ZICOS - A new project for neutrinoless double beta
decay using Zirconium complex in organic liquid
scintillator
Y. Fukuda1∗ , Y.Kamei1 , Narengerile1 , A.Obata1 , S. Moriyama2 , I.
Ogawa3 , T. Gunji4 , R. Hayami4 and S.Tsukada4
1

Department of Physics, Faculty of Education, Miyagi University of Education, Sendai, Miyagi
980-0845, Japan
2
Kamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo, Hida,Gifu, 506-1205, Japan
3
Applied Physics, Faculty of Engineering, University of Fukui, Fukui, Fukui, 910-8507, Japan
4Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo
University of Science, Noda, Chiba, 278-8510, Japan
E-mail: fukuda@staff.miyakyo-u.ac.jp
Abstract. A liquid scintillator containing 96 Zr has been developed for ZICOS experiment.
We will use 180 tons of liquid scintillator with 64 % photo coverage of 20 inch photomultiplier.
0ν≥ 1027 years, we have to achieve 50 % enrichment of 96 Zr
In order to reach the sensitivity T1/2
208
and reduce 95 % ofTl decay backgrounds. Using Monte Carlo simulation, we could reduce
93 % of 208 Tl background with 78 % eﬃciency for 0νββ signal using diﬀerence of Cherenkov
hit pattern. For the separation of Cherenkov and scintillation signals, we measured the timing
pulse shape of Zr loaded liquid scintillator using FADC digitizer, and we found an inconsistentpulse shape at the rising time with the template of scintillation. Also the event rate with aninconsistent pulse shape seems to have a directionality.

1. ZICOS experiment
We have proposed a new experiment for neutrinoless double beta decay (0νββ) search using
nucleus 96 Zr [1]. A 96 Zr has third highest Q-value (3.35 MeV) among possible 0νββ isotopes,
therefore, in general, some radioactive backgrounds such as 214 Bi in Uranium series and 10 C,
which is spallation product of the energetic cosmic muon, could be avoidable due to their lowerenergy.
We will use total 180 tons of liquid scintillator containing 10 wt.% concentration of a tetrakis(isopropyl acetoacetato) zirconium (Zr(iPrac)4 ), which corresponds to 72 kg (45 kg in fiducialvolume) of 96 Zr assuming 2.6 % natural abundance, inside of the balloon. The inner balloon(3.5 m in radius) is located in the spherical detector (4.0 m in radius) which has 64 % of photo
coverage of 20 inch photomultiplier (PMT) as shown in Fig.1. The outside of the inner balloon
will be filled with pure water which has almost same density as Anisole. This is very important
for reducing backgrounds due to 208 Tl decay which may adhere to the surface of inner balloon
as described later. The energy resolution is expected to 2.6 % at 3.35 MeV[2].

inner balloon
total volume : 180ton
fiducial vol. : 113ton

4.0 m

4.0m
3.5m

10 m

Zr(iprac)4 10wt.%
PPO 5wt.%
in Anisole

pure water

20” PMD with QE~0.4 and TTS <1ns @ 1peTotal PMT : 650 Photo coverage : 64%

10 m

Scintillation (energy) + Cherenkov (BG reduction)

Figure 1. The left panel shows the global design of the ZICOS detector. The inner balloon will
be filled with a liquid scintillator which contains 10 wt.% of Zr(iPrac)4 . The right panel shows
the conceptual design of inner detector.
2. Background reduction using Cherenkov light0ν ≥ 1027 years, which corresponds to m ∼ 0.01eV, we have toIn order to reach the sensitivity T1/2
νachieve 50 % enrichment of 96 Zr and reduce the radioactive backgrounds significantly. According
to recent results form KamLAND-Zen[3], there found backgrounds around 3.35 MeV, and they
were events of 208 Tl decay which occurred on the surface of balloon. In the case of KamLANDZen, they also used a liquid scintillator at outside of the balloon, therefore they succeeded toreduce 208 Tl backgrounds at 2.5MeV (Q-value of 136 Xe) even though whose vertexes remain
within their fiducial volume due to measure all energies from 208 Tl decay (Q-value is 4.99 MeV).
On the other hands, ZICOS detector will use pure water instead of liquid scintillator at
outside of the balloon, therefore almost half of 208 Tl backgrounds observed in KamLAND-Zen
should be reduced by missing the energy. However, another half still exist inside of balloon. Inorder to remove those backgrounds, we have been developed techniques which use Cherenkov
light for discrimination of signal and background[2]. Basically 208 Tl decays into the exited stateof 208 Pb with emission of beta, and the transition to ground state with the emission of several
gammas including 2.6145 MeV gamma after the beta decay. Therefore some electrons emitted
by Compton scattering are able to emit Cherenkov lights from diﬀerent positions, while theelectrons from 0νββ also emit Cherenkov lights but from same position. This diﬀerence could
be used for the reduction of 208 Tl events.
In fact, the Monte Carlo simulation with EGS5, which simulates 208 Tl beta decay (Emax =1.7963 MeV) and two gammas (Eγ = 2.6145 MeV and 0.5832 MeV), indicates that 95 % of 208 Tl
events, whose averaged position of vertexes exist within fiducial volume, could be reduced by thedistance of each electrons as shown in left panel of Fig.2. Instead of the vertex reconstruction
using Cherenkov light, we could consider the diﬀerence of PMT hit pattern between signals
and backgrounds as shown in middle panel of Fig.2. Once we use the vertex position obtainednhit∑−−→→by scintillation light, then we can define an averaged direction as dˆ ≡d i , where d i is unit
i=1vector to i-th PMT from the vertex, and nhit is the number of PMT hit which received Cherenkov
lights larger than 1 pe. Using the averaged direction, we can also define an averaged angle asnhit∑→
1ˆ and the vector −θi , where θi is an angle between the averaged direction (d)d i . If weθ̂ ≡ nhit
i=1

