ZICOS – NEW PROJECT FOR NEUTRINOLESSDOUBLE BETA DECAY EXPERIMENT USINGZIRCONIUM COMPLEX IN ORGANIC LIQUIDSCINTILLATOR –TAUP 2017 XV International Conference onTopics in Astroparticle and Underground Physics
Sudbury Canada
25 July, 2017

Miyagi University of Education

Y. Fukuda, Narengerile, A.Obata,Y.Kamei

Kamioka Observatory, ICRR, Univ. of Tokyo 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
Tetrakis(isopropylacetoacetate)zirconium :Zr(iprac)43.5m

MW : 663.87
LS: Zr(iprac)4 10wt.% andPPO 5 wt.% solved in anisole.

10m

10m

Estimated energy resolution ~2.8%@3.35MeV
assuming 64% photo coverage.
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Neutrino mass sensitivity of ZICOS experiment
Total mass : 180ton (fiducial volume : 113ton)
Measurement time: 2years
10wt.% Zr(iprac)4 = 12.６ton includes 1.7ton ofZirconium= 45 kg of 96Zr (natural abundance 2.6%)
T1/20ν > 4×1025y ← Not enough for 0νββ search1) Zr enrichment
50% enrichment of 96Zr (e.g. 57.3% for NEMO-3)
96Zr will be 865kg then T 0ν > ~2 ×1026y1/22) BG (208Tl ) reduction
BG level < 1/20 ×KL-Zen
then T1/20ν > ~1 ×1027y Today’s talk
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Backgrounds around signal region
Measured by KamLAND-Zen
208Tl onsurface ofballoon

Phys.Rev.Lett. 117 (2016) 082503

Necessary sameclean material as
KamLAND-Zen,but … need further
BG (208Tl ) reduction0νββ signal region for 96Zr
Require an additional technique other than theenergy spectral shape obtained by scintillation.
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4

How to distinguish 208Tl and DBD
0νββ event
e-Reconstructed vertex byescintillation light

β decay
2.6MeV γ
Reconstructed vertex
by Cherenkov light
Surface ofBalloon
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e-July 27, 20175

Reduction of 208Tl decay
Simulated by EGS5

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1) E : 3.0-3.7MeV
17925 events
2) Fiducial volume
628 events
3) Multi events
263 events
4) Closer events(d≦10cm)
35 events~1/20 BG reduction
could be achieved byusing the informationfrom Cherenkov light.
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6

Averaged angle of Cherenkov hit
Simulated by EGS5

Vertex position obtainedby scintillation

Averageddirection = Σ diedi θi

Averagedangle= Σ θi
Nhit

Averaged angle distribution with respect to averaged
direction for single electron has a peak at ~48 degree,which is almost same as Cherenkov angle.
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Hit pattern of 208Tl decay and DBD
Simulated by EGS5

Simulated by EGS5

78%
remainVertex position
Double beta decay event sampleSimulated by EGS593%
cut

Averaged angle of 208Tl
Hit pattern of Cherenkov lights decay is smaller values
for 208Tl decay looks different
than that of DBD.
Vertex position

from DBD .

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Separation of Cherenkov and Scintillation
arXiv:1609.0986(simulation)• Rise time of Cherenkov light :an order of a few 100 picosecond due to the electromagnetic process• Rise time of Scintillation light:
an order of nano seconds ingeneral.
Possible to extract PMT hits
received Cherenkov lights byPulse Shape Discrimination.

・CAEN V1721 8 channel 8bit 500MS/s FADC・CAEN V2718 VME-PCI Optical Link Bridge
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Pulse shape of scintillation light• Templates of FADC timing pulse shape for
scintillation light were obtained for both case.
• Both decay time of scintillation light are same,and it was about 7ns.
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Observation of Cherenkov lights
NaI detector

BS γ 30deg 60Co
γangleCherenkovlightLSelectron (1.02MeV)TAUP 2017 Sudbury, Canada

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Cherenkov lights fromO(1MeV) electron seem tohave a directionality.

Pulse shape observed for Anisole• Most of observed events havea different pulse shape fromthat of scintillation light.
• It is faster rise time and decaytime.
Those events mainly consist ofCherenkov lights, because of low
QE for wave length of scintillation
light (300nm).
Same pulse shapes were also
observed in H2O.
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Pulse shape observed for Zr loaded LS

Mainly two types of pulse shape (Scintillation-onlyand Scinti.+Cherenkov-like) were observed.
We maybe use pulse shape discrimination for theselection of events which include Cherenkov lights.
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Pulse shape discrimination
Typical pulse shape ofScinti.+Cherenkov-like

Comparison of excess pulse
shape and signal for anisole

Excess pulse shape seems to be consistent with thepulse which was seen in Anisole (see p12).
Excess of sigma between data and template wereaccumulated for first 3 bins : accumulated sigma
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Accumulated sigma distributionpreliminary

Scintillation-only

Scinti.+Cherenkov-likepreliminary

Ratio of Scintillation+Cherenkov
 90deg 25.3 ± 3.3 % 10deg 17.2 ± 4.5 %Significance : 1.5σ

Scinti.+Cherenkov-like might
be discriminated by acc. sig.
method… but
The difference of ratio was
smaller than expected by
Anisole.
There seems to exist somedifferent conditions between
anisole and LS.

