ZICOS – NEW PROJECT FOR NEUTRINOLESSDOUBLE BETA DECAY EXPERIMENT USINGZIRCONIUM COMPLEX IN ORGANIC LIQUIDSCINTILLATOR –Supported by Grant-in-Aid for Scientific Research on Innovative Areas 26105502

TAUP 2015 XIV International Conference onTopics in Astroparticle and Underground Physics
Torino, Italy
9 September, 2015

Miyagi University of Education

Y. Fukuda, Narengerile, A.Obata

Kamioka Observatory, ICRR, Univ. of Tokyo S. Moriyama
Fukui University I. Ogawa
Tokyo University of Science T. Gunji, S. Tsukada, R. Hayami

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 energy
resolution
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Zirconium Complex in Organic Liquid
Scintillator for neutrinoless double beta
decay (ZICOS) experiment
Goals for development of LS ：(1) high solubility(2) 3.5% at 3.35MeV of energy
resolution, if ZICOS have PMTs
with 40% photo coverage andlong attenuation length (~10m)

1.5m

Pure water surrounding innerdetector to veto external
backgrounds.
Inner detector with 40% photo
coverage 10” PMT including
Zirconium loaded 14.1 tons LSTAUP 2015 Torino, Italy

5.0m
5.4m
September 7, 2015

3Backgrounds around 3.35MeV
Measured by KamLAND-Zen•

I.Shimizu@Neutrino2014

208Tl β + γ ’s

γ 2.6MeV +① βmax 1.89 MeV+ γ 0.58 MeV(8.45×10-01)② βmax 1.29 MeV+γ 1.09 MeV(3.97×10-03)③ βmax 1.53 MeV+γ 0.86 MeV(1.24×10-01)

0νββ signal region for 96Zr
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Neutrino mass sensitivity of ZICOS experiment
Results from NEMO-3 (96Zr) : T1/20ν > 9.2 ×1021y<mν> 7.2 – 10.8 eV (gA=1.25,gpp=1.11,QRPA)(Ref: M.B.Kauer Doctor thesis for UCL(2010))

Assuming same energy resolution, BG rate andmeasurement time as KamLAND-Zen(T1/20ν > 2.6 ×1025y ) (Ref: I.Shimizu arXiv:1409.0077 (2014))
Mass : 14.1ton

10wt.% Zr(iprac)4 = 1.57ton
includes 216kg of Zirconium= 6.5kg of 96Zr(= 9.2kg of 136Xe = 0.03 ×KL-Zen)

T1/20ν > 4.4 ×1024y ← Not enough for 0νββ search
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Neutrino mass sensitivity of ZICOS experiment
1) Zr enrichment
58.5% enrichment of 96Zr (e.g. 57.3% for NEMO-3)
then 96Zr will be 126kg (0.56 times 136Xe 320kg)
T1/20ν > 1.9 ×1025y ; <mν> < 0.16 – 0.3 eV (QRPA)
2) Lowering BG ratei.e. < 1/30 ×KL-Zen
T1/20ν > 1.0 ×1026y ;<mν> < 0.04 – 0.09eV
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Development of Liquid scintillator
Zirconium β-keto ester complex

Zr(CH3COCHCOOCH(CH3)2)4 Zr(CH3COCHCOOCH2CH3)4= Zr(iprac)4mw : 663.87= Zr(etac)4mw : 607.76

Synthesized byProf. T.Gunji

Stable solid powder was obtained.
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Solubility of Zirconium β-keto ester
complex for anisoleZr(iprac)4 : 31.2wt.%

Zr(etac)4 : 32.7wt.%

Very huge solubility !
> 70g/L of Zirconium could be solved in anisole.
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Absorbance spectra for zirconium
β-keto ester complex
Absorption peaks ofZr(iprac)4 and Zr(etac)dwere found around at278nm.
.

Part of region of emission
and absorption spectrum
was overlapped.
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9Light yield quenching by Zr complex
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 could help recovering the performance.
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BACK SCATTERING METHODNaI detector
60Co

sourceLiquid scintillator

PMTTAUP 2015 Torino, Italy

Single peak could be usedeven in liquid scintillator.
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Recovering the light yield
Several conditions of PPO concentration

expected

5wt.% PPO helps
actually recovering thescintillation light yield.

expected

fitted48.7±7.1% light yield
to standard cocktail
was obtained at
10wt.% concentration.
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Recovering the energy resolution
Several conditions of PPO concentration
fittedexpected

5wt.% PPO helps again
the energy resolution
35% → 13%. at 10wt.%
of Zr(iprac)4.
13.0±2.0%

expected√(40%/9.2%)X(3.35MeV/1.03MeV)= 3.5±0.5% at 3.35MeV
Achieved goal for ourdevelopment of LS !

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Measurement of backgrounds of 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 10-16g/g)
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Background reduction
Residual backgrounds as shown in KamLAND-Zen
are 208Tl β decay + γ 2.6MeV+ γ 0.58 MeV/γ 1.09
MeV/γ 0.86 MeV

Need additional technique except energy spectral
shape obtained by scintillation light in order toreduce those backgrounds efficiently.
Can we use Cherenkov light forbackground reduction ?
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Property of Cherenkov light Refractive index of anisole : n=1.518
 Cherenkov angle is determined bycosθ= 1/nβ Assuming 1.65MeV electron, then
β=0.951 and Cherenkov angel θ=46.2
degree are expected.
 Cherenkov light should be measured.
(350nm – 550nm : 120 photon/MeV)

c.f. Light yield of Scintillation : 12000photon/MeV
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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 2015 Torino, Italy

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

400nm 500nm 600nm

Cherenkov light yield (>400nm)
Scintillation light yield of std. LS
115.2/0.183== ~0.01
298.6/0.046/0.098
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Consistent with expectation.
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ZICOS proto-type detector
Performance test :• Energy resolution• BG reduction study
using Cherenkov light
Physics goal :• 96Zr : 12g (NEMO-3 :10g) using naturalabundance Zirconium.
• Measure 2νββ half-life.
• Obtain limits beyond
the NEMO-3 results.
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MPPC (Multi-Pixel Photon Counter)
Hamamatsu S13360-6050CS•••

effective area : 6.0mm × 6.0mm# of pixels /channel : 14400
Time resolution (FWHM) : 200~300ps

Emission time of• Cherenkov light : ~3ps
• Scintillation light : ~10ns

Scintillation

Cherenkov
TAUP 2015 Torino, Italy

Using
difference oftiming shape,
we might beable to extract
Cherenkov light.
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SUMMARY ZICOS sensitivity : T1/20ν > 1.0 ×1026y; <mν> < 0.1eV (QRPA) using 58.5% enriched 96Zr andreduced BG at 1/30 × KamLAND-Zen.

 Liquid scintillator containing zirconium β-ketoester complex has been developed.

 Liquid scintillator with 10 wt.% concentration ofZr(iprac)4 has 48.7±7.1% for light yield to BC505
and 3.5±0.5% at 3.35MeV (assuming 40% photocoverage) for energy resolution.

 ZICOS proto-type will demonstrate not only

good energy resolution but also ability ofbackground reduction using Cherenkov light.

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