400
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104

103
all events
10400r ≦ 3.0m

2400
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multi eventsd ≦ 10cm0-200 -400
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11.5

2.0

2.5

3.0 3.5 4.0 4.5 5.0
Total energy in MeV

DBD
140

208

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Tlvertex located within
fiducial volume

100

93% cut

80

78% remain

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(a) 208Tl beta-gamma multi events sample

1010-10

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Number of events/2degree

Number of events/0.01MeV/10000events

set the averaged angle 48 degree as a cut point, we can reduce 93 % of 208 Tl events even though
we keep 78 % of 0νββ events as shown in right panel of Fig.2.

0-200 -400-4002000-2004002000-200 -400-4002000-200(b) Double beta decay event sample

001020304050 60 70 80 90
averaged angle [degree]

Figure 2. The left panel shows the reduction step of 208 Tl events using Monte Carlo simulation.
The middle panel shows PMT hit pattern for typical (a) 208 Tl and (b) 0νββ in case of vertex
located within the fiducial volume. The right panel shows that the averaged angle distribution
for each case.

3. Separation of Cherenkov and Scintillation
Since it is important to measure the energy as precise as possible using scintillation light forthe discrimination of signal and backgrounds, we have to distinguish PMT whether received
Cherenkov light or not. The separation of Cherenkov and scintillation signals is realized by
the diﬀerence of emission mechanism. Cherenkov radiation is generated by the vibration ofmolecular polarity due to the electron-magnetic interaction, so that the time spread corresponds
to passing time of the electron (a few 100 pico seconds). On the other hands, the scintillation is afluorescence from the transition of orbital electron between exited state and lower state, therefore
the timing spread with a few tenth of nano seconds in case of an organic liquid scintillator. Suchdiﬀerence of spectral shape at the rising time due to Cherenkov lights emitted from cosmic muon
was really observed in Linear Alkyl Benzene [4].
We had tried to measure such timing using CAEN FADC V1721 digitizer. In order tocompare the spectral shape, we made a template for scintillation light. For the measurement ofscintillation, we collimated 60 Co source using lead block, and entered gammas at far edge of thevial in parallel direction with respect to the PMT surface in order to avoid incident of Cherenkov
light. The template of scintillation was made by following steps. At first, we accumulated pulses
around the Compton edge. Then we got both Mean and RMS using FADC count distribution
for each timing bin. At this time, we conventionally fixed the peak position at 60 ns for allpulses. Actually there were two types for the shape of template, which was fast timing anddelayed timing due to the resolution of FADC (2 ns). However we showed here only the shapewhich has better chi square in order to avoid the confusion. The crosses shown in Fig.3 andFig.4 show the template of scintillation obtained by Anisole and Zr loaded liquid scintillator,respectively. Both templates have about 7 ns for the decay time.
Next we measured the timing spectral shape of Anisole. In this time, gammas from 60 Co
source were also collimated by lead block, and scattered gammas which had 150 degree of thescattering angle with respect to the incident gamma were detected by NaI scintillator. Therefore
the scattering electrons generated inside Anisole have both an unique direction (4.45 degree forscattering angle) and energy (1.02 MeV). The PMT was located at 90 degree, which is anangle between the incident gamma and the PMT surface, because the PMT is easy to receive

FADC count

FADC count

event sample #1

15

template ofscintillation
data

10

event sample #2

10

55015

5060

70

80

90050607080

time [ns]90time [ns]Figure 3. Observed timing pulse shape in Anisole. The left panel of shape is consistent with
template of scintillation, however typical shape is similar to right panel.

