ZICOS - Neutrinoless Double Beta Decay experimentusing Zr-96 with an organic liquid scintillator XXVIII International Conference on Neutrino Physics and Astrophysics (Neutrino2018)
4 – 9 June 2018 Heidelberg, Germany
Y.Fukudaa , D.Anzaia, Y.Kameia,e, K.Hiraideb, S.Moriyamab, Narengerilea, A.Obataa, I.Ogawac, T.Gunjid, S.Tsukadad , R.Hayamida Miyagi University of Education b Kamioka Observatory, ICRR, University of Tokyo c University of Fukui dTokyo University of Science eRCCN Tohoku University

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 ofsingle 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
Simulated by EGS5

Vertex position
Double beta decay events sample

93%
cut

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 for64% 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.

◆ 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 of
Zirconium = 45 kg of 96Zr (using natural abundance 2.6%)

The averaged angle of208Tl decay is smaller
than that of DBD.

78%
remainPossible to reduce
208Tl BG to be order of
1/20, if we can extract
Cherenkov lights fromscintillation.

4. Observation of Cherenkov lights◆ Pulse shape measured by FADC V1729 digitizer
Pulse shape of scintillation lights
Specification
Zr-LSAnisole•4channel•12bit ADC•500MS/s
sampling•Auto/External
trigger

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 ×1026y2) 208Tl background reduction
BG level < 1/20 ×KL-Zen
T1/20ν > 1 ×1027y◆ Cherenkov lights observed by cosmic muons (Anisole with UV cut filter)
Observed Cherenkov
light yield depends
on muon tracks.

◆ Development of Zr loeaded Liquid Scintillator

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

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 eventχ2 distribution ofpulse shape for
Zr-LS using
scintillation
template looks
different between
vertical andinclined muons.

(subtracted)

Pulse shape discrimination could
be useful for identification of PMT
which receives Cherenkov lights.
Pulse shape of Cherenkov lights isdifferent from that of scintillation
lights.

5. Results and Future0νββ event
e-The pulse shape ofCherenkov lights
should be observed
directly.

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 of Cherenkov lights using cosmic muons
was done, and the shape was quite different from that of scintillation lights.
 Pulse shape of Zr (iPrac)4 loaded Liquid Scintillator looks different whether thepulse includes Cherenkov lights or not.
 Pulse shape discrimination could be useful for the identification of PMT which
receives Cherenkov lights.
 Real energy resolution will be measured using 64% photo coverage of PMT
within this year.
 Pulse shape and directionality of Cherenkov light from O(1MeV) electrons will
be measured by much faster PMT (Hamamatsu H2431-50 TTS~0.37ns) andCAEN V1751 digitizer (10bit ADC 2GS/s sampling) soon.