Neutrino-less Double Beta Decay
Experimental Review

大阪大学核物理研究センター
梅原さおり
Umehara, Saori
RCNP, Osaka University
umehara@rcnp.osaka-u.ac.jp

Contents
 Double beta decay◼ [Metter dominated universe] and [double beta decay]◼ Test of Majorana nature Double beta decay measurements◼ Half-life measurement◼ Requirement for measurements◼ Projects for double beta decay measurements
 CANDLES project◼ Current operating detector system : CANDLES III◼ Next detector system Summary
Umehara, Saori、11th Sep. 2021、 TGSW20212

[double beta decay] and[matter dominated universe] Double beta decay : Key of mystery of [matter dominated universe]◼ Early universe : same amount of matter and anti-matter◼ But Current universe : asymmetry between matter and anti-matter◼ Leptogenesis：Explain by [Majorana nature] and [heavy neutrino]◼ We can test [Majorana nature] by 「neutrino-less double beta decay」◼ Majorana nature of neutrino : particle = anti-particle

Key of mystery
Early universe
Antimatter

Double beta decay
electronMatter
dominateduniverseenergymatter
Anti-mattermatterAnti-particle⇔particle

梅原さおり、レーザー素子開発、2021年08月18日

3Test of Majorana nature Another way for test of Majorana nature : for example . . .
◼ Neutrino beam experimentννν

But not realistic
to collide neutrinos by beam control

Neutrino converts to anti neutrino→Collision◼ Double beta decay : realistic test
electron enucleus

Anti-neutrino 𝜈ҧ

in nuclear sizein short time
two anti-neutrino

Beta decay is forbidden(A,Z) → (A,Z+1) + e- + 𝜈ҧDouble beta decays(A,Z) → (A,Z+2) + 2e- + 2𝜈ҧ(A,Z) → (A,Z+2) + 2e-

Umehara, Saori、11th Sep. 2021、 TGSW2021(beta decay is forbidden
by energy conservation)

Binding energyνodd‐odd nuclei
even‐even
Beta decay nuclei

ZDouble betadecay
Z+2 Z+44

Neutrino-less double beta decay

 Two neutrino double beta decay and Neutrino-less double

beta decay

In standard model

Two neutrino double
beta decay（2νββ）(A,Z) → (A,Z+2) + 2e- + 2𝜈ҧelectron e-nucleus

Anti-neutrino 𝜈ҧDirac neutrinoν ≠νPaul DiracUmehara, Saori、11th Sep. 2021、 TGSW2021

Lepton number nonconservation

Neutrino-less double
beta decay（0νββ）(A,Z) → (A,Z+2) + 2e-nucleus
electron e-electron e-

Anti-neutrino 𝜈ҧneutrino 𝜈particle⇔anti-particle
Majorana neutrino

ν=ν

Ettore Majorana
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Double beta decay measurement
 Double beta decay◼ Decay rate：very rare event2νββ◼ 2νββ：half-life 𝑇1/2 1018 ~ 1021 year0νββ

◼ 0νββ：half-life 𝑇1/21026 year～212◼ 0νββ ∝ 𝐺0ν 𝑀0ν 𝑚𝛽𝛽
𝑇1/2Two neutrino double
beta decay（2νββ）(A,Z) → (A,Z+2) + 2e- + 2𝜈ҧeenucleus

0+48Ca> 1.1 x 1020 年

Neutrino-less double
beta decay（0νββ）(A,Z) → (A,Z+2) + 2enucleus

6+48ScT1/2 ~ 4 x 1019 年
Qββ= 4.27MeV0+48Ti𝜈ҧ𝜈ҧ

48Ca decay scheme

Umehara, Saori、11th Sep. 2021、 TGSW2021

e-e𝜈ҧ

𝜈6

Double beta decay measurement

 Double beta decay measurement : by detection of two beta-rays

 Energy spectra of double beta decays0νββ/2νββ = 10-6
FWHM = 5% @ QββSum energy spectraof two electrons
2e-no 𝜈ҧnucleus

2e2𝜈ҧ2νββContinuous spectrum

0νββ
Mono energy
sum energy of 2e-/ Qββ

Ref : S.R.Elliot and P.Vogel, Ann.
Rev.Nucl.Part.Sci.52(2002)115.

