ZICOS - New project for neutrinoless double beta
decay experiment using zirconium complex in liquid
scintillator Yoshiyuki Fukuda
Department of Physics, Faculty of Education, Miyagi University of Education, Sendai, Miyagi
980-0845, Japan
E-mail: fukuda@staff.miyakyo-u.ac.jp
Abstract. A liquid scintillator containing a tetrakis (isopropyl acetoacetato) zirconium
has been developed for new project of neutrinoless double beta decay experiment (ZICOSexperiment). We have synthesized a tetrakis (isopropyl acetoacetato) zirconium, which havehigh solubility (over 31.2 wt.%) in anisole. We measured the performance of liquid scintillator
containing 10 wt.% concentration of a tetrakis (isopropyl acetoacetato) zirconium, and obtained
48.7 ± 7.1 % of the light yield of BC505 and the energy resolution of 4.1 ± 0.6 % at 3.35 MeV
assuming 40 % photo coverage of the photomultiplier, respectively. We also estimated that
ZICOS experiment should be sensitive to ⟨mν ⟩ < 0.1 eV assuming gA = 1.25, gpp = 1.11 andQRPA model, if a radius of the inner detector is 1.5 m and the detector is filled with this liquid
scintillator with an enriched 96 Zr nucleus and we can reduce 208 Tl backgrounds to be one tenthorder of magnitude of KamLAND-Zen using Cherenkov lights.

1. Introduction
To determine the Majorana neutrino mass from neutrinoless double beta decay (0νββ), we mustmeasure the half-life. The half-life of 0νββ is given by following formula.
0ν2[T1/2(0+ → 0+ )]−1 = G0ν M0ν

⟨mν ⟩2m2e(1)

Here G0ν is the kinematic phase space factor, M0ν is the matrix element of the target nucleus
including Fermi, Gamow-Teller and tensor contributions, me is the electron mass, and ⟨mν ⟩is the eﬀective Majorana neutrino mass. According to Eq.(1), we must be able to measure a
half-life of the order of 1025 years assuming the neutrino mass to be below 0.1 eV. On the other
hand, the half-life can also be experimentally expressed by following relation.
√0ν
T1/2
∼aMT
∆E B(2)

Here a is the abundance of the target isotope, M is the target mass, T is the measurementtime, ∆E is the energy resolution, and B is the background rate. According to Eq.(2), nextgeneration 0νββ experiments should have tonnes of target isotope, a background rate of 0.1–1
counts/(tonne·year), and an energy resolution of 3.5 % at 3.35 MeV. (Alternatively we could
combine a relatively low target mass with very high energy resolution.)2. ZICOS experiment
We are going to search for 0νββ signal using nucleus 96 Zr in the liquid scintillator. Thisexperiment is named Zirconium COmplex in liquid Scintillator (ZICOS) experiment forneutrinoless double beta decay search. The spherical detector will be filled with a liquid
scintillator which contains a high concentration of tetrakis (isopropyl acetoacetato) zirconium(Zr(iprac)4 ). If the radius of this detector is 1.5 m, then total volume is 14.1 m3 . This detector
is located in an outer cylindrical tank as shown in the left panel of Fig 1, which is 5.4 m indiameter and is 5 m in height, assuming EGADS tank which located in the Kamioka mine [1].
This tank should be filled with a pure water in order to identify the passing muons and toexclude external γs and neutrons. Photomultiplier will be mounted on the wall of both an innerdetector and an outer cylindrical tank. The photo coverage of the inner detector should be 40% in order to collect the scintillation light eﬃciently.

5.0m

5.4m

Figure 1. The left panel shows the conceptual design of the ZICOS detector. The spherical innerdetector will be filled with a liquid scintillator which contains high concentration of Zr(iprac)4 .
The middle panels shows the chemical structure formula of Zr(iprac)4 and the right panel showsa sample liquid scintillator containing 10 wt.% concentration of Zr(iprac)4 .
A nucleus 96 Zr has a Q-value of 3.35 MeV, which is the third largest value in possible double
beta decay nuclei, and 3 % natural abundance. We choose an anisole (methoxybenzene) as asolvent, because it can dissolve Zr(iprac)4 with high concentration. Our standard scintillator
cocktail is produced by dissolving 100 mg PPO (2,5-Diphenyloxazole) and 10 mg POPOP (1,4bis(5-phenyloxazol-2-yl) benzene) in 20 mL anisole. The light yield of the standard cocktail isalmost same as BC505 and the decay time is 20 ns.
The complex Zr(iprac)4 is not commercial product, therefore we have to synthesize it byourselves. The chemical structure formula of Zr(iprac)4 is shown in the middle panel of Fig. 1,
and the chemical formula of Zr(iprac)4 is Zr(CH3 CCOCHCOOCH(CH3 )2 )4 (MW=663.87).
Synthesized Zr(iprac)4 was a white powder. We measured the solubility of Zr(iprac)4 in anisoleand they were over 31.2 wt.%. This corresponds to 70g/L for the solubility of Zr metal. Thisquite high values makes us to realize tons scale experiment with small size of detector for low
backgrounds and low systematic errors. Indeed a quit transparent scintillator cocktail even for
the 10 wt.% concentration of Zr(iprac)4 was obtained as shown in the right panel of Fig. 1.
An absorption peak of Zr(iprac)4 was found at 278 nm. The overlap region between theabsorption spectrum of Zr(iprac)4 and the emission spectrum of anisole is smaller than the caseof Zirconium (IV) Acetylacetonate [2], therefore we could expect good performance of liquid
scintillator even if 10 wt.% concentration of Zr(iprac)4 dissolved in anisole.

