REDUCTION OF TL-208 BACKGROUND FOR
ZR-96 NEUTRINOLESS DOUBLE BETA DECAY
EXPERIMENT USING TOPOLOGICALINFORMATION OF CHERENKOV LIGHTS
41st International Conference on High Energy Physics
ICHEP2022

July 9, 2022
Grant-in-Aid for Scientific Research on Innovative Areas
19H05093 and 20H05241
Scientific Research (B) 22H01243

Miyagi University of Education

Y. Fukuda, T.Shimizu

Kamioka Observatory, ICRR, Univ. of Tokyo S. Moriyama, K. Hiraide
University of Fukui I. Ogawa
Tokyo University of Science T. Gunji, S. Tsukada, R. Hayami
Institute for Materials Research, Tohoku University S. KurosawaConceptual design of ZICOS detector
Phys.Rev.Lett. 117 (2016) 082503

0𝜈𝜈
NEMO3 : 𝑇𝑇1/2> 9.1 × 1021 yrs

96Zr : 45 kg (nat.) → 865 kg(50 % enrich)→1/20 BG

0𝜈𝜈
𝑇𝑇1/2> 4 × 1025 yrs → 2 × 1026 yrs → ~1 ×1027yrs

Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

2

Discrimination of signal and BG
0νββ event
e-Reconstructed vertex byescintillation lightβ decay
2.6 MeV γ
Reconstructed vertex
by Cherenkov lightBalloon or
surface ofdetector
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

e-July 9,2022

3

Verification of 208Tl background reduction
using Cherenkov lights
1. Selection of PMTs which receive Cherenkov
lights among huge Scintillation lights.
• Pulse shape discrimination
2. Confirm topology of Cherenkov lights• Directionality of Cherenkov lights• Direct measurement of topological information
averaged angle
3. Demonstrate BG reduction using beta-gamma
sources with topological information (averagedangle) of Cherenkov light.
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

4

Pulse shape of Cherenkov and scintillation
arbitary scale• Pulse shape of 90Sr using
H2431-50 measured byV1751 with DES mode
(2GS/s)• Decay time of scintillation :4.57 ns and 8.38 ns• Rise time of scintillation :1.45 ns• Rise time of Cherenkov :0.75 ns
Use the charge ratio Qtime/Qtotal. Here, Qtimeis FADC
count in each time, and Qtotalis sum count of FADCbetween 55 ns and 80 ns.
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,20225

Measured by Compton edge event and BG sample

χ2 ＜0.1(scintillation like)
403/3970 = 10.4±0.5% forCompton edge event

1772/3606=49.1±1.4 %for BG sample
The difference between
Compton edge events andBG sample is 2.9 σ.
Topology of Cherenkov
lights for ~1 MeV e- wasstrongly indicated.

Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

6

Direct measurement of topologicalinformation using HUNI-ZICOS

2645%

Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

7

Electron with fixed direction and fixedenergy using 88Y gamma source
Average direction=- Σ diedi θiTopological information of Cherenkov lights
could be presented as averaged angle.
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

Averagedangle= Σ θiNhit
July 9,2022

8

Setting hemisphere flask to jig and locate
on supporting stand HUNI-ZICOS was putted on flask
clip and the chimney was pinchedby clamp.
 Assuming e- generated position to be center of the truncatedicosahedron jig (not hemisphere flask).

Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

9

Measured averaged angle and simulation•••

Averaged angle measured by HUNIZICOS has a peak at 40 degree. Thisis due to PMT geometry of HUNIZICOS detector.
Averaged angle obtained by EGS5
simulation of Cherenkov light has apeak at 40 degree. This is consistent
with above measurement.
Averaged angle obtained by EGS5
simulation of scintillation has a peakaround 50 degree. This is quite
different from Cherenkov light.

Verified Cherenkov lights emitted
from 1.484 MeV electron reallymaintain their topology.
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,202210Demonstration of 208Tl BG reduction

Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

11

Demonstration of 208Tl BG reduction

Due to March earthquake in Japan, we lost 5 PMTs.
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

12

Setup for the measurement using UNI-ZICOS

Source holder
has a hole, and it
can be coveredby Al plate toterminate electron.
The vertex position could be reconstructed by assuming that all PMTs should havesame effective charge which is corrected by the distance between PMT and vertex.

Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

13

Observed averaged angle using FDFE events
•
•
•

•

•

The vertex position was reconstructedby charge information as explained.
Extracted PMTs which include
Cherenkov light by PSD technique.
Using those PMTs and the vertex, theaveraged angle of fixed direction fixedenergy electron from 88Y source wasmeasured, and a peak was foundaround 50 degree.
The averaged angle obtained by MC
simulation has a peak around 48degree. Within the statistical error, thepeak position was almost reproduced.
Measurement time was limited byMarch earthquake in Japan.

Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

14

Events selection for beta-rich samples

Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

15

Observed averaged angle using β−γ events••••
preliminary

Averaged angle is obtainedby z>0 and by subtracting β+γand γ dominant data.
Obtained averaged angle
looks two peaks around 50and 60 deg.
Events should consists not
only β+γ events but β onlyand γ only events.
Averaged angles of MC forβ+γ and β only events wouldreproduce those data.

It would be possible to reduce β+γ evens suchas 208Tl backgrounds using the averaged angle.
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment usingtopological information of Cherenkov lights

July 9,202216

Observation of 2νββ signal using 2ν-ZICOS• 20 cm diameter flask
using Ultra-pure quartz
and 31 low ＢＧ 2” PMT
Hamamatsu R3378-50(R2083) or SiPM arrays• Filled 3L of ZICOS-LS• Loaded 300g Zr(iPrac)4
which contains 1.1 g96Zr• 200 2νββ events peryear is expected• Location: Kamioka mine
• Start time: FY2024
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,202217Observation of 2νββ signal using 2ν-ZICOS• 20 cm diameter flask
using Ultra-pure quartz
and 31 low ＢＧ 2” PMT
Hamamatsu R3378-50(R2083) or SiPM arrays• Filled 3L of ZICOS-LS• Loaded 300g Zr(iPrac)4
which contains 1.1 g96Zr• 200 2νββ events peryear is expected• Location: Kamioka mine
• Start time: FY2024
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,202218

Summary••••Topological information (averaged angle) of Cherenkov
light was directly measured by UNI-ZICOS using fixed
direction / fixed energy (1.484MeV) electron from 88Y
source, and the averaged angle has a peak around 48
degree as expected by MC simulation. This is consistent
with the Cherenkov angle.
The averaged angle of β+γ events from 60Co source wasmeasured by UNI-ZICOS, and the averaged angle has apeak around 60 degree as expected by MC simulation.
We would be able to reduce β+γ event such as 208Tl BG
using the averaged angle.
Next program to observe 96Zr 2νββ decay will start inFY2024 after the screening of low BG materials, thesynthesis of Zr(iPrac)4, and the construction of low BG
environment using Pb shield in Kamioka mine. Stay tuned!
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

19

backupReduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

20

BG reduction using topological information
Average direction=- Σ diedi θi

Simulated by EGS578%
remain93%
cut

Averagedangle= Σ θi
Nhit
Topological information (averaged angle) of Cherenkov
lights could be used for reduction of 208Tl backgrounds.
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,202221

Qtotal distribution for γ sources••••

60Co (1.17 MeV/1.33 MeV)

Compton edge： 1.04 MeV
137Cs (662 keV)
Compton edge： 478 keV
133Ba (356 keV)
Compton edge： 207 keV
57Co (122 keV which is underCherenkov threshold“168 kev”)

Pulse shape with charge
ratio in each FADC time.
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using

July 9,202222χ2distribution using 57Co template• Most of backgrounds
have lower χ2 than 1.0• Most of backgrounds
have lower energy than
Cherenkov threshold,then only scintillation
was seen.

It seems to events with
Cherenkov lights shouldhave large χ2 value.
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using

July 9,2022

23

Charge ratio in rise time using ZICOS LS•••

There is difference inshape between t = 57 nsand 58 ns
Charge ratio looks
depend on the energy
For t > 58.5 ns, all shapes
were almost same.
Cherenkov looks dominant
between 57 ns and 58 ns.
Template waveform ofscintillation between 57 ns
and 58 ns for 57Co.

Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using

July 9,2022

24

Mounting PMTs on jig for hemisphere flask

 Total 26 H3164-12 PMTs
were used for HUNI-ZICOS.
 In order to remove
scintillation light (~300nm),
SC-37 filter was coveredaround the hemisphere flask
in this time.
July 9,2022
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

25

Cable connection to FADC and HV
LeCroy 1182 ADC

All channels are connected
to FADC V1742 digitizer.
Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

26

Measurement of averaged angle using 88Y
La-GPS88Y• Scattering gammas
were detected by wellcalibrated La-GPS
scintillator through thePb collimation.

Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,202227

Event selection for scattered electron
Event selection criteria :••••Number of PMT hit for BG areclustered below 5 hit.
1) Nhit ≧ 5
Qtotal for BG are also
clustered below 1000.
2) Qtotal ≧1000
Observed energy of scattering
gamma is clustered around352keV.
Due to SC-37 (UV cut) filter,
we observed almost only
Cherenkov light in this time.

Reduction of Tl-208 background for Zr-96 Neutrinoless Double Beta Decay experiment using
topological information of Cherenkov lights

July 9,2022

28