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Study of Double Beta Decay of 48 Ca 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 48 Ca. ■Largest Q value (4.27 MeV) next largest; 150 Nd (3.3 MeV) almost background free - PowerPoint PPT Presentation
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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%