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

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平成29年6月24日
東京大学柏キャンパス
宮城教育大学 教育学部 福田善之, 亀井雄斗*,ナリンゲルラ*, 小畑旭*
東京大学宇宙線研究所 森山茂栄
福井大学 工学部
小川 泉
東京理科大学理工学部 郡司天博, 塚田 学, 速水良平

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

Zr(iprac)4: 10 wt.% PPO: 5wt.% POPOP: 0.05wt.%
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ZICOS- Zirconium Complex in Organic
Liquid Scintillator for neutrinoless double
beta decay
Liquid Scintillator：
49% of BC505 for Light Yield
and 3.5% at 3.35MeV for energy
resolution assuming 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
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3Design of ZICOS detector

Natural abundance of 96Zr : 3%
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4Detector :1) 180tons LS : 10 wt.%
Zr(iprac)4 with 5wt.%
PPO in anisole
2) Need 500 of 20” PMTwith high QE >0.3 and
TTS < ~1ns = 64%
photo coverage
Expected performance :1) Energy resolution~2.8%@3.35MeV
2) T1/2(0νββ) > 1026years, if
both 1/20 BG reduction
and 50% 96Zr enrichment
could be achieved.

Backgrounds around signal region
Measured by KamLAND-Zen
208Tl onsurface ofballoon

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

Phys.Rev.Lett. 117 (2016) 0825030νββ signal region for 96Zr

T1/20ν > 5 ×1025y

Need additional technique other than the energy
spectral shape obtained by scintillation.
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Radiation branch of Thallium-208

The vertex reconstructed byscintillation lights could locate
within fiducial volume due to
mis-fitting of gammas.
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6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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Reduction of 208Tl decaySimulated by EGS5

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 byvertex reconstruction.

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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 sample
Simulated by EGS5

93%
cut

Averaged angle of 208Tl
decay is smaller values
Cherenkov hit pattern of 208Tl
decay looks different from DBD . than that of DBD.
Vertex position

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Separation of Cherenkov and Scintillation
arXiv:1609.0986(simulation)• Rise time of Cherenkov light :an order of 100 ps due to theelectro-magnetic process• Rise time of Scintillation light:
an order of ns in general.

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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Directionality

Anisole only
Simulated by EGS5

NaI detector

BS γ30deg 60

angle

CherenkovlightCoγ
LSelectron (1.02MeV)

Cherenkov light from
~1MeV electron seems
to have a directionality.
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MC reproduced such a directionality.

Pulse shape of timing information
Anisole onlyslow
FastZr(iprac)4 loaded LS

Fast
宇宙線将来計画タウンミーティングslow• Templates of pulse
shape of timing
information for
scintillation light wereobtained by FADC.
• Fast and slow risetime component wereobserved due to
FADC resolution.
• Both decay time of
scintillation light areabout 7ns.
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Pulse shape observed in anisole only
90deg
• 70% of events have a
different pulse shape oftiming from scintillation.
• It is shorter rise time anddecay time.
Those events might mainly
consist of Cherenkov lights,
because of low QE for wave
length of scintillation.
Same pulse shapes were also observed in H2O.
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Pulse shape observed in Zr loaded LS
Scintillation-like

Cherenkov-likeTwo types of pulse shape (Scintillation-like andCherenkov-like) were observed.
Differences of sigma between data and template
were accumulated by first 3bins.
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Accumulated sigma
 90deg (total 305 events)
Cherenkov-like 49 events
preliminary
Cherenkov

16.1 ± 2.3 % 10deg (total 63 events)
Cherenkov-like 6 events

9.5 ± 3.9 %Scintillation

preliminary

Stat. Significance : 2.2σ(need more study)

maybe Cherenkov for slow

preliminary

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Probably Cherenkov light
could be discriminated bypulse shape of timing.
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pp solar neutrino measurement using
115In● Advantage
R.S.Raghavan Phs.Rev.Lett37(1976)259

νe → 115Sn* + e115Sn*(4.76µs) → 115Sn +γ1(115keV) + γ 2(497keV)
115In +・ large natural abundance (95.7%)
・ large cross section (~640SNU)
・ direct counting (real-time measurement)
・ sensitive to low energy solar neutrinos(Eν ≧ 125keV)・ energy measurement (Ee = Eν -125keV)・ triple fold coincidence to extract neutrinosignal from BG (e1 +γ2 + γ3 )● Disadvantage
・ natural β-decay of 115In(τ1/2 = 4.4 × 1014 yr , Ee≦498keV)・ possible BG due to correlated
coincidence by radiative Bremsstrahlung

Requirement :1. Good energy resolution : ~10%@100keV(FWHM)
2. High efficiency γ detection
3. (High granularity)
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ZICOS : Zirconium - Indium Complex in
Organic liquid Scintillator
Tris(isopropylacetoacetate)indium :In(iprac)3
MW : 544.3

Liquid Scintillator：10 wt.%
In(iprac)3 loaded in anisole.

3.5m

2.53 ton of 115In loaded LS in 113ton
fiducial volume. (0.73 event/day)

Estimated energy resolution~16%@100keV (not so bad!)

10m

10m
pp solar ν is possible physics target .

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

Zr(iprac)4 loaded Liquid Scintillator with 64% photo
coverage of 20” PMT) for 96Zr DBD search (and115In pp solar ν measurement).
 Monte Carlo simulation indicated the directionalityof Cherenkov light even from O(1MeV) electron.
 Further 1/20 backgrounds reduction for 208Tl (andalso 214Bi) could be achieved by the vertex positionor the hit pattern obtained by Cherenkov light.
 Probably we will be able to extract Cherenkov

signals using the pulse shape discrimination.
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