ZICOS - Neutrinoless Double Beta Decay experimentusing Zr-96 with an organic liquid scintillator Yoshiyuki FUKUDA∗
Department of Physics, Faculty of Education, Miyagi University of Education
E-mail: fukuda@staff.miyakyo-u.ac.jp

Shigetaka MORIYAMA, Katsuki HIRAIDE
Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo

Izumi OGAWA
Department of Applied Physics, Faculty of Engineering, University of Fukui

Takahiro GUNJI, Ryohei HAYAMI, Satoru TSUKADA
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo
University of Science

Shunsuke KUROSAWA
Institute for Material Research, Tohoku UniversityA liquid scintillator containing a tetrakis (isopropyl acetoacetato) zirconium has been developed0ν ≥ 1027 years, we have to use tonefor ZICOS experiment. In order to reach the sensitivity T1/2
scale of 96 Zr and have to reduce 95 % of 208 Tl decay background. Using Monte Carlo simulation,
we could demonstrate new method using the topological information of Cherenkov light, andcould reduce 93 % of 208 Tl background with 78 % efficiency for 0νβ β signal. For the discrimination of Cherenkov light, the precise spectral pulse shape from Cherenkov lights was directly
measured by using sub-MeV electrons from 90 Sr/90 Y beta source. The observed shape was clearly
different from the shape of scintillation light. The pulse rise and fall (decay) time for Cherenkov
light were 0.8 ns and 2.5 ns, respectively. They were actually shorter than those times of scintillation light which were also measured by 1.6 ns and 6.5 ns, respectively. This clear differenceof rise time will be used for the pulse shape discrimination in order to select PMT which receive
Cherenkov lights, and the topological information due to Cherenkov light will be used for thereduction of backgrounds from 208 Tl decay which should be major backgrounds observed aroundQ-value (3.35MeV) of 96 Zr neutrinoless double beta decay.

European Physical Society Conference on High Energy Physics - EPS-HEP2019 10-17 July, 2019
Ghent, Belgium∗ Speaker.

c Copyright owned by the author(s) under the terms of the Creative Commons⃝
Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0).

https://pos.sissa.it/

ZICOS experiment

Yoshiyuki FUKUDA

1. ZICOS experiment
ZICOS is one of the future experiment for neutrinoless double beta decay. The target nuclei is96 Zr and the Q-value is 3.35MeV, therefore the radioactive backgrounds such as 214 Bi in Uranium

series and 10 C, which is spallation product of energetic cosmic muons, could be removed by theirlower energy. The conceptional design of ZICOS detector is shown in left panel of Fig.1. The
detector consists of spherical frame mounted by 650 of 20 inch photomultiplier (PMT) and innerballoon filled with a liquid scintillator, therefore it is almost similar structure as KamLAND-Zendetector. As reported by KamLAND-Zen[1], non-negligible events were found around 3 - 4 MeV,and those were the decay products from 208 Tl which was adhere on the surface of inner balloon.
Fortunately the Q-value of 136 Xe is 2.479MeV so that those were not backgrounds due to out ofrange for signal region.
However those events should be quite serious backgrounds for ZICOS experiment. The ZICOS detector will use liquid scintillator containing a tetrakis (isopropyl acetoacetato) zirconium(Zr(iPrac)4 ) inside of inner balloon, and will use pure water for outside of inner balloon. Therefore almost half of 208 Tl events observed in KamLAND-Zen should be reduced by missing theenergy. However, another half will be unavoidable backgrounds for 96Zr 0νβ β signal. In order to0ν ≥ 2 × 1027 years, we have been developed
remove those backgrounds to reach the sensitivity T1/2
the reduction technique using topological information from Cherenkov light [2].

averaged angle with respect to averaged direction
Number of events/2degree

Conceptual design of ZICOS detectorinner balloon
total volume : 180ton
fiducial vol. : 113ton
4.0m
3.5m
Zr(iprac)4 10wt.%

160DBD
140

208

120Tlvertex located within
fiducial volume10078% remain

93% cut

80
60

PPO 5wt.%
in Anisole

40

pure water

2020” PMD with QE~0.4 and TTS <1ns @ 1pe

0

Total PMT : 650 Photo coverage : 64%

010Scintillation (energy) + Cherenkov (BG reduction)

2030

4050 60 70 80 90
averaged angle [degree]

Figure 1:

2. Discrimination of Cherenkov light using pulse shapeAs described in our previous papers [3] [4], there is a difference of hit pattern of Cherenkov
lights between 208 Tl backgrounds and 0νβ β signals in case of Monte Carlo simulation, and wecould reduce about 93 % of 208 Tl beta decay events with 78 % efficiency for 0νβ β events using
an adequate topological information (defined by averaged angle) from Cherenkov light as shownin right panel of Fig.1.
ZICOS detector also should measure the energy as precise as possible in order to reduce background from 2νβ β signals, so that we have to observe scintillation lights for the precise energy1

ZICOS experiment

Yoshiyuki FUKUDAmeasurement. Therefore it is necessary to extract the hitted PMT by Cherenkov light among thelarge yield of scintillation. In this points of view, we have to discriminate PMT whether including
Cherenkov light or not using the pulse shape. Generally speaking, Cherenkov radiation is generated by the vibration of an electromagnetic dipole moment, so that the timing spreads duringpassing time of the charged particle (a few 100 pico seconds). On the other hands, scintillation isthe radiation from transition between the exited state and lower state of scintillator atoms, therefore
the timing should be decided by nuclear property with a few tenth of nano seconds. This differenceof spectral shape at both rise and decay time could be observed. In particular, the difference of risetime should be important because of poor amount of photon from Cherenkov light.