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Summary Conceptual design of ZICOS detector (10 wt.%

Zr(iprac)4 loaded Liquid Scintillator has 2.8%
@3.35MeV energy resolution assuming 64% photo
coverage of 20” PMT) for next generation DBD
experiment.(T1/2(0νββ) > 1027years).
 96Zr : 45kg (nat.)865kg(50% enrich) Further 1/20 reduction of 208Tl backgrounds using

PMT hit pattern of Cherenkov lights.
 PSD could be useful for the extraction of Cherenkov

lights, however still need to study for confirmation.
 If PSD works, then BG reduction using PSD should

be checked by prototype for next step.
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BACKUPTAUP 2017 Sudbury, Canada

July 27, 201717ZICOS- Zirconium Complex in Organic
Liquid Scintillator for neutrinoless double
beta decay
1.5wt.% Zr loaded Liquid Scintillator :Light yield of 48.7±7.1% for BC505,
and an energy resolution of 2.8±0.4%
at 3.35 MeV assuming 64% photo
coverage of the photomultiplier

3.5m

Inner detector : 64% photo
coverage with 20” ultra-high spec.
PMT including 1.7ton Zirconium
loaded 113 tons LS in fiducial
volume. (Total vol. : 180 tons)
Outer detector : active veto using
pure water surrounding innerdetector in order to veto muonsand external γ-ray backgrounds.

10m10mTAUP 2017 Sudbury, Canada

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Future 0νββ experiments

1027 yrs~tons of target and ~zero BG detector will benecessary for next generation 0νββ experiment.
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Neutrinoless double beta decay[T1/20ν(0+ ->0+)]-1 =G0ν(E0,Z)|M0ν|2<mν>2 /me2
T1/2~a(Mt/∆E・B)1/2

a: abundance

M: target masst: measuring time ∆E: energy resolution B: BG rate

Requirement : Low BG, Large target mass, High E-resolution
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Pulse shape of timing information
Anisole onlyslow
FastZr(iprac)4 loaded LS

FastTAUP 2017 Sudbury, Canadaslow• Templates of pulse
shape of timing
information forscintillation light wereobtained by FADC.
• Fast and slow risetime component wereobserved due to
FADC resolution.
• Both decay time ofscintillation light areabout 7ns.
July 27, 201721

Pulse shape observed in Zr loaded LS
Scintillation-only

Fastslow

Cherenkov-included

Fastslow

Two types of pulse shape (Scintillation-only andCherenkov-included) were observed.
We can use pulse shape discrimination for
selection of events which include Cherenkov lights.
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Design of ZICOS detector
Detector :1) 180tons LS : 1.5 wt.% Zr
and 5wt.% PPO inAnisole .
2) Need 500 of 20” PMTwith high QE ~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
Natural abundance of 96Zr : 2.6%
could be achieved.
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tetralkis(isopropyl acetoacetate)zirconiumZr(CH3COCHCOOCH(CH3)2)4 : Zr(iprac)4
Molecular weights : 663.87

LS = Zr(iprac)4: 10 wt.% PPO: 5wt.%(POPOP: 0.05wt.% )TAUP 2017 Sudbury, Canada

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Zr loaded liquid scintillator
Feb. 27,2015 Mar. 14, 2016

Light yield of 48.7±7.1% forBC505, and an energy
resolution of 4.1±0.6% at 3.35
MeV assuming 40% photo
coverage of the photomultiplier
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Decay branch of Thallium-208

The vertex reconstructed byscintillation make it within
fiducial volume due to misfitting of gammas.
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Pulse shape observed in H2O90deg• Same pulse shape oftiming as anisole wasobserved.
• This signal should becaused by Cherenkov
light, because of no
scintillation in H2O.
This pulse shape ismade by Cherenkov
lights.
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Monte Carlo Simulation
Number of event received
Cherenkov light in Anisole
has a clear difference
between 90deg and 10deg,
because of directionality ofCherenkov light.
Simulated by EGS5