 Main backgrounds : 2nbb and radioactive contamination◼ Good energy resolution to reduce BG from 2nbb
◼ High purity materials and Particle identification
Umehara, Saori、11th Sep. 2021、 TGSW2021

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Double beta decay Neutrino-less double beta decay◼ Key of mystery of the matter dominated universe◼ Majorana nature（𝜈 ↔ 𝜈）ҧ◼ Lepton number non-conservation◼

10νββ
𝑇1/2 ∝<𝑚ν >2Two neutrino double Neutrino-less double
beta decaybeta decay
Sum energy of electron

continuous

Mono energy

Half-life

1018 ～ 1021 years

1026 years～

experiments

Observed

Not observed◼ Requirements for measurement
Long half-life→very rare decay, need low background condition,
To avoid background from 2nbb decay
resolution
Umehara,→goodSaori、11thenergy
Sep. 2021、
TGSW2021

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Double beta decay source

Natural abundance vs Qββ◼ Need a few 100 kg ~ tonne scale source◼ For >1026 years half-life
 Large Isotopic abundance

208Tl

Qβ=5.0 MeV

Qββ(MeV) Large detector scale

214Bi

Qβ=3.3 MeV◼ large natural abundanceor isotopic enrichment
 Large detection efficiency◼ Detector types
Source ⊆ Detector→ large detection efficiency
detector
double beta
nucleusbb
measured

Umehara, Saori、11th Sep. 2021、 TGSW2021

208Tlγ

2.6 MeV
Natural abundance(%)

Source ≠ Detector→ small efficiency,but BG rejection is gooddetector source detector

measuredbb

measured9Backgrounds

 Background condition for double beta decay measurements◼ Low natural radioactivity : in U-chain, Th-chain◼ required level ~ 1 μBq/kg ⇔ ordinary materials 1~100Bq/kg
Ordinary materials
DBD detector materials
Rock, concrete

10-4

10-2

1102

104 radioactivity

Start from material selection

Some metal plates
Pb bricks etc.
◼ Cosmic muon, neutrons
Muon flux in underground Labs
◼ Underground laboratory

Cosmic-rays
rocks
detector
Muon flux : <1/100000 of sea level lab.
Neutronflux(thermal~fast): < 100th Sep.
Ref : Data from Phys Rep 187(2000)203
10Umehara,Saori、112021、 TGSW2021

Detector technology

 Technology◼ legacy technique : scintillator, semi-conductor detector, bolometer◼ New technique◼ Light & charge detection
Light sensor◼ Scintillating bolometerHeatsensor

Light sensor

scintillator
Light detection

Tracking detector
detector＝ bb source
Light&charge detection
Light&heat detection
detectorbb nucleus
Light sensor
scintillating
bolometereebolometer
Heat detection
Charge sensor
Charge detection
Semiconductor detector
Heatsensor
ACharge sensor
11Umehara, Saori、11th Sep. 2021、 TGSW2021

DBD ProjectsEuropeCOBRA CUORE
116Cd
130Te
CdZnTe Bolometer
GERDA
NEXT
76Ge
136Xe
High-PTPC HPGe
SuperNEMO
82Se
Tracking
China
PandaX-III
136Xe
High-P TPC

Fluid type

Crystal type
CUPID
100Mo,82Se
Bolometer
Korea
AMoRE
100Mo
40Ca100MoO
4136Xe130Te,96Zr

76Ge(scintillating)bolometer
North America
EXO-200 SNO+
136Xe
130Te
TPC
Te-LS
Majorana
76Ge
HPGe
KamLAND
136Xe
Japan
Xe-LS
LEGENDCANDLES AXEL (R&D)
76Ge
136Xe
48Ca
HPGe
High-P TPC
CaF2
DCBA
ZICOS (R&D)
150Nd
96Zr
Drift chamber
Zr-LS

Many projects have been carried out so far and are proposed.

Umehara, Saori、11th Sep. 2021、 TGSW2021

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KamLAND, SNO+ etc.

Target nuclei in liquid scintillator

 KamLAND-ZEN◼ Large detector for liquid scintillator◼ 13m liquid scintillator area, 2-3.5m Xe area
◼ Large target mass : 350kg(KamLAND-Zen400) → 720kg◼ Energy resolution : 4.5%(σ) @ Qbb◼ Result : T1/2 > 1.07 x 1026 year, 60-160meV◼ Backgrounds:2nbb, radioactivity in balloon, liquid scintillator

Y. Gando et al.,J. Conf : 1468(2020)012142
Result of KamLAND-Zen800Detector setup

13m2-3m

Umehara, Saori、11th Sep. 2021、 TGSW2021

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Ge semiconductor detector

GERDA, Majorana,LEGEND GERDA◼ Ge detectors immersed in instrumented Liquid Ar◼ Good energy resolution : < 0.2%(FWHM) @ Qbb◼ →low background condition◼ Detector mass: 44.2kg◼ Result : T1/2 > 1.8 x 1026 year, 79-180 meV◼ Backround : 5.2 x 10-4 count/keV/kg/year (lowest)
Detector setup

M. Agostini et al., PRL 125(2020)252502
Resultth Sep.
TAUP2019
slidebyTGSW2021
C. WIESINGER
Umehara, Saori、112021、

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Scintillating bolometer

CUORE(bolometer), CUPID,AMoRE, CANDLES etc.