3. Pulse shape of Cherenkov light and Scintillation using sub-MeV electrons90

Sr/90Y

20deg

FADC counts

In order to observe the rise time of pulse shape precisely, we introduced two equipments.
One is photomultiplier which should have a fast timing response for both rise time and transittime spread (TTS). We used both fast PMT Hamamatsu H2431-50 (R2083) and CAEN V1721
digitizer for this measurement.In this paper, we used 1 GS/s ADC for Zero Length Encording(DPP-ZLEplus). These two fast timing spectral measurement would separate the shape of rise time
between Cherenkov light and scintillation clearly.
We used 1 MBq 90 Sr (half life 28.79 year) for sub-MeV electrons source. The end point energyof electrons from 90 Sr is 0.546 MeV so that all electrons could not emit Cherenkov light becauseof under Cherenkov threshold. On the other hand, 90 Y (half life 64 hour) beta decay should bea radioactive equilibrium, then same radiation yield could be expected. The end point energy ofelectrons from 90 Y is 2.280 MeV, so that about half of electrons could emit Cherenkov lights. Usingelectrons above 0.679 MeV, we measured the pulse shape for Cherenkov light.

100

Scintillation

8060

Cherenkov (fast) scaled

40

Vial filled byLiquidScintillatororAnisole

200556065707580859095

100

FADC time [ns]Figure 2:The left photograph of Fig.2 shows the setup for this measurement. The incident electrons
were colimated by Lead block and the direction was fixed by 20 degree with respect to the direction of PMT, because it is easy to induce Cherenkov photon to the PMT. For the measurement ofscintillation, this setup was same even though Cherenkov photon was induced to PMT, because oflarge amount of light yield of scintillation.
2ZICOS experiment

Yoshiyuki FUKUDA

4. Averaged Pulse shape of Cherenkov light and Scintillation
In order to obtain the spectral shape, we have to collect signal events and have to make a
distribution of FADC counts for each timing bin. Of course, same distribution for backgrounds incase of no source should be subtracted in each timing bin.
At first, we measured the pulse shape for scintillation.Because of high statistics, we could
select events with almost same pulse height. Next we measured the pulse shape from Cherenkov
light. In this time, the vial which was filled with only Anisole was covered by SC-37 filter in orderto observe Cherenkov photons only above 400 nm. For real experiment, Cherenkov photons below
400 nm should be absorbed by the secondary scintillator such as PPO, therefore only photons above
400 nm should be observed. Due to environmental backgrounds, we have selected events withalmost same pulse height for signal and background. Again FADC count distributions for eachtiming in case of Cherenkov light were obtained by subtracting the distribution from no source.
Using mean and RMS for each distribution, we made averaged pulse shape of Cherenkov light.
The right panel of Fig.2 shows the averaged pulse shape for scintillation and Cherenkov light,respectively. In this figure, the shape of Cherenkov light was scaled to same peak height as scintillation. The observed Cherenkov pulse shape was clearly different from the shape of scintillation.
The pulse rise and fall (decay) time for Cherenkov light were 0.8 ns and 2.5 ns, respectively. They
were actually shorter than those times of scintillation light which were also measured by 1.6 ns and6.5 ns, respectively. This clear difference of rise time will be used for the pulse shape discrimination in order to select PMT which receive Cherenkov lights, and the topological information suchas averaged angle which was described in our previous paper [3] will be used for the reduction ofbackgrounds from 208 Tl decay which should be major backgrounds observed around Q-value (3.35MeV) of 96 Zr neutrinoless double beta decay.

5. Conclusion
Using sub-MeV electron from 90 Sr/90 Y beta source, we successfully measured the actual pulse
shapes of Cherenkov light. The measured pulse rise and decay time for Cherenkov light wereactually shorter than those times of scintillation. We will develop the pulse shape discriminationin order to distinguish PMT whether receive Cherenkov light or not within 2019 fiscal year. Also
we are going to measure an actual energy resolution using special light guide which has almost 60% photocoverage, and will measure the topological information using hemisphere detector HUNIZICOS to directly obtain an averaged angle for fixed directional electron.

References[1] A.Gando et al. (KamLAND-Zen collaboration), Physical Review Letters 117 (2016) 082503.
[2] Y.Fukuda, Journal of Physics: Conference Series 718 