ジルコニウム９６を用いたニュートリノを放出しない二重ベータ崩壊事象の探索 XI
日本物理学会 2017年秋季大会
2017年9月13日
宮城教育大学教育学部

福田善之、亀井雄斗*、那仁格日楽＊、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

Zr loaded liquid scintillator
𝐸𝐸𝛾𝛾 = 225.0 ± 16.0 keV

20172013~2015
2015
Measured on this August.
An energy resolution is obtainedby 2.61±0.14% at 3.35 MeV
assuming 64% photo coverageof the photomultiplier.
日本物理学会2017年秋季大会

2017年9月13日∆𝐸𝐸= 0.125𝐸𝐸(𝐸𝐸 = 1.02MeV)

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ZICOS- Zirconium Complex in Organic
Liquid Scintillator for next generation ofDBD experiment

96Zr : 45kg (nat.) → 865kg(50% enrich)0𝜈𝜈21𝑇𝑇>
9.1×10ys0𝜈𝜈 NEMO3 : 250𝜈𝜈
1/2
𝑇𝑇1/2 > 4 × 10 ys →𝑇𝑇1/2 > 2 × 1026 ys

日本物理学会2017年秋季大会

2017年9月13日

3Backgrounds around signal region
Measured by KamLAND-Zen

208Tl onsurface ofballoon

Phys.Rev.Lett. 117 (2016) 082503

0νββ signal region for 96Zr

Even though using
ultimate cleanness
like KamLAND-Zen,need further 1/20
208Tl reduction for0𝜈𝜈> ~1 × 1027 ys
𝑇𝑇1/2An additional technique other than the energyspectral shape obtained by scintillation is necessary.
日本物理学会2017年秋季大会

2017年9月13日

4Hit pattern of Cherenkov lights
Simulated by EGS5

Simulated by EGS578%
remainVertex position
Double beta decay event sample
Simulated by EGS5

93%
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 .

日本物理学会2017年秋季大会

2017年9月13日

5Separation of Cherenkov and Scintillation

arXiv:1609.0986(simulation)

Difference of light emission
mechanism• Cherenkov : an order of a few
100 pico second due to theelectro-magnetic process• Scintillation : an order of tenthnano seconds in general.
Possible to extract PMT hits
received Cherenkov lights byPulse Shape Discrimination.

・CAEN V1721 8 channel 8bit 500MS/s FADC digitizer
・CAEN V2718 VME-PCI Optical Link Bridge
日本物理学会2017年秋季大会

2017年9月13日

6Pulse shape of scintillation light• Templates of FADC timing pulse shape for
scintillation light were obtained for both anisoleand Zr loaded liquid scintillator.
• Both decay times are almost same, and it wasabout 7ns.
日本物理学会2017年秋季大会

2017年9月13日

7Observation of Cherenkov lights
NaI detector

NaI detector

BS γ 30deg 60Co
γBS γangle

Cherenkov
lightγ30deg

angle

LSLSelectron (1.02MeV)

90deg
日本物理学会2017年秋季大会

60Co

Cherenkov
light10deg
2017年9月13日

8Pulse shape observed in Anisole• Most of observed events have adifferent shape from scintillation.
• Same shape was observed inpure water.
• Recently same shape was foundeven though using UV cut filter.
• Observed directionality : 6 events(90deg) and 1 events (10deg)

日本物理学会2017年秋季大会

2017年9月13日

9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
日本物理学会2017年秋季大会

2017年9月13日

10Pulse shape observed in Anisole• Most of observed events have a
different shape from scintillation.
• Same shape was observed inpure water.
• Recently same shape was foundeven though using UV cut filter.
• Observed directionality : 6 events(90deg) and 1 events (10deg)

These pulse shapes might consistof Cherenkov lights.
日本物理学会2017年秋季大会

2017年9月13日

11Pulse shape observed in Zr loaded LS

Mainly two types of shape (Scintillation-only andScintillation + Cherenkov-like) were observed.

Is this excess caused by Cherenkov light?
日本物理学会2017年秋季大会

2017年9月13日

12Excess of rise time shape
Typical shape of Scinti.
+ Cherenkov-like

Excess shape and shapeobserved in anisole

Excess shape is similar to the one observed in anisole.
It might be caused by Cherenkov lights!
Pulse shape discrimination whether the event includes
Cherenkov-like or not using excess could be possible.
日本物理学会2017年秋季大会

2017年9月13日

13Pulse shape discrimination
preliminary
16.15days

Scintillation-only

Scinti.+Cherenkov-like

Count rate for events included
Cherenkov–like 90deg 4.6 events/day
 10deg 0.7 events/day
Significance : 6.0σ

Same directionality as
observed in anisole.
preliminary

24daysScintillation-onlyHowever, the number of events
for scintillation-only also
decreased at 10 degree.