 CUORE(bolometer) → CUPID(scintillating bolometer)◼ energy resolution : 0.3 %(FWHM) @ Qbb◼ low background condition◼ TeO2 bolometer (not scintillating bolometer)◼ Detector mass: 206kg of 130Te,◼ Result : T1/2 > 3.2 x 1025 year, 75-350meV◼ background : 1.49 x 10-2 counts/keV/kg/year
Detector setup

a-rays and g-rays from 208Tl(2.6MeV)th Sep. 2021、TAUP2019
slideby S. Pozzi
Umehara, Saori、11TGSW2021

ResultD. Q. Adams et al., PRL 124(2020)122501
15 Detector system for Double beta decay : CANDLES III◼ Main detector : CaF2 scintillator

light censer

Inside photo of CANDLES
CaF2 scintillators

Umehara, Saori、11th Sep. 2021、 TGSW2021

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 Why 48Ca ? : advantage of 48Ca
◼ higher Qββ value (4.27MeV) . . .

Natural abundance vs Qββ
Qββ(MeV)

Double beta decay of 48Ca

208Tl

Qβ=5.0 MeV

214Bi→low backgroundQβ=3.3 MeV
because Q ββ value is higher than BG
208Tlγ
Emax=2.6MeV(208Tl, γ-ray)
2.6 MeV214
3.3MeV( Bi,β-ray)
Natural abundance(%)◼ But small natural abundance 0.19%
 Double beta decay of 48Ca by using CaF2
◼ CANDLES system◼ CANDLES III : current detector system◼ Enrichment + scintillating bolometer : for next system

Umehara, Saori、11th Sep. 2021、 TGSW2021

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CANDLES III

Ref : K. Nakajima et al, Astroparticle Phys,
100, (2018), 54-60
Ref : T. Iida et al, Nucl. Inst. Meth. A986,(2021), 164727

 CaF2 scintillator (CaF2 (pure))◼ 305kg (96modules×3.2kg)◼ 48Ca：350g

 Liquid scintillator (LS)4m◼ 4π active shield(2m3) 62 Large photomultiplier tube

CaF2

 Shielding system

Liquid scintillator
waterPMTs
3m

Umehara, Saori、11th Sep. 2021、 TGSW2021◼ Pb : 10-12cm◼ B4C sheet : 5mm
CANDLES tank(stainless steel)Pb（γ-ray shield）
B sheet（neutron shield）
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Detector construction:shielding system
 Shielding system

Side photo of CANDLES◼ Pb shield, B4C sheet

Main tankWork in the main tankfor setting Pb bricks
Pb bricks +B4C sheet

Pb total mass : ~50ton
Umehara, Saori、11th Sep. 2021、 TGSW2021

19Detector construction : Crystals
In a clean wear
To avoid dust

By using high purity wire,
acrylic frame, sleeves etc.

CaF2 modules
Before installationin CANDLES

CaF2 setting

Laser for height adjustment for CaF2
Umehara, Saori、11th Sep. 2021、 TGSW2021

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Result Result of measurement for 130days
Result with 21 high purity CaF2
結果
0νββ efficiency

0.36(21CaF2)

Num. of eve.(exp)0Expected BG

1.02

Half life of 48Ca

>5.6×1022year

Sensitivity

2.8×1022year

experimental data
simulation(total)
γ-ray from N capture
contamination in CaF22νββEnergy spectrum andBG simulation

21 crystals

Ref : Phys. Rev. D103, (2021), 092008＊comparable to most stringent limit of 48Ca
ELEGANT VI
measurement time：4947kg・day(2 years <)half life limit ：5.8×1022 year

93 crystals*Achieved background rate
・< 10−3 events/keV/year/(kg of natCa)
・comparable to lowest background measurements
Umehara, Saori、11th Sep. 2021、 TGSW2021

