1. Double Beta Decay of 48Ca with CaF2(Eu) - ELEGANT VI -
Saori Umehara
ELEGANT VI Collaboration
Department of Physics, Osaka University
T. Kishimoto, I. Ogawa, R. Hazama, H. Miyawaki,
S. Yoshida, K. Mukaida, K. Matsuoka, Y. Ishikawa, K. Kishimoto,
A. Katsuki, H. Sakai, D. Yokoyama, S. Tomii, Y. Tatewaki,
T. Kobayashi, A. Yanagisawa

2. Double Beta Decay of 48Ca with CaF2(Eu)
Outline
Double Beta Decay of 48Ca with CaF2(Eu)
ELEGANT VI System
Detector Design (4p active shield)
Backgrounds
Background Origins
Performance of Background Rejection by FADC
Double Beta Decay Measurement
Analyses for Event Selection
Half Life of 0nbb
Summary
14th Sep 2007, Saori Umehara

3. ELEctron Gamma-ray Neutrino Telescope
ELEGANT VI
ELEGANT VI
ELEctron Gamma-ray Neutrino Telescope
Schematic Drawing of ELEGANT VI
CaF2(Eu)
CaF2 Scintillator (CaF2(Eu))
23 Crystals(45×45×45cm3:290g)
Source of bb Decay : 48Ca
(Qbb=4.27MeV)
veto counters
46 CaF2(pure)
38 CsI(Tl)
→　4p Active Shield
Passive Shields
for g-ray
Cu : 5cm,Pb : 10cm
for Neutron
LiH+Paraffin :15mm
Cd sheet : 0.6mm
H3BO3 loaded water
CaF2(pure)
CsI(Tl)
Signal
readout
from same PMTs
LiH Cu
Air-tight Box Pb
14th Sep 2007, Saori Umehara

4. ELEGANT VI Scintillators in ELEGANT VI System CaF2 Module PMT PMT
CaF2(pure)
. . . UV Light
CaF2(Eu)
. . . Visible Light
CaF2 Module
CsI(Tl)
Active Shielding Technique
in ELEGANT VI system
14th Sep 2007, Saori Umehara

5. Active Shielding Technique
Discrimination between CaF2(Eu) and CaF2(pure)
Light Transmission from each Scintillator
Visible Light
Light from CaF2(Eu)
PMT
PMT
Left PMT VL
CaF2(pure)
CaF2(Eu)
CaF2(pure)
Right PMT VR
Light from CaF2(pure)
Absorption with CaF2(Eu)
PMT
PMT
UV Light
Distribution
of Roll-off Ratio
CaF2(Eu):
not transparent
to UV light
2000
1500
1000
500
CaF2(Eu)
Energy(keV)
VR-VL
Roll-off Ratio =
VR+VL
CaF2(pure)
CaF2(pure)
VL = Charge
from Left PMT
VR = Charge
from Right PMT
Roll-off Ratio
Clear Discrimination
between CaF2(Eu) and CaF2(pure)
. . .well act as veto counter
14th Sep 2007, Saori Umehara

6. Oto Cosmo Observatory ELEGANT VI bb Decay of 48Ca Dark Matter Search
with CaF2(Eu)
Oto Cosmo Observatory
Cosmic Ray /m2/sec
(1/10000 of sea level lab.)
Neutron /cm2/sec
Rn Bq/m3
Japan
Oto Cosmo Observatory(1400m.w.e.)
EL845m
500m
EL435m
PICO-LON
ELE-VI
EL323m
29/1000
14th Sep 2007, Saori Umehara
5036m

7. Backgrounds Background Origins in ELEGANT VI System
External Backgrounds
. . . strongly limited by 4p active shield
Internal Backgrounds within CaF2(Eu) crystals
Background Process
U-Chain b a
214Po
238U
214Bi
210Pb
206Pb
stable
Qb = 3.27MeV
Qa = 7.83MeV
T1/2 = 164msec
Th-Chain b a
212Po
232Th
212Bi
208Pb
stable
64%
Qb = 2.25MeV
Qa = 8.95MeV
T1/2 = 0.299msec
Pile-up Events
b+a
because . . .
CaF2(Eu) decay constant
: ~1ms
Emax = 4.7MeV for U-chain
4.3MeV for Th-chain
(Qbb = 4.27MeV)
Pile-up Events Characteristic Background Origins
for ELEGANT VI System
14th Sep 2007, Saori Umehara

8. Previous Result and Background Study
First Result from ELEGANT VI
From 1998 to 1999
Simulated Background
Energy Spectra
Measured Energy Spectrum
100
Experimental data
total
Qbb of 48Ca
Counts 10
212Bi
BG (sim)
Pile-up
Event 1
0nbb window
214Bi
10-1
2nbb (sim)
208Tl
10-2
3000
4000
5000
Energy(keV)
Pile-up Events
Observed Events : 0
Expected BG : 1.3
Half-Life : >1.4 × 1022 year
(90% C.L.)
Ref: I. Ogawa et al.,
NPA730(2004)215
Main Background Origins
94% of expected BG at Qbb region
Rejection by FADC
14th Sep 2007, Saori Umehara

