivities
• Th-chain(β-α sequential decays) → Bolometer
• Th-chain(208Tl)

→ 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 48Ca : 4267.98(32) keV @ arXiv : 1308.3815
• Q value of 238U (α-decay) : 4270 keV

Impossible to avoid → required particle ID → Developing CaF2 Scintillating Bolometer

10

Background Candidates for CaF2

(Goal : 0.5 % [FWHM] energy resolution @ 4.27MeV)

11

CaF2 Scintillating Bolometer

• The technique (scintillating bolometer) was already established
• CRESS-Ⅱ(CaWO4), AMoRE (CaMoO4): Ca crystal
• CaF2(Eu) scintillating bolometer was also demonstrated.

Ref : NIMA 386 (1997) 453, small size (~0.3 g) of CaF2(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 238U
• Q-value ; 4.27 MeV = Q-value of 48Ca 0νββ

→ Chance to achieve “BG” free measurement”

12

Development for CaF2 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

CaF2 Scintillating Bolometer Setup

Crystal : CaF2(pure) Volume : 300g (5cmφ × 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 CaF2 crystal for R&D.
• ~ 30 mBq of 226Ra (U-chain) within crystal
• Delayed coincidence (222Rn → 218Po → 214Pb)
• Position dependence make

E-resolution worse.

Excellent energy resolution !

16

Summary

• Status and future prospect of CANDLES 48Ca double beta

decay search.

• The results obtained by CaF2 scintillator gives the best

lifetime limit.

• T1/2 0νββ : 48Ca(yr) > 6.2 × 1022
• Scintillating bolometer of undoped CaF2 was firstly

demonstrated, and evaluated performance of detector.

• ΔE(σ) = 1.8 % @ ~5MeV, not good due to position

dependence.

• PID ~5σ separation
• ΔE(σ) = 0.18 % @ ~5MeV w/o position dependence

Backup

17

18

19

20

21

22

23

24

25

26

27