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Future CANDLES

 Next step of double beta decay measurement
CANDLES III

Next detector system

48Ca Abundance

0.187%

50%

48Ca Weight0.35 kg

600 kg ~Energy Resolution

6%

1.0% (required)𝑚𝜈 sensitivity

500meV~5 meV

FeatureCooling CaF2
Low BG

Massive 48Ca & high energy resolution
IH ⇒ NH𝒎𝝂 = 𝟓𝒎𝐞𝐕
2νββEnergy resolution
 Large amount of 48Ca
4%(FWHM)
2.8%◼ Current CANDLES :0.5%

limited by mass of 48Ca

0νββ
Umehara, Saori、11th Sep. 2021、 TGSW2021

 Higher energy resolution◼ To reduce 2nbb events
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Next detector system：enrichment

 48Ca◼ Natural abundance is low：0.19%→We can improve the detector sensitivity by enrichment◼ But enrichment of 48Ca is difficult
 New enrichment techniques◼ Crown-ether, laser enrichment, Electrophoresis
Chemical enrichment
by crown-ether

Laser enrichment

Umehara, Saori、11th Sep. 2021、 TGSW2021

Electrophoresis

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Fukui Univ.: H.Niki, I.Ogawa

Next detector system：enrichment introduction of laser isotopic separation(LIS)
setup
Deflection method
48Ca

Recoveryplatecollimator

48Ca

Atomic beam

Deflection
laser

Momentum
photontransfer
Absorption
from laser direction

Beam oven

48Ca

photonabsorption wave spectrum
Emit (isotropic)
for Ca40Ca
One laser for~0.001nm enrichment
・for deflection48CaRepeating excitation/deexcitation
Umehara, Saori、11th Sep. 2021、 TGSW2021

ref：ionization method
Ionization
potential
Excitedstate

Laserabsorption
Laserabsorption

Grand state
Target isotope
Two laser forenrichment
・for excitation
・for ionization
Korea group is also
developing24

1

laser off
laser on

40Ca

0.8

44Ca

0.6
0.4
0.2
6.1

6.3

6.5

48Ca

Recovery efficiency
concentration
48Ca concentration[%]Ion signal[mV] Separation effect

48Ca recovery efficiency[%]Laser isotope separation

Ref :K. Matsuoka et al, J. of Phys. :Conf. Ser. 1468, (2020), 012199
Ref : Presentation
by I. Ogawa in SPLG2021

setup
Recovery plate

Atomicbeam

48Calaser

6.7 6.9
Set position[mm]
TOF[μs]When Recovery plate is set at 6.5mm・・・
Recovery effi. 19.6%
concentration 5.5%oven

 For high-concentration・high-recovery effi.

◼ Large deflection◼ optimize irradiation system for deflection laser◼ High intensity blue laser

Umehara, Saori、11th Sep. 2021、 TGSW2021

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Next detector system：scintillating bolometer
Particle identificationby scintillating bolometer

Scintillating bolometer at low
temperature 10mK Lightsensor

Heatsensor
CaF2

Scintillation0νββ region
4.27MeV

ThermalsignalLightsignal

β-α pile-up eventsβ/γ Expected BG： 2νββ events, α-rays

α peaks
4.27MeVHeat

 bolometer：good energy resolution◼ For reduction of BG affects from 2νββ events
 Scintillating bolometer：good PI ability◼ For reduction of BG affects from α-ray
Umehara, Saori、11th Sep. 2021、 TGSW2021

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IBS Kim Yong-Hamb
AMoRE sub group
CANDLES sub group

Scintillating bolometerCounts

Energy spectrum of α-events
Energy resolution 1.86%
222Rn
226Ra

Energy ratio between two
events at the same position

218Po

0.18 % (σ) @ ~5 MeV

214Bi→214Po→210Pb(β+α)
Energy [MeV]

222Rn→218Po(3min)→214Pb

 First result of CaF2(pure) scintillating bolometer◼ Energy resolution(σ)：1.86±0.11%◼ But not best by position dependence◼ Additional sensor for removing position dependence
Ref :K. Tetsuno et al, J. of Phys. :1468(2020)012132
Umehara, Saori、11th Sep. Conf.
2021、Ser.
TGSW2021

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Summary Neutrino-less Double beta decay◼ Crucial process for nuclear/particle physics◼ Testing of Majorana nature◼ Very rare decay : half-life > 1026 years◼ not observed yet Double beta decay measurements◼ Many projects have been carried out and proposed◼ Requirements : low background condition◼ Many techniques are applied◼ Scintillating bolometer, enrichment◼ To observe 0nbb with next-generation experiments

Umehara, Saori、11th Sep. 2021、 TGSW2021

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