Study of 48Ca Double Beta
Decay with CANDLES
OGAWA, Izumi(小川
泉)
for the CANDLES collaboration
July 3, 2009
TAUP2009
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Double beta decay of
48Ca
Largest Q value (4.27 MeV)



next largest; 150Nd (3.3 MeV)
large phase space factor
almost background free (g: 2.6 MeV, b: 3.3 MeV)
Low Natural abundance → 0.187%


large detector
enrichment
Next generation detector : fight against BG!


July 3, 2009
m  T 1/ 2 
Mdet-1/2
m  T 1/ 2  Mdet-1/4
TAUP2009
if background free
if background limited
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CANDLES
CAlcium fluoride for studies of Neutrino and Dark matters
by Low Energy Spectrometer
undoped CaF2 (CaF2(pure))



48Ca
(Qbb=4.27 MeV)
Atten. length > 1 m
Low radioactive impurities
Low background detector



4p active shield (LS)
Passive shield (Water, LS)
Pulse shape information
Good energy resolution


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large photo-coverage
Two phase LS system
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BG reduction / rejection
ー 4p active shield ー
July 3, 2009
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CANDLES I
- POP (Proof of Principle) Detector
CaF2(pure) crystal (~1 msec)
in liquid scintillator (~10 nsec)
(with w.l. shifter)
viewed by 4 PMTs (5 inch)
LS : mineral oil
+ DPO
(3 g/l)
+ Bis-MSB (0.3 g/l)
PMT(5")
~ 4
liquid Scintillator
CaF2(pure)
4p active shield
July 3, 2009
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Performance Test (4p active shield)
2 ADCs with
different gate width
40K
events from CaF2
Clear separation
between CaF2 and LS
July 3, 2009
events from LS
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BG reduction / rejection
ー Energy resolution ー
(BG from 2bb events)
July 3, 2009
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Improve light collection efficiency
Keep high transparency for both(CaF2(UV),
LS(vis.)) scintillation light

CaF2 crystal, LS, pure water, acrylic vessel,…
Undoped CaF2 (attenuation length > 1m)
 cf. CaF2(Eu) ～10 cm

Shift wavelength of scintillation light from CaF2
scintillators; UV ⇒ visible
Large photo-coverage

Large (13,17 inch) PMT
July 3, 2009
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Two Phase System
Concept of Method

Conversion Phase
 large conversion eff.
CaF2(Pure)
 good transparency for UV

Veto Phase
 large light output with
aromatic solvent
(absorb UV light)
 good transparency for
visible light
CaF2 Emission (~285nm)
Conversion by WLS (350~400nm)
propagate
PMT
July 3, 2009
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Performance of two phase system
PMT(5")×4
Liquid Scintillator for
Conversion Phase
conversion
phase only
137Cs
(5 cm thickness)
CaF2(pure)
PMT(5")×4
Liquid
Scintillatorfor
for
Liquid Scintillator
Conversion
Phase
Conversion Phase
5 mm
Two Phase
system
no
reduction
Liquid Scintillator for
Veto Phase
July 3, 2009
Liquid Scintillator for
Veto Phase
CaF2(pure)
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BG reduction / rejection
ー Internal BG (U, Th)ー
July 3, 2009
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Succesive decays in CaF2 scintillator
BG

Qb = 3.27 MeV

Successive decays in U, Th
b
Pb
214Bi
200
(U)
Bi
214
214
250
Pulse Height(10 mV/ch)

b
214
Po
164.3 ms

99.98%
19.9 m

0.02%
26.8
. m
Tl
Q = 7.83 MeV
b
210
b
Pb
22.3 y
210

1.30 m
150
212Bi
100
Qb = 2.25 MeV 212
(Th)
b

50
0
212
Pb
50 100 150 200 250 300 350 400
Time(10 nsec/ch)
Bi
212
b
July 3, 2009
64 %
Q

60.55 m

36 %
10.64 h
Tl
208
3.053 m
208Tl

Po
0.299 m s
= 8.95 MeV
208
b
Pb
stable
Qb = 5.00 MeV
bg
Emax=5.8 MeV (U)
5.3 MeV (Th)
: b－g
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Development of High Purity
CaF2 Crystals
CaF2(Eu) in ELEGANT VI
U-chain(214Bi) :1100 mBq/kg
Th-chain(220Rn) : 98 mBq/kg
U and Th
(ICP-MS)
Raw Materials
CaCO3, HF
CaF2
Powder
Radioactivities in CaF2 Powder
(HPGe measurement)
101 crystals
Fused
CaF2
CaF2
Crystal
Radioactivities in CaF2(pure) Crystal
(-ray measurement)
Powder selection
Crystal growing
U-chain(214Bi) ~36 mBq/kg …1/30 of Previous Crystals (14±5 mBq/kg ;Best)
Th-chain(220Rn) ~28 mBq/kg …1/3 of Previous Crystals ( 6±1 mBq/kg ;Best)
July 3, 2009
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Rejection of Double Pulse(DP)
810ns
Delayed
Prompt
pulse height (ch)
pulse height (ch)
Typical Pulse Shapes
8.8ns
time(2 nsec)
PMT
PMT
time(2 nsec)
LinFI/O
PM AMP
recorded shape
fitted shape
FADC
‘Dirty’ CaF2 crystal
July 3, 2009
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Pulse Shape Discrimination
Pulse Shape discrimination