Scinti.+Cherenkov-like

Need more data and detailed
analysis to confirm above results.
日本物理学会2017年秋季大会

2017年9月13日

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

Zr(iprac)4 loaded Liquid Scintillator has 2.6%
@3.35MeV energy resolution assuming 64% photocoverage of 20” PMT) for next generation DBD0𝜈𝜈> ~1027 years).
experiment.(𝑇𝑇1/2 96Zr : 45kg (nat.)865kg(50% enrich) Further 1/20 reduction of 208Tl backgrounds using

PMT hit pattern of Cherenkov lights.
 Cherenkov might have a different pulse shape fromscintillation even though O(1MeV) electron.
 PSD could be useful for the extraction of Cherenkov

lights, however still need to study.
日本物理学会2017年秋季大会

2017年9月13日

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BACKUP日本物理学会2017年秋季大会

2017年9月13日

16Pulse shape observed in H2O
90deg• 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.
日本物理学会2017年秋季大会

2017年9月13日

17Pulse shape observed in anisole with
UV cut filter SC-37
Observed events except
large pulse height (only2 events) has same
pulse shape (or muchnarrow) as before.
Scintillation should notbe measured due to UV
cut filter, so confirmedthat this shape consistsof Cherenkov light.
日本物理学会2017年秋季大会

2017年9月13日

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Reduction of 208Tl decay
Simulated by EGS5

日本物理学会2017年秋季大会

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.
2017年9月13日

19Averaged angle of Cherenkov hit
Simulated by EGS5

Vertex position obtainedby scintillation

Averageddirection = Σ diedi θiAveragedangle= Σ θiNhit

Averaged angle distribution with respect to averaged
direction for single electron has a peak at ~48 degree,which is almost same as Cherenkov angle.
日本物理学会2017年秋季大会

2017年9月13日

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

β decay
2.6MeV γ
Actual reaction
position

e-Surface ofBalloon
日本物理学会2017年秋季大会

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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
日本物理学会2017年秋季大会

2017年9月13日

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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.

10m
10m
日本物理学会2017年秋季大会

2017年9月13日

23Future 0νββ experiments

1027 yrs~tons of target and ~zero BG detector will benecessary for next generation 0νββ experiment.
日本物理学会2017年秋季大会

2017年9月13日

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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
日本物理学会2017年秋季大会

2017年9月13日

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Pulse shape of timing information
Anisole only

slow
Fast
Zr(iprac)4 loaded LSFast
日本物理学会2017年秋季大会

slow• Templates of pulse
shape of timinginformation for
scintillation light wereobtained by FADC.
• Fast and slow risetime component wereobserved due to
FADC resolution.
• Both decay time of
scintillation light areabout 7ns.
2017年9月13日

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Pulse shape observed in Zr loaded LS
Scintillation-only

Fast

slow

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.
日本物理学会2017年秋季大会

2017年9月13日

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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.% )日本物理学会2017年秋季大会

2017年9月13日

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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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2017年9月13日

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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 of
Cherenkov light.
Simulated by EGS5

Maybe different situation inLiquid Scintillator.
Photon could be scattered by high concentration of solute.
We have to simulate such kind of effect.
日本物理学会2017年秋季大会

2017年9月13日

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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
日本物理学会2017年秋季大会

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Multiple scattering

Initial direction

Even though multiple scatteringof electrons, Cherenkov photons
look have some clusters.
日本物理学会2017年秋季大会

2017年9月13日

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kinetic energy spectrum of electron
For calculation of 2νββ,

2e emit Cherenkov
lights日本物理学会2017年秋季大会

ki, 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.
2017年9月13日

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

Hard to detect Cherenkov events below 0.5MeV.
日本物理学会2017年秋季大会

2017年9月13日

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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-.
日本物理学会2017年秋季大会

2017年9月13日

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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.
日本物理学会2017年秋季大会

2017年9月13日

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ATTENUATION LENGTH OF ANISOLE
Attenuation length ofscintillation light from
POPOP (~450nm) wasobtained as ~6m.

No problem for radiusof ZICOS detector.

日本物理学会2017年秋季大会

2017年9月13日

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Natural radiative U/Th decay chain

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2017年9月13日

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Backscattering method

NaI detector
sourceLiquid scintillator

PMT
日本物理学会2017年秋季大会

Single peak could be usedeven in liquid scintillator.
2017年9月13日

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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)日本物理学会2017年秋季大会

2017年9月13日

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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.

日本物理学会2017年秋季大会

2017年9月13日

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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.
日本物理学会2017年秋季大会

2017年9月13日

42Physical constants of Liquid Scintillator

LY of NaI(Tl) : 4×104 photon/MeV
日本物理学会2017年秋季大会

LY of BC505 : 1.2×104 photon/MeV
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Absorbance spectra for Zr(iprac)4
Absorption peaks ofZr(iprac)4 was foundaround at 278nm.
However, overlapped
region with emissionof anisole was existed.
Zr(iprac)4 works as aquencher for the liquidscintillator system.
日本物理学会2017年秋季大会

2017年9月13日

44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.
日本物理学会2017年秋季大会

2017年9月13日

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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.
日本物理学会2017年秋季大会

2017年9月13日

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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 at
10wt.% concentration.
日本物理学会2017年秋季大会

2017年9月13日

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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 3.35MeVAchieved goal !
日本物理学会2017年秋季大会

2017年9月13日

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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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2017年9月13日

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