ZICOS - Neutrinoless Double Beta Decay experiment using Zr-96 with an organic liquid scintillator The XXIX International Conference on Neutrino Physics and Astrophysics (NEUTRINO 2020)
22 June – 2 July 2020 (online-only)
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Y.Fukuda , D.Anzai , Y.Kamei , Narengerile , A.Obata , K.Hiraide , S.Moriyama , I.Ogawa , T.Gunji , S.Tsukada , R.Hayami , S.Kurosawa
a Miyagi Univ. of Education b Kamioka Observatory, ICRR, Univ. of Tokyo c Univ. of Fukui dTokyo Univ. of Science eRCCN Tohoku Univ. fIMR Tohoku Univ.

Supported by Grant-in-Aid for Scientific Research on Innovative Areas 20H05241,19H05093 and Scientific Research (C) 18K03664

1.

ZICOS experiment◆ Detector design for ZICOS experiment

2. How to reduce backgrounds

3. How to discriminate Cherenkov lights◆ Main backgrounds around Q-value◆ Pulse shape discrimination using FADC and PMT0νββ event

Phys.Rev.Lett. 117 (2016) 082503

CAEN V1751
Specification• 4/8 channel• 10bit ADC• 2GS/s sampling• Self/Externaltrigger

e2.6MeV γe-β decay
Detector :1)180tons LS : 1.5 wt.% Zr
and 5wt.% PPO in Anisole .
2)Need 500 of 20” PMT withhigh QE ~0.4 and TTS ~0.3ns@1pe for 64% photo
coverage.

Expected performance :1)Energy resolution ~2.8%@3.35MeV2)T1/2(0νββ) > 1027years if both 1/20
BG reduction and 50% 96Zr
enrichment could be achieved.
3)Start experiment ~2027.

◆ Neutrino mass sensitivity for ZICOS experiment・Q-value of 96Zr : 3.35MeV Abundance : 2.80%
・Total mass : 180ton (fiducial volume : 113ton)
・Measurement time: 2years
・10wt.% of Zr(iPrac)4 = 12.6ton of Zr(iPrac)4 includes 1.7ton of Zirconium= 45 kg of 96Zr
T1/20ν >4×1025y← Not enough for 0νββ search◆ Requirements in order to realize 0νββ GEN-III experiment
1) 50% enrichment of 96Zr (e.g. 57.3% for NEMO-3)
then 96Zr will be 865kg
T1/20ν > 2 ×1026y
2) 208Tl background reduction
BG level < 1/20 ×KL-Zen
T1/20ν > 1 ×1027y◆ Development of Zr loeaded Liquid Scintillator

Reconstructed vertex
by scintillation light

KamLAND-Zen observed
208Tl β-decay backgroundaround Q-value of 96Zr.

Surface ofBalloon

Zr-LS: Zr(iPrac)410wt.%, PPO 5 wt.%
and POPOP 0.2wt%
solved in Anisole.

1) Light yield : 48.7 ±7.1% of BC5052) energy resolution :13.0±2.0%√(64%/9.2%)X(3.35MeV/1.03MeV)= 2.7±0.4% at 3.35MeV (6.4% FWHM)

Need to measure real energy resolution• Measured by V1751 with DES mode (2GS/s)• Decay time of scintillation : 4.57ns and8.38ns• Rise time of scintillation lights : 1.45ns• Rise time of Cherenkov light : 0.75ns208Tl β decay event◆ Pulse Shape measured by various energy of gammas

Topological information from Cherenkov light could be used for 208Tl reduction.

◆ Definition of averaged angle
Vertex position obtainedby scintillation
Averageddirection = Σ di

Simulatedby EGS5

Cherenkov

e-Averaged
angle= Σ θi
Nhit

di θi

Scintillation

Photomultiplier
The averaged angle distribution for single electron has a peak at ~48
degree, which is almost same value as Cherenkov angle in Anisole.

Simulated by EGS5

Shorter wavelength ofabsorption for Zr(iPrac)4 andamount of PPO recover bothlight yield and energy
resolution of Zr-LS .

◆ Pulse Shape of Cherenkov lights and scintillatione-◆ Reduction of 208Tl events using Topological informationfrom Cherenkov Lights

tetrakis (isopropylacetoacetate)
Zirconium :Zr(iPrac)4
MW : 663.87

Hamamatsu H2431-50
Specification• Line focus 8 steps• HV : 3000V• Gain :2.5×106• TTS : 0.37ns• Rise time : 0.7ns

Vertex position
Double beta decay events sample

93%
cut

78%
remainVertex position

The averaged angle
distribution of 208Tl beta
decay is clearly different
from that of DBD events .

Possible to reduce 208Tl BG to beorder of 1/20, if we can extract PMT
which receives Cherenkov lights
among the scintillation signal.

Using χ2 for first 3bins, possible to discriminate scintillation signal
whether including Cherenkov lights or not on PMT by PMT basis.

4. Summary and Future plan Conceptual design of ZICOS detector with 10 wt.% Zr(iPrac)4 loaded Liquid
Scintillator has 2.7% @3.35MeV energy resolution assuming 64% photo
coverage of 20” photomultiplier.
 A technique further 1/20 reduction of 208Tl backgrounds using thetopological information of Cherenkov lights was developed.
 Pulse shape discrimination whether including Cherenkov lights or not in
scintillation signal was also developed.
 BG reduction will be directly demonstrated by using beta-gamma eventsfrom 60Co with dedicated detector UNI-ZICOS in next year.
Budget has
been funded by
Grant-in-Aid for
Scientific
Research on
Innovative
Areas
20H05241.