Study of Double Beta Decay of 48Ca with CANDLES

Y.Hirano, T.Kishimoto, I.Ogawa, R.Hazama, S.Umehara,K.Matsuoka, G.Ito, Y.Tsubota, for the CANDLES Collaboration

Double Beta Decay of 48Ca

■Largest Q value (4.27 MeV) next largest; 150Nd (3.3 MeV)almost background free

(γ: 2.6 MeV, β: 3.3 MeV) large phase space factor

■ Low Natural abundance → 0.187%large detector Enrichment

→CANDLES system

CANDLES system

Undoped CaF2 (CaF2(pure))

Long Attenuation Length (>10m@350nm) Peak Emission at UV Region (280nm)→ Wave Length Shifter

Liquid Scintillator (LS)

4π active shield

Passive shield

Wavelength shifter for CaF2

Photomultiplier

large photo-coverage

CaF2(Pure)

Liquid Scintillator(Veto Counter)

Buffer Oil

Large PMT

Active Shielding Technique

Short Gate （ 200nsec ）Long Gate （ 4μsec ）　

CaF2

Liq.Scintil.

Ra

tio=

Sho

rt /

Lo

ng

Liq ＋ CaF2

Clear Discrimination !

Different Time Constants　　 CaF2(pure) : ~1μsec

　　 Liquid Scintillator : a few 10 nsec

CaF2

Liq

208Tl

40K

Project to Y axis

Expected Background in CANDLES

■External Background

Strongly Suppressed

Because of Large Qββ of 48Ca(4.27MeV)

4π Active Shielding System

■Remaining Background

22 Decay Event Decay Event

Improve Energy Resolution

Natural Radioactivities in CaFNatural Radioactivities in CaF22(pure) Crystal(pure) Crystal

Improve Purity of CrystalRejection by Offline Analyses

Backgrounds from Natural Radioactivitiesin Crystals

Th-Chain

U-Chain

Sequential Pulse

212Bi and 208Tl(T1/2=3min) . . . rejected by Space-Time Correlation Cut

CaF2(pure) Decay Constant :1μsec rejected by FADC

4μsec Gate

Rejection of Sequential Pulse

Typical Pulse Shape (500 MHz FADC)

recorded shapefitted shape

99% of sequential pulse events will be rejected

…under test

Delayed890ns

68ns

Pul

seH

eigh

t(ch

)

0 1000 2000 3000 4000 5000

Time(nsec)

0 1000 2000 3000 4000 50000 100 200 300 400 500

Prompt

CANDLES III

■Constructed at Osaka Univ. (sea level)small version for R&Dcheck the performance of CANDLES

■ CaF2 modules103 cm3 × 60 crystal; 191 kgwith conversion phase

■ Liquid scintillator f1m×h1m acrylic container

■ H2O Buffer : passive shield　 f2.8m×h2.6m

■ PMTs15” PMT (× 8) 13” PMT (×32)Total 40PMTs

33.4% photo-coverage

CANDLES III (prototype)

2007/03/25 日本物理学会春の大会＠首都大学東京

2.8 m

h2

.6

m

Top View

Side View

PMTs

Photomultiplier Tube (13inch)

Inside View

CaF2 module

half filled filledIndex 1.44@586nm (CaF2)

Index 1.46@586nm (Mineral Oil)

CaF2 + conversion phase + acrylic casePeak Emission at UV Region (280nm) → Wave Length Shifter

Liquid Scintillator Tank

59 CaF2 modules installed

Experimental Condition

■Measurement at Osaka Univ. (sea level)

■CaF2 modules 103 cm3 x 59 crystal; 189kg

1dirty crystal : U:65 mBq/kg, Th:28 mBq/kg

■PMTs

15” PMT( x 6)

13” PMT( x 29) Total 35 :~30% photo-coverage

■DAQ

ADC (40ch) x 2 (Long Gate and Short Gate)

1 module FADC under test

■Live Time 30 Hours

Position Reconstruction

∑i ADC(i) x PMT(i)

∑i ADC(i)R =

Preliminary analysis

Position Reconstruction Parameter

１0cm

20cm

σ=~ 5cm (position dep.)

Enough to Pos. Resolution to

reduction 208Tl Events

Know Where prompt α occur

Top View＞ 2MeV

＞ 2MeV

208Tl Event

212Bi36%

Prompt α-ray Delayed β-ray

208TlT1/2 = 3.05min

Qb = 5.0MeV

CaF2 Discrimination (Top View)

Dirty Crystal

No Crystal

(59 crystals installed)

1st(Top) 2nd

3rd 4th 5th(Bottom)

x

Y

CaF2 Spectra

CaF2No 214Bi(U) 220Rn(Th)

― 60 65 28 (Dirty Crystal)

― 59 0.049 0.149

― 33 0.034 0.020

Contami. [mBq/kg]

40K(1.4MeV)

Compton(1.24MeV)

208Tl(2.6MeV)

Compton(2.38MeV)

Cosmic Ray Events??

