ZICOS – A NEW EXPERIMENT FOR
NEUTRINOLESS DOUBLE BETA DECAY
WITH ZIRCONIUM-96
Supported by Grant-in-Aid for Scientific Research on Innovative Areas
No.24104501 and No.26105502

International Workshop on “Double Beta Decay
and Underground Science” DBD16Miyagi University of Education Y.Fukuda, Y.Kamei,
Naringelile*, A.Obata*
University of Tokyo, ICRR
S.Moriyama
Fukui University
I.Ogawa
Tokyo University of Science T.Gunji, S.Tsukada, R.Hayami

ZICOS- Zirconium Complex in Organic
Liquid Scintillator for neutrinoless double
beta decay
Liquid Scintillator：(1) 10 wt.% Zr(iprac)4 loaded inanisole(2) 49% of BC505 for Light Yield
and 3.5% at 3.35MeV for energy
resolution with 40% photo
coverage.

3.5m

Pure water surrounding innerdetector in order to veto muons andexternal backgrounds.
Inner detector with 64% photo
coverage with 20” PMT including
1.7ton Zirconium loaded 113 tons
LS in fiducial volume. (Total vol. :180 tons)

10m
10m
DBD16

November 9th, 20162

tetralkis(isopropyl acetoacetate)zirconium
Zr(CH3COCHCOOCH(CH3)2)4 : Zr(iprac)4
Molecular weights : 663.87

Solubility was obtained by 31.2 wt.% inanisole. (usually 10wt.% for LS)
DBD16

November 9th, 20163Liquid scintillator containing Zr(iprac)4Light yield of 48.7±7.1% for BC505, and an energy
resolution of 4.1±0.6% at 3.35 MeV assuming 40%
photo coverage of the photomultiplier are obtained at10wt.% concentration of Zr(iprac)4 with 5 wt.% of PPO
and 0.5wt.% of POPOP in anisole.
DBD16

November 9th, 2016

4

Design of ZICOS detector

DBD16

November 9th, 2016

5Detector :1) 180tons LS : 10 wt.%
Zr(iprac)4 withPPO/POPOP in anisole
Need 500 of 20” PMTwith high QE >0.3 and
TTS < ~1ns = 64%
photo coverage
Expected performance :1) Energy resolution~2.8%@3.35MeV2) T1/2(0νββ) > 1026years, if
both 1/20 BG reduction
and 50% 96Zr enrichment
could be achieved.

Neutrino mass sensitivity of ZICOS experiment
Total mass : 180ton (fiducial volume : 113ton)
10wt.% Zr(iprac)4 = 12.６ton
includes 1.7ton of Zirconium= 45 kg of 96Zr (natural abundance 2.6%)
T1/20ν > 1.2 ×1025y ← Not enough for 0νββ search1) Zr enrichment
58.5% enrichment of 96Zr (e.g. 57.3% for NEMO-3)
96Zr will be 1.0 ton then T 0ν > 5 ×1025y
1/22) Lowering BG level (208Tl /214Bi)
i.e. < 1/20 ×KL-Zen (~1.0events/ton/year)
then T1/20ν > 5 ×1025y Today’s talk
DBD16

November 9th, 2016

6

Backgrounds around Q-value
Measured by KamLAND-Zen
208Tl

In balloon

Lowering BG (208Tl )< 1/20 ×KL-Zen(~1.0events/ton/year)

Phys.Rev.Lett. 117 (2016) 082503

0νββ signal region for 96Zr

T1/20ν > 5 ×1025y

Need additional technique other than the energy
spectral shape obtained by scintillation.
DBD16

November 9th, 20167Radiation branch of Thallium-208

The vertex reconstructed byscintillation lights could becontaminated within fiducial
volume due to gammas.
DBD16

November 9th, 20168

How to distinguish 208Tl decay and DBD?
0νββ event
e-Reconstructed vertex byescintillation lightβ decay
2.6MeV γ
Reconstructed vertex
by Cherenkov light
Balloon or
surface ofdetector
DBD16

e-November 9th, 2016

9Reduction of 208Tl decay
1) E : 3.0-3.7MeV
17925 events
2) Fiducial volume628 events
3) Multi events
263 events
4) Closer events(d≦10cm)
35 events
1/20 BG reduction
could be realized byidentifying vertex.
DBD16

November 9th, 201610

Separation of Cherenkov light and scintillation
arXiv:1609.0986(simulation)• Rise time of Cherenkov light :an order of 100 pico seconds
due to electro-magnetic
process• Rise time of Scintillation : anorder of a few nano seconds
due to energy transport offluorescent material.
Possible to extract hits of PMT
received Cherenkov lights byPulse Shape Discrimination.

DBD16・CAEN V1721 8 channel 8bit 500MS/s FADC・CAEN V2718 VME-PCI Optical Link Bridge

November 9th, 201611

Light yield of Cherenkov lights
90%
SC-37 filter
50%

400nm 500nm 600nm

Cherenkov light yield (λ>400nm)
Scintillation light yield of anisole= ~0.02 ≡ ~200 photon/MeV
DBD16

November 9th, 201612

Multiple scattering

Initial direction

Even though multiple scatteringof electrons, Cherenkov photons
look have some clusters.
DBD16

November 9th, 201613

Angular dependence
MC reproduced angular dependence.

NaI detector
60Co

BS γ 30deg γangleCherenkovlightLS

Electron (1.02MeV)

Cherenkov light emitted
by 1MeV e- actually has
an angular dependence.
DBD16

November 9th, 201614

Simulation of Cherenkov lights
Simulated by EGS5 (kinetic energy 1.675MeV)

Hard to detect Cherenkov events below 0.5MeV.
DBD16

November 9th, 201615

Reconstruction of vertex position• Vertex position for
single e- could bereconstructed by only
Cherenkov light withresolution σ=10cm.
• However, it needs
non-overlap PMT hits
for multi events.
Vertex position reconstructed by scintillation lightis almost same as the position reconstructed byCherenkov lights for single e- and DBD event.
DBD16

November 9th, 201616

Averaged angle
Vertex position obtainedby scintillation

Averageddirection = Σ diedi θi

Averagedangle= Σ θi
Nhit

Averaged angle with respect to averaged direction
for single e- seems to have a peak at ~48 degree
which is almost same as Cherenkov angle.
DBD16

November 9th, 201617

Hit pattern of DBD (opposite and half E)
Simulated by EGS5 (kinetic energy 1.675MeV)

Generate position of DBD

Multi events from DBDtend to have a slightly
larger values of averaged
angle than single e-.
DBD16

November 9th, 201618

Hit pattern of 208Tl (2.6MeVγ+ β+γ)
77.7%
remain
92.8%cut

Cherenkov hit pattern canbe used for BG reduction.
DBD16

Multi events from 208Tl
decay tend to have asmaller values ofaveraged angle than DBD.
November 9th, 201619

Summary Conceptual design for ZICOS detector using 20”

PMT with 64% photo coverage for detecting
Cherenkov light is presented.

 Simulated Cherenkov lights with EGS5 show somedirectionalities and it could reproduce the angulardependence which was observed by real data.

 Need to develop PMT with high QE (>30%) and fasttiming (TTS < ~1ns) in order to separate Cherenkov
hits and obtain vertex resolution σ～10cm.

 Further 1/20 reduction of 208Tl (and 214Bi) decay

backgrounds could be realized by Cherenkov hit
pattern even using vertex obtained by scintillation.
(we should confirm using proto-type detector.)

DBD16

November 9th, 201620