Energy resolution at 1.03MeV electron [%]Relative light yield to the standard cocktail

3. Performance of liquid scintillator
The performance of a liquid scintillator from the point of view of 0νββ should be evaluated by
its energy resolution. To distinguish between 2νββ and 0νββ and avoid energetic γ rays from214 Bi and 208 Tl which are the progeny of 238 U and 232 Th chains, our initial goals should bethat (a) the light yield should be larger than 60% that of BC505, and (b) the energy resolution
should be 3.5 % at 3.35 MeV for a 10 wt.% concentration of Zr(iprac)4 .
We measured liquid scintillator samples with several concentrations of Zr(iprac)4 . The left
panel of Fig. 2 shows the measured light yield fraction for the standard cocktail as a functionof the concentration of Zr(iprac)4 . The light yield could be fitted by using following Eq.(3).
PPO 0.5 wt.%
1.0

PPO 1.5 wt.%
PPO 4.8 wt.%

0.80.6

0.40.2PPO 1.5 wt.%
30.0

2.0

4.0

6.08.0

PPO 4.8 wt.%

25.0
20.0
15.0
10.000

PPO 0.5 wt.%

35.0

10.0

concentration [wt.%]

5.000

2.0

4.0

6.0

8.0

10.0

concentration [wt.%]

Figure 2. The left panel shows the measured light yield fraction to standard cocktail as afunction of the concentration of Zr(iprac)4 in case of the concentration of PPO as 0.5 wt.%, 1.5
wt.% and 4.8 wt.%. The right panel shows the measured energy resolution as a function of theconcentration of Zr(iprac)4 in case of same concentration of PPO. Both light yield and energy
resolution are recovered by increasing the concentration of PPO with an order of 5 wt.%

Light yield fraction =η1 NP P O
η1 NP P O + η2 NZr(3)Here, η1 and η2 show the absorbance of PPO and zirconium β-keto ester complex per mole, andNP P O and NZr show the amount of PPO and zirconium β-keto ester complex in mole unit. We
assume the emission photon could be absorbed by both PPO and Zr(iprac)4 with a probability
in proportion to the number of molecular and the absorbance, because each absorption spectral
shape of PPO and Zr(iprac)4 are almost similar. According to the solid line in left panel ofFig. 2 which is fitted by Eq(3), the light yield fraction is expected to be almost 15 % at 10 wt.%
concentration of Zr(iprac)4 . This number is quite smaller than our goal.
The right panel of Fig. 2 shows the measured energy resolution as a function of theconcentration of Zr(iprac)4 . It appears that the energy resolution obeys the usual expectation
σ = √ σ0 , where E, E0 , and σ0 correspond to the electron energy, the reference energy, andE/E0

the energy resolution for the reference energy, respectively. The energy should be proportional
to the light yield so that we used same relation of Eq.(3) as a function of concentration ofZr(iprac)4 . The energy resolution obtained by solid line around 10 wt.% concentration was 35% at 1.03 MeV. The photo coverage of the measurement setup was estimated to be about 9.3% using Monte Carlo simulation. On the other hand, the ZICOS detector will have 40 % photo
coverage of the photomultiplier, so that the energy resolution for the ZICOS detector should be
9.4 % at 3.35 MeV. This value is also quite larger than our goal. Therefore, we have to improve
the liquid scintillator system in order to get both larger light yield and better energy resolution.

According to Eq.(3), the light yield also should be in proportion to fraction of the amount
PPO molecular with respect to the amount of Zr(iprac)4 molecular. In other words, we could be
able to modify the light yield of our liquid scintillator system, if we add more PPO in samples.
The dotted and dashed lines in the left panel of Fig. 2 show the expected light yield fraction
as a function of the concentration of Zr(iprac)4 using above equation in the case of PPO 1.5wt.% and 4.8 wt.%, respectively. Also the energy resolution will be modified by same equation
as shown in the right panel of Fig. 2. Actually, if we use the concentration of PPO as 5 wt.% in10 wt.% concentration of Zr(iprac)4 , the energy resolution is estimated by 4.1 ± 0.6 % at 3.35
MeV assuming 40 % photo coverage of photomultiplier. Therefore we achieved our goal for theliquid scintillator system.
4. Sensitivity of ZICOS experiment for 0νββ search
An experimental results of 0νββ for 96 Zr were obtained by NEMO-3 experiment [3]. A lower
0ν > 9.2 × 1021 years and an upper limit of the neutrino eﬀective
limits of the life-time was T1/2
mass was also ⟨mν ⟩ < 7.2 − 10.8 eV, if nuclear parameters of gA = 1.25 and gpp = 1.11, and thenuclear matrix model QRPA were used.
ZICOS experiment will use 216 kg of zirconium which includes 6.5kg of 76 Zr. This corresponds
to 9.2 kg of 136 Xe which means that 0.03 times to KamLAND-Zen[4]. Assuming same energy
resolution, background rates, and the measurement time as those of KamLAND-Zen, we can0ν > 4.4 × 1024 years for ZICOS
estimate the sensitivity for the lifetime measurement to be T1/2experiment. However, this is not enough for 0νββ search.
In order to increase sensitivity, we have to use some improvements. One improvement is anenrichment. NEMO-3 experiment used 7g of 96 Zr with an enriched to 57.3 % for their target [5].
If we can use 58.5 % enrichment of 96 Zr, which is a commercial grade, then the amount of 96 Zr
will be 126 kg. This corresponds to 0.56 times 136 Xe 320 kg of KamLAND-Zen, and the lifetime0ν > 1.1 × 1025 years. This is quite 1155 times longer than NEMO-3
limits is obtained by T1/2
limits. Using Eq.(2), it corresponds to ⟨mν ⟩ < 0.16 − 0.3 eV assuming same parameters ofgA = 1.25, gpp = 1.11 and QRPA model.
Another improvement is to reduce backgrounds around 3.5 MeV region. According to recent
analysis of KamLAND-Zen, they found that those backgrounds consist of decay products from
208 Tl (β and 2.6 MeV γ) both inside of liquid scintillator and the balloon film. In order toremove those backgrounds, we have to not only increase the energy resolution but also use
another technique such as Cherenkov lights.
5. Cherenkov lights using for background reduction
The number of photons generated by the Cherenkov radiation is calculated by following formula.
dN= 2πz 2 α sin2 θcdx

∫ 550nm
400nm

dλ= 277z 2 sin2 θc photon/cm
λ2(4)A refractive index of anisole is 1.518, so that the light yield of Cherenkov light is 118 photon/cm
above 400nm. The Cherenkov light below 400nm should be absorbed by PPO for a re-emissionof scintillation. On the other hands, the light yield of scintillation for anisole is almost 12,000
photon/MeV, and the range of 1 MeV electron in anisole is almost 0.8cm, so that the yield ratioof Cherenkov light and scintillation light should be order of 1 %.
In order to confirm the light yield of Cherenkov lights, we used UV cut filter (SC-37) whichcuts lights below 400 nm for the measurement of only Cherenkov light. The left panel ofFig. 3 shows the pulse height distribution of no filter, which include not only scintillation
lights from anisole (most of them should be less 400 nm) but also Cherenkov lights. Theright panel shows only Cherenkov lights above 400 nm using filter. The diﬀerence betweenthose amounts corresponds to the light yield of scintillation from anisole, and then the yieldratio is experimentally obtained by 2.8 %. The measured value is almost consistent with the30

20

SC-37 filter off(observed both Scintillation lights andCherenkov lights)
Mean : 314.9
Sigma : 127.7

2016

Data

15

BG Data
Subtracted

10SC-37 filter on(observed only Cherenkov lights)

18

number of observed events

number of observed events

2514

Data

Mean : 158.9
Sigma : 44.712

BG Data108

Subtracted
6452000100

200

300

400

500

600

700

800

900

1000

ADC channel

0100

200300

400

500

600

700

800

900

1000

ADC channel

Figure 3. The left panel shows the pulse height distribution of no filter, therefore thelight includes both scintillation and Cherenkov. The right panel shows only Cherenkov light
distribution above 400 nm, which is obtained by using SC-37 UV cut filter.
expectation within a statistical error. If we can select photomultiplier which detects Cherenkov
lights among all hits, then we may be able to distinguish 0νββ events and the backgrounds dueto the inconsistency of vertexes. For the purpose, we will use the diﬀerence of time spread ofCherenkov light (less than 1 ns) and scintillation light (20 ns), which is also argued in Ref.[6].
6. Conclusion
A tetrakis (isopropyl acetoacetato) zirconium has a absorption peak at 278 nm and the narrowabsorption spectrum makes less overlapping with the emission spectrum of anisole, so that
we have succeeded to obtain that the liquid scintillator containing 10 wt.% concentration ofZr(iprac)4 has 48.7 ± 7.1 % relative to BC505 for the light yield and the 4.1 ± 0.6 % at 3.35
MeV for the energy resolution in case of 40 % photo coverage of photomultiplier. We could
estimate the ZICOS experiment should have the sensitivity of ⟨mν ⟩ < 0.1 eV, if we use both58.5 % enriched 96 Zr and a further background reduction method using Cherenkov light in orderto achieve one tenth order of magnitude of KamLAND-Zen.
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), and a Grant-in-Aid for Scientific Research (C) (No.24540295) fromthe Japanese Society for the Promotion of Science (JSPS). We would like to thank to Prof.
T. Muramatsu who lent us the UV-visible spectrometer and helped us the measurement ofan absorbance of the complex, and Prof. T. Gunji who synthesized many samples of tetrakis(isopropyl acetoacetato) zirconium.
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