CaF2No. 33, 59, 60(Dirty Crystal)1st 2nd 3rd 4th 5th

No33No59 No60

Q-Value Region(4MeV~4.5MeV)

Sequential Pulse

Position 2σ region

Ratio 2σ region

Background of Q-Value Region

No59 No60

Bottom

CaF2No.60 － No.59 Spectrum

Contribution only from contaminations inside crystal

Sequential Pulse4.0~4.5MeV

Contaminations in crystals

CaF2No.60 (dirty crystal: av. 46mBq/kg)

　 8.6 Events/Hour

Other Clean Crystals: (av. 42μBq/kg)

　→ 8x10-3 Events/Hour/ Expected

Sequential Pulse Rejection efficiency 99% (by FADC )

Other Background (Cosmic Rays ?)

　 Several Events/Hour (e.g. CaF2No.59: 3events)

　 → move to under ground lab. negligible

Summary

■ CANDLES III under the performance check background measurement at surface lab.

Several Event Remained in Q-value region ■ Level of Contaminations (U,Th) sufficiently low for Under ground Measurement

We expect further reduction by PSD using FADC : ■ Other Background (Cosmic Ray Events…under investigation)

Move to Under grand lab.

■ Improvement of trigger systemGo.Ito Poster session. TAUP2007

■ Improvement of energy resolution (install Light Guide )Y.Tsubota Poster session TAUP2007

Mile Stone

(Prototype )CANDLES III (at Sea level)– CaF2 (10cm3) 191 kg

•CANDLES III (U.G.)–Kamioka underground lab. (summer 2008)–CaF2 300 kg ~30 Bq/kg for ~0.5 eV

•CANDLES IV –Several tons; –~3 Bq/kg for ~0.1 eV in 6 yrs

•CANDLES V–100 t; Kamland or ??? for ~30 meV in 7 yrs–another option; enrichment ( under investigation)

Enrichment of 48Ca

Cosmic Ray Spectrum::Ratio (Simulation)

Experiment SimulationRatio

Upper Thre.

CaF2 Ratio RegionCaF2 Ratio Region

Liq.Scinti.

Sequential Pulse Rejection×98.8% (by FADC )

PSD efficiency>99% at 4MeV (Energy dep.)

Cosmic Ray::Spectrum(simulation)

Spectrum : BESS Experiment @ KEK in JapanAngular Distribution : cos2θFlux : 0.026 /cm2/sec

[MeV]

[MeV]

―Liq. Cut

―Liq No Cut

Cut eff. ~99%

CandidatesNucleus Q-value Reaction20F 7.0MeV 19F(n,γ)20F16N 10.4MeV19F(n,α)16N19O 4.6MeV 18O(n, γ)19O, (n,n)

CaF2 Spectra

CaF2No 214Bi(U) 220Rn(Th)

60 ― 65 28 (Dirty Crystal)

14 ― 0.026 0.013

46 ― 0.051 0.356

Contami. [mBq/kg]

40K(1.4MeV)

Compton(1.24MeV)

208Tl(2.6MeV)

Compton(2.38MeV)

CaF2No. 14, 46, 60(Dirty Crystal)

Calibration and Resolution

Position Region 2σ

Position Region 0.5σ

Compton edge decrease

One Photo-electron = 10ch

1461keV = 4930ch → 493p.e → 10% (@1461 40K) → 6%(Q-value)

Peak Channel

Development of High Purity CaF2 Crystals

Radioactivities in CaF2(pure) Crystal(-ray measurement)

Radioactivities in CaF2 Powder(HPGe measurement)

U-chain(214Bi) ~36 Bq/kg …1/30 of Previous Crystals (14±5 Bq/kg ;Best)Th-chain(220Rn) ~29 Bq/kg …1/3 of Previous Crystals ( 6±1 Bq/kg ;Best)

CaF2 Powder

FusedCaF2

Raw MaterialsCaCO3, HF

CaF2 Crystal

Powder selection Crystal growing

CaF2(Eu) in ELEGANT VI U-chain(214Bi) :1100

Bq/kg Th-chain(220Rn) : 98

Bq/kg

101 crystals

U and Th (ICP-MS)

Pulse Shape Discrimination

• PSD (Event by Event Analysis)PSD (Event by Event Analysis)– FADC (100MHz)– Fit with Two Exponential Function

(fixed time constants)– Ratio ; Afast/Aslow

　　　 (Intensity of Fast and Slow Component)

between and rays

Clear Discrimination between αand γ(β) Events　　 Background Rejection Efficiency > 99.7%