event sample #1

60

40

event sample #2

100

template for
scintillation
sample data

FADC count

FADC count

FADC countCherenkov lights. Figure 3 shows typical timing pulse shape observed in Anisole. Most ofevents had the shape of the right panel of Fig.3. This shape is not agree with the templateof scintillation, and it has faster both rising and decay time. Backgrounds such as Cherenkov
occurred in the vial glass or the PMT surface had same shape but smaller charge than above
events. Same shape was also observed in pure water. Recently we observed same shape inAnisole even though using the UV cut filter (product of FUJIFILM Corporation: SC-37), whichcuts oﬀ the light shorter than 400 nm, and no scintillation event shown in left panel of Fig.3 wasobserved, because the peak of wavelength for scintillation is 300 nm. Therefore we assume thatan observed shape in Anisole shown in right panel of Fig.3 mainly consist of Cherenkov light.
At last, we measured the timing pulse shape for Zr loaded liquid scintillator. Figure 4 shows
typical pulse shape mainly observed in the liquid scintillator. The left panel of Fig.4 is consistent
with the shape of scintillation, and about 70 % of events look like this shape. The middle panelof Fig.4 has statistically inconsistent rising time shape with the template. If the diﬀerence oftemplate for
scintillation
data

15

subtract signal
to template10

signal observedin anisole50
52005060708090time [ns]05060708090time [ns]05060708090time [ns]Figure 4. Typical timing pulse shape observed in Zr loaded liquid scintillator. The left panelof shape is consistent with template of scintillation, however, the middle panel diﬀer from thetemplate at the rising time. The template shape was overlaid by fitting at decay tail.
shape at the rising time is caused by Cherenkov lights, we could extract the spectrum using
subtraction of data and template. The dashed line in the right panel of Fig.4 shows subtracted
shape which corresponds to the shaded region in middle panel, and the typical pulse shapeobserved in Anisole as shown in Fig.3 is overlaid as a solid line with adjusting the peak position.
Those shapes look consistent with each other. This means that the inconsistent shape at risingtime might be caused by Cherenkov lights.
In order to estimate statistically this inconsistency, we could define an accumulated sigma(ACS) which is a summation of sigma between data and template at first 3 bins of the risingtime. Figure 5 shows the ACS distribution in case of 90 degree and 10 degree, which is the angle
between incident gamma and the PMT surface, respectively. Latter case means that the PMTmight not be easy to receive Cherenkov lights. In the left panel of Fig.5, there is peak aroundConstant
MeanSigma(a) 90 deg

11.12
4.389
1.024

16
14

Total : 292 events
Cherenkov included: 74 events

12108

16.15days

6(b) 10 deg

7

Total : 99 events
Cherenkov-included: 17 events

65424days

312-10.08240

Number of observed events

Number of observed events

18-5.0

0.0

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15.0

20.0

Accumulated Sigma

0-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

Accumulated Sigma

Figure 5. The accumulated sigma (ACS) distribution for (a) 90 degree and (b) 10 degree,which is the angle between incident gamma and the PMT surface.
4.4 on the ACS. Looking at event by event, those events have actually inconsistent pulse shapeat the rising time with the template of scintillation.
If we assume that the event which has greater ACS value than 3.0 possibly include Cherenkov
lights at the rising time, then the event rate with ACS ≥ 3.0 is 4.6 events per day for 90 degreeand 0.7 events per day for 10 degree, respectively. The significance is almost 6 sigma. Samesituation was also observed in case of Anisole. There were 5 events for 90 degree and 1 eventfor 10 degree for the event shown in right panel of Fig.3 with 7days measurement.
Therefore, Cherenkov lights emitted by 1 MeV electron seems to have a directionality eventhough such a low energy electron could be multiply scattered. However, the event rate of only
scintillation received in Zr loaded liquid scintillator also seems to have a directionality. We will
have to check this using statistically suﬃcient data.
4. Conclusion
A conceptual design for ZICOS experiment with 180 tons of liquid scintillator loaded 10 wt.%
Zr(iPrac)4 was presented. The energy resolution is expected by 2.6 % at 3.35 MeV assuming 640ν ≥ 1027 years, we% photo coverage using 20 inch PMT. In order to reach the sensitivity of T1/2
have to achieve 50 % enrichment of 96 Zr and reduce 95 % of 208 Tl backgrounds.
We tried to demonstrate the method for 208 Tl background reduction using Cherenkov hit
pattern, and got 93 % reduction with 78 % signal eﬃciency. For the separation of Cherenkov
and scintillation signals, we measured a timing pulse shape using FADC digitizer, and foundthe diﬀerence at the rising time. The pulse shape discrimination using the accumulated sigma
was also demonstrated. To confirm observed directionality, we need more statistics and detailed
analysis.
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas(No.24104501 and No.26105502) from the Ministry of Education, Culture, Sports, Scienceand Technology (MEXT), a Grant-in-Aid for Scientific Research (C) (No.24540295) from theJapanese Society for the Promotion of Science (JSPS), and the Foundation for High Energy
Accelerator Science. We would like to thank Dr. K.Hiraide for helping us to take data using
CAEN V1721 digitizer.
References[1] Y.Fukuda, S.Moriyama and I.Ogawa, Nuclear Instruments and Methods A 732 (2013) 397.
[2] Y.Fukuda, Journal of Physics: Conference Series 718 (2016) 062019.
[3] A.Gando et al. (KamLAND-Zen collaboration), Physics Review Letters 117 (2016) 082503.
[4] J.Caravaca et al., Physical Review C 95 (2017) 055801.