9. Rejection of Pile-up Events
T1/2 = 0.299msec
Th-Chain
decay constant of CaF2(Eu)
：~1msec b a
232Th
T1/2 = 1.1 x 1010year
212Bi
Qb=2.2MeV
212Po
Qa=7.8MeV
64%
Prompt
Delayed
Typical Pulse Shape(100MHz FADC)
with long time difference
with short time difference
2ms
Clearly identified
as Pile-up events
60ns
Delayed
Clearly identified
as Pile-up events
Prompt
Background Rejection by 100MHz FADC
　　DT > 30ns(3ch)
14th Sep 2007, Saori Umehara

10. Data Analysis for bb Study
Run Summary for bb Measurement
Date
Analysis
Live Time
kg・day
Jun1998-Nov1999
without FADC
233 day
1553 kg・day
Jan2003-Dec2003
with FADC
183 day
1114 kg・day
Jan2004-Sep2006
463 day
2280 kg・day
FADC
installed
Trigger system
improved
for Dark Matter
Search
I. Ogawa et al., Nucl. Phys. A730(2004)215
This Talk . . .
Data Analyses with FADC
Data from 2003 to 2006
Obtained Half-Life for 0nbb Decay of 48Ca
Data from 1998 to 2006
14th Sep 2007, Saori Umehara

11. Event Selection(1) Event Selection(1) . . . Base Analysis
No hit with CsI(Tl)
Hit with only one CaF2
No hit with CaF2(pure) < Roll-off Ratio < 0.3
Position Dependence of Roll-off Ratio
Roll-off= +0.3
CaF2(Eu)
Roll-off= -0.3
CaF2(pure)
(bar = s)
14th Sep 2007, Saori Umehara

12. Event Selection(2) Pile-up Event Rejection by 100MHz FADC
Total energy of pile-up event > 3MeV
Time Difference
Energy Spectrum
of Delayed Events
Energy Spectrum
of Prompt Events
Counts
212Bi Qb
272±10nsec
Pile-up Event
Consistent
214Bi Qb
212Po
214Po
Time(nsec)
Energy(keV)
Energy(keV)
212Po:T1/2=299nsec
214Po:T1/2=164msec
212Po a:Eee=2.0MeV
214Po a:Eee=1.6MeV
212Bi b:Qb=2.2MeV
214Bi b:Qb=3.3MeV
Pile-up Event
Pile-up Event Rejection
Th-Chain b a
Effectively Rejected
Rejection Efficiency :
94% for Th(T1/2=0.3msec)
99.9% for U(T1/2=164msec)
232Th
212Bi
212Po
208Pb
stable
64%
14th Sep 2007, Saori Umehara

13. Energy Resolution Energy Resolution of a Peak #10
Measuring Live Time : days
Source : a rays from contamination
in crystals itself
(216Po a-ray 6.78MeV)
6.9 %
FWHM
Energy Resolution
% at 1.3 MeV with each CaF2 module
→　Corresponding to 4-6 % at Qbb region
14th Sep 2007, Saori Umehara

14. Preliminary Preliminary Double Beta Decay Energy Spectrum
after FADC installed
Live Time: days・kg
Energy Spectra after Event Selections
(Multi Hit Cut)
Energy Spectra after Event Selections
(Multi Hit Cut +FADC Cut)
Experimental Data
Experimental Data
214Bi
(Sim)
212Bi(Sim)
212Bi
(Sim)
Preliminary
Preliminary
Qbb of 48Ca
208Tl(Sim)
208Tl
(Sim)
No events in 0nbb Energy Window
14th Sep 2007, Saori Umehara

15. Double Beta Decay Half-Life for Neutrino-less bb Decay of 48Ca
Run Summary
Date
Analysis
Number of Event
Expected BG
(212Bi,214Bi,208Tl)
Detection Efficiency
Live Time
kg・day
Jun
without FADC
1.30
0.55
1553
Jan
with FADC
0.27
0.53
1114
Jan
0.43
2280
No events in the energy region
0nbb Half-Life of 48Ca : >6 × 1022 year (90% C.L.) Preliminary
Previous Result : >1.4 × 1022 year (90% C.L.)
Ref: I. Ogawa et al., NPA730(2004)215
14th Sep 2007, Saori Umehara

16. Future Perspective (Next Project)
ELEGANT VI → CANDLES
No event not limit detector sensitivity
by small natural abundance 0.187%
ELEGANT VI
CaF2(Eu)
CANDLES III
CaF2(pure)
Mass
6.7kg
300kg
Radioactivities in Crystal(Average)
1.2mBq/kg(U)
0.11mBq/kg(Th)
0.039mBq/kg(U)
0.026mBq/kg(Th)
Background Rejection
Pile-up Event Rejection(94%)
Pile-up Event Rejection(98.8%)
Pulse Shape Discrimination
between a and g rays(99.7%)
Sensitivity for Neutrino Mass
0.5eV
(~3 ×1024 year)
Achieved
×45
1/30 for U
1/4 for Th
BG Rej.
=1/1000
Achieved
with prototype
detector
CANDLES
14th Sep 2007, Saori Umehara
Scale-up Detector from ELEGANT VI

17. Summary ELEGANT VI System Double Beta Decay Experiment
CaF2(Eu) system for bb decay study
4p active shield by CaF2(pure) and CsI(Tl) scintillators
Active shield technique
CaF2(Eu) . . .not transparent to UV light
Main backgrounds around Qbb-value region Pile-up events
FADC Installed for background rejection
Double Beta Decay Experiment
at Oto Cosmo Observatory
Live Time : 4947 kg・day
No events at Qbb-value region
Half-Life : 6 × 1022 year (Preliminary)
14th Sep 2007, Saori Umehara