1. Sr-84 0n EC/b+ decay search with SrCl2 crystal
Self Introductions. Talking about the today’s topics and simple contents of my talk.
International Workshop on Double Beta Decay at Seoul National University
J. H. So (KNU)

2. What is the 0n EC/b+ decay ?
Q-value : 2400-keV
Double b decay
Double b+ decay
2n DBD mode
0n DBD mode
b+ decay has three decay mode, such as EC/EC for less than 1-MeV Q-value,
EC/ b+ for 1-MeV to 2-MeV Q-value, b+/b+ for more than 2-MeV Q-value.
EC/EC, EC/b+, b+/b+ had never been observed until today. 2

3. Experimental Method (Passive method)
Tracking detectors : foil + tracking + detector
Source
Detector
511-keV gamma
Source ≠ Detector
High resolution gamma detector to tag 511-keV gamma from EC/b+, b+ b+ elements
Ex) HPGe detector
1. Good energy resolution.
2. Low efficiency (The effective Z number is relatively low.)
3. Impossible to separate 0n, 2n process.

4. Experimental Method (Active method)
511-keV gamma
Source = Detector
511-keV gamma
Using the active detector made of rare decay candidates.
Ex) Crystal scintillators
1. Worse energy resolution.
2. Higher efficiency (The effective Z number is relatively higher.)
3. Possible to separate 0n, 2n process.
We used SrCl2 crystal scinitllator for active method with 4p CsI tagging detector.

5. Active method with 4p tagging system
We could take triple coincidence signal by using the 4p tagging system.
Source = Detector
511-keV gamma
4p CsI tagging detector

6. Potential candidates for 2b+ decay
Nuclide
Decay channel
Qββ, (keV)
Natural
abundance
Decay mode
Half-life limits (yr)
50Cr
Εβ+
1171
4.345
0v+2v
1.8*1017
58Ni
Εβ+,2E
1925
68.07
7*1020
4.0*1019
84Sr
1786.8
0.56 0v
7.3*1013
96Ru
2β+,Eβ+
2719
5.54
3.1*1016
6.7*1016
106Cd
2β+, Eβ+
2771
1.25
2.4*1020
3.7*1020
112Sn
1922
0.97
6.1*1013
124Xe
2β+,kβ+
2865
0.09
4.2*1017
1.2*1018
136Ce
2400
0.185
6.9*1017
3.8*1016
78Kr
2866
0.35
2*1021
5.1*1021
92Mo
1649
14.84
1.9*1020
3.0*1018
EC/EC and EC/b+ decay mode is possible in 84Sr.

7. Development of SrCl2 crystal scintillator
Pure SrCl2 single crystal grown by CZ-method at KNU.
Cut and polished in the dimensions of Φ20ⅹ15 (㎟).
The total weight is 16.3 g with 0.05 g of 84Sr.
Gul Rooh. et al.
IEEE ; Tran. Nucl. Sci. VOL.55 (2008)
The light output of SrCl2 single crystal is 77% of the light output of the CsI(Tl).
Fast decay time component is 91 ns (12%) and slow decay time component is 907 ns(88%).

8. YangYang underground Laboratory ; Y2L
Y2L was established by KIMS (Korean Invisible Matter Search) group for WIMP Search.

9. Installation of 4p CsI tagging detector

10. 4p CsI tagging detector No No
Yes
511keV
511keV
511keV
511keV
511keV
511keV
Fig:1
Fig:2
Fig:3
Put the SrCl2(EC/b+) inside of the 4p CsI(Tl) tagging detector.
Annihilation of b+ produces a pair of 511-keV back to back gamma rays.
Using coincidence technique to tag these signals.
Signal in figure 3 is of the choice.

11. Experiments (4p Schematics)

12. Experimental Setup • High Voltage ☞ -670 V for SrCl2
☞ ~ V for 4 π CsI tagging detector
• Trigger pulse width : 200 ns
Threshold : 25 mV
For SrCl2
Preamp
400MHz FADC PC
For 4 π CsI(Tl)
64MHz FADC

13. Calibration ; SrCl2
Number of counts
Arbitrary channels
22Na gamma ray source was used for energy calibration of SrCl2 and 4p CsI
tagging detector.
The FWHM resolution was 35% at 511-keV.

14. Calibration ; CsI CsI #10 CsI #9 CsI #8 CsI #7 CsI #11 CsI #14 CsI #6

15. Efficiency of 4p CsI tagging system
Front
Back
It is very complicate to consider about back to back coincidence between each
CsI channels.
Only considering for front and back coincidence, and then the efficiency was
12.6 % for these two channels.

16. Background reduction Number of counts 766-keV keV Raw data
0 Hit events : Inner background
(Not for bg reduction)
2 Hits events
2 Hits and back to back coincidence
Number of counts
766-keV
keV

17. Results The half-life limit was 4.6 x 1017 years.
(The previous result was 7.3 x 1013 years.)
24 events
We could obtain 224 events with 511-keV back to back gamma coincidence
on total energy range.
There were 24 events on 766-keV with 35% of energy resolution.
(which is our interesting region)

18. Summary and Future Plans
We set up the 4p CsI tagging detector for the search of 0n EC/b+ decay of 84Sr at Y2L and the data was taken for 60 days.
4p CsI tagging system is very useful to reduce the background for the 0n EC/b+ decay search with 84Sr.
We could obtain 4.6 x 1017 years of half-life limit for 0n EC/b+ decay in 84Sr.
This system requires more analysis steps, such as hit position correlation and tag time correlation for better sensitivity.

19. Inner background study
G4 simulation
Inner background of SrCl2
Number of Counts
Number of Counts
MeV
keV
We tried to make a Monte-Carlo simulation for inner background study.
-90Sr makes two b background.(546-keV, 2282-keV)
Inner background study with real data
Data taking time : 60 days.
Background spectrum without any signal in 4p CsI tagging detector.