Shape Indicator (PRC 67(2003) 014310)
Difference in decay shape
between  and g rays
-event
gevent
1s
mean value:
no energy dependence (>1 MeV)
July 3, 2009
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CANDLES III
July 3, 2009
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CANDLES III (prototype)
Constructed at Osaka Univ. (sea level)


small version for R&D
check the performance of CANDLES
CaF2 modules


103 cm3 × 60 crystal; 191 kg
with conversion phase
Liquid scintillator

f1000×h1000
1 “calibration” crystal (#60)
(High Contamination in U, Th)
65 mBq/kg (U-chain),
28 mBq/kg (Th-chain)
acrylic container
H2O Buffer : passive shield

f2800×h2600
PMTs


15” PMT (× 8) : R2018
13” PMT (×32) : R8055
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h 2.6 m
CANDLES III (prototype)
f 2.8 m
Reference
crystal (#60)
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CaF2 module
CaF2 + conversion phase + acrylic case
filled
half filled
Index 1.44@586nm (CaF2)
Index 1.46@586nm (Mineral Oil)
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LS tank
60 CaF2 modules
installed
July 3, 2009
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Rejection of LS Events
Rejection by using Pulse shape information

Typical Pulse Shapes
CaF2
Charge Ratio =
July 3, 2009
CaF2+
Liquid Scintillator
Liquid Scintillator
charge in partial gate
charge in full gate
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Identification of CaF2 signal (2)
Position reconstruction

Total charge of each PMT
X
X-Z
July 3, 2009
Z
X-Y
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Event Rate dependence on z-axis
Background Rate near Q-value for each
CaF2 Crystal
July 3, 2009
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2nd layer
after rough ratio cut
(LS events are rejected)
1st layer

3rd – 5th layer
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Obtained Energy Spectra
@sea level laboratory
LS veto & position cut
DP rejection & PSD
not applied
Counts/50 keV/day/crystal

@ Surface lab.
remaining BG
If all originate from
cosmic ray…
×10-5 @ underground lab.
Lower layer
(#25 - 59)
further study
origin and process of BG
July 3, 2009
Calib. crys.
(#60)
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Energy (MeV)
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CANDLES III (U.G.)
@Kamioka
July 3, 2009
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CANDLES III(U.G.)
CaF2(pure)

103 cm3 × 96 crystals; 305 kg
Liquid scintillator


PMT
two phase system
Purification system
H2O Buffer

passive shield (larger tank)
PMTs


17” PMT (×14) : R7250
13” PMT (×56) : R8055
liquid scintillator
photon trans. simulation
energy res. ~4.0 % @ Qbb
Kamioka underground lab.
CaF2
pure
water
φ 3000.00
July 3, 2009
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conversion
phase
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Kamioka new exp. room
experimental room D
San-chika
room C
XMASS
SK
KamLAND
July 3, 2009
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CANDLES III (U.G.)
July 3, 2009
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R&D
for future large detector
July 3, 2009
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To reach IH mass region
Enlarge the detctor

Purification of CaF2 crystals ; <1 mBq/kg
further R&D is underway
Enrichment of

48Ca
Chemical processing with Crown Ether
July 3, 2009
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Crown Ether
Held by electrostatic
attraction between
negatively charged Oof the C-O dipoles &
ion (Ca2+)
How well the ion fits
into the crown ring
Liquid (aq-salt)-liquid
(org-crown) extraction
in isotopic equilibrium
July 3, 2009
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Crown Ether
C
C
- -O C
C
O- Ca2+ - O
O
C
C O
- -O
C C
Dicyclohexano
18-crown-6
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Chromatography
1m Glass Column
Experimental Setup
Breakthrough method
Ca Solution
CaCl2
+ HCl
Measurement of
• Ca concentration
• isotopic ratio
fixed flow rate
by pump
Crown-ether resin
packed in column
Water pump
Sampling
by fraction collector
July 3, 2009
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Water
thermo. bath
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Summary
CANDLES project

Study of
48Ca
double beta decay
CANDLES III @Osaka University


R&D study for underground experiment
further study of BG
CANDLES III(U.G.) @Kamioka


Under construction
Expected BG: 0.18 ev/year
R&D efforts for future large detector


purification of CaF2 crystal
enrichment of 48Ca
 Optimizing the parameters (temp., flow rate, …)
July 3, 2009
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CANDLES Collaboration
Osaka U. （大阪大学）
T. Kishimoto, I. Ogawa, S. Umehara, K. Matsuoka, Y. Hirano, Y. Tsubota, G. Ito,
K. Yasuda, H. Kakubata, M. Miyashita, M. Nomachi, Y. Kohno, M. Saka, S. Ajimura
Fukui U. （福井大学）
Y. Tamagawa, T. Hayashi, Y. Maekawa, S. Isogai, T. Sato, T. Jinno
Hiroshima U. （広島大学）
R. Hazama
Kyoto Sangyo U. （京都産業大学）
K. Okada
Saga U. （佐賀大学）
H. Ohsumi
Tohoku U. （東北大学）
S. Yoshida
Tokyo Institute of Technology （東京工業大学）
Y. Fujii
U. Tokushima （徳島大学）
K. Fushimi
July 3, 2009
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