Maybe different situation in
Liquid Scintillator.
Photon could be scattered by high concentration of solute.
We have to simulate such kind of effect.
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Property of Cherenkov light Refractive index of anisole : n=1.518
 Cherenkov angle is determined bycosθ= 1/n’β (Ee>0.7MeV) n’>n
 Assuming 1.65MeV electron, then
β=0.972 and Cherenkov angel θ=47.3
degree are expected.
 Number of Cherenkov photon :100 photon/MeV (400nm – 600nm)

c.f. Light yield of Scintillation : ~12000photon/MeV

Cherenkov light = ~1% of scintillation light
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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
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Multiple scattering

Initial direction

Even though multiple scatteringof electrons, Cherenkov photons
look have some clusters.
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kinetic energy spectrum of electron
For calculation of 2νββ,

2e emit Cherenkov
lightsTAUP 2017 Sudbury, Canadaki, electron momenta
εi =sqrt(ki2+me2) : electron energy
W0=Q+2me : total release energy
Q : Q value me: electron mass
θ : opening angle F : Fermi func.
εi can generate independently
within energy conservation.
For calculation of 0νββ,Same calculation but εi can only
generates with ε1+ε2=W0.
July 27, 201732

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

Hard to detect Cherenkov events below 0.5MeV.
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Hit pattern of DBD (opposite and half E)
Simulated by EGS5 (kinetic energy 1.675MeV)

Generate position of DBD

Multi events from DBD
tend to have a slightly
larger values of averaged
angle than single e-.
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Emission and absorption spectra forsolvent and solute in standard cocktail
PPO absorbed mostof emission lights fromanisole.

Effectively the energy
was transferred to thesecondary scintillator.
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ATTENUATION LENGTH OF ANISOLE
Attenuation length ofscintillation light from
POPOP (~450nm) wasobtained as ~6m.

No problem for radiusof ZICOS detector.

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Natural radiative U/Th decay chain

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Backscattering method

NaI detector
sourceLiquid scintillator

PMT
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Single peak could be usedeven in liquid scintillator.
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Light yield of scintillation in anisole

300nm 400nm

Relative scintillation lightyield of anisole is 9.8% tostandard cocktail (due todifference of quantum
efficiency of PMT)TAUP 2017 Sudbury, Canada

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LIGHT YIELD COMPARISON BETWEEN
BC505 AND STANDARD COCKTAILLight yield of BC505
and our standard
cocktail (100mg PPO
and 10mg POPOP
solved in 20mL anisole)is almost same quality.

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ENERGY SPECTRA FOR SEVERALCONCENTRATION OF ZR(IPRAC)4
Peak values decreased
as a function of theconcentration ofZr(iprac)4.
Energy resolutions arealso getting worth as afunction of theconcentration ofZr(iprac)4.
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Physical constants of Liquid Scintillator

LY of NaI(Tl) : 4×104 photon/MeV
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LY of BC505 : 1.2×104 photon/MeV
July 27, 201742

Absorbance spectra for Zr(iprac)4Absorption peaks ofZr(iprac)4 was foundaround at 278nm.
However, overlapped
region with emissionof anisole was existed.
Zr(iprac)4 works as aquencher for the liquid
scintillator system.
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Solubility of Zr(iprac)4 for anisole

Solubility > 31.2 wt.%

Zr(iprac)4 2242mg, PPO 999mg
and POPOP 10mg solved in
20mL Anisole> 70g/L of Zirconium could be solved in anisole.
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Light yield quenching by Zr(iprac)4
Light yield = L0 ×

σ1Nppoσ1Nppo + σ2NZr

L0 : Light yield of anisole
Nppo : Number of PPO molecular in mole
NZr : Number Zr complex molecular in moleσ1 : absorbance of PPO (mol-1)
σ2 : absorbance of Zr complex (mol-1)
PPO would help the recovering light yield.
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Recovering the light yield
Measured at several conditions of PPO concentration
5wt.% PPO helps
actually recovering thescintillation light yield.

48.7±7.1% light yield
to standard cocktail
was obtained at10wt.% concentration.
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Recovering the energy resolution
Measured at several conditions of PPO concentration
5wt.% PPO helps again
the energy resolution
35% → 13%. at 10wt.%
of Zr(iprac)4.
13.0±2.0%√(40%/9.2%)X(3.35MeV/1.03MeV)= 3.5±0.5% at 3.35MeV
Achieved goal !
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Measurement of backgrounds from LS
1.120MeV Measured by lowBG CsI(214Bi)0.609MeV(214Bi)+0.583MeV(208Tl)1.765MeV(214Bi)
2.204MeV(214Bi)2.615MeV(208Tl)Using subtracted # of events around 2.6MeV and 2.2MeV
214Bi < 4.9x10-20g/g
208Tl < 2.7x10-22 g/g(238U < 6.4x10-6 g/g) (232Th < 7.4x10-7 g/g) (c.f. KL 10-18g/g)
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