ZICOS - Neutrinoless Double Beta Decay experimentusing Zr-96 with an organic liquid scintillator International Symposium on Revealing the history of the universe with underground particleand nuclear research 2019, 7 – 9 March 2019 Sendai Japan
Y.Fukudaa , D.Anzaia, Y.Kameia,e, K.Hiraideb, S.Moriyamab, Narengerilea, A.Obataa, I.Ogawac, T.Gunjid, S.Tsukadad , R.Hayamid, S.Kurosawafa 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 26105502 and Scientific Research (C) 18K03664

1.

96
ZICOS ( Zr DBD experiment)

3. Development of reduction technique◆ Reduction of 208Tl events using Cherenkov Lights◆ Neutrinoless double beta decay

Vertex position obtainedby scintillation● Lifetime and neutrino mass

[T1/20ν(0+ ->0+)]-1 =G0ν(E0,Z)|M0ν|2<mν>2● Energy spectrum and lifetime measurement・monochromatic energy at Q-value
・T1/2~a(Mt/∆EB) a: abundance M: masst: meas.time ∆E: energy res. B: BG rate

Simulated by EGS5

Averageddirection = Σ diedi θThe averaged angle
distribution withrespect to theaveraged direction of
single electron has apeak at ~48 degree
which is almost samevalue as Cherenkov
angle in Anisole.

iAveragedangle= Σ θi
Nhit

Low background rate, Large target mass and High energy resolution
Simulated by EGS5

◆ Detector design for ZICOS experiment
The averaged angle of208Tl decay is smaller
than that of DBD.

Simulated by EGS5

Vertex position
Double beta decay events sample

93%
cut

78%
remainPossible to reduce
208Tl BG to be order of1/20, if we can extract
Cherenkov lights fromscintillation.

Vertex position

Detector :1)180tons LS : 1.5 wt.% Zr and5wt.% PPO in Anisole .
2)Need 500 of 20” PMT with highQE ~0.4 and TTS ~300ps@1pe for
64% photo coverage.

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

◆ Neutrino mass sensitivity for ZICOS experiment・Total mass : 180ton (fiducial volume : 113ton)
・Measurement time: 2years
・10wt.% of Zr(iPrac)4 = 12.6ton of Zr(iPrac)4 includes 1.7ton ofZirconium = 45 kg of 96Zr (using natural abundance 2.6%)
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

4. Observation of Cherenkov lights◆ Pulse shape measured by fast FADC and PMTCAEN V1751
digitizer

Hamamatsu
H2431-50

Specification• 4channel• 10bit ADC• 1GS/s sampling• Auto/Externaltrigger

Specification• Line focus 8• 3000V• 2.5×106 gain
• 0.37ns (TTS)• 0.7ns (rise time)◆ Cherenkov lights observed by electrons from 90Sr/90Y source
Liquid Scintillator
Anisole with UV filter
Averaged waveform◆ Development of Zr loeaded Liquid Scintillator

tetrakis (isopropylacetoacetate)
Zirconium : Zr(iPrac)4
MW : 663.87◆ FADC counts plot at each FADC time
Anisole with UV filter
Liquid Scintillator

Zr-LS: Zr(iPrac)410wt.%, PPO 5 wt.%
and POPOP 0.2wt%
solved in Anisole.
Shorter wavelength ofabsorption for Zr(iPrac)4
and amount of PPO
recover both light yieldand energy resolutionof Zr-LS .

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

2. How to reduce backgrounds◆ Conceptional idea using Cherenkov lights

Reconstructed vertex
by scintillation light e-β decay
2.6MeV γ
Event generated
Surface of position
Balloon
208Tl decay event

PSD could be used for IDof PMT received Cherenkov .

5. Results and Future0νββ event
e-There is clear difference
for both rise and fall time
between Cherenkov andScintillation light.

e~1/20 BG reduction could beachieved by using a topology
information of Cherenkov light.

Cherenkov light could be used for tool of 208Tl BG reduction.

 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” Photo-multiplier.
 A technique further 1/20 reduction of 208Tl backgrounds using PMT hit patternof Cherenkov lights was developed.
 Direct measurement of pulse shape ofCherenkov and Scintillation light using
electrons from 90Sr/90Y beta source wasdone, and both rise and fall time werequite different from each others.
 Pulse shape discrimination could be
used for the identification of PMT
which receives Cherenkov lights.
 Averaged angle will be measured byreal proto-type detector HUNI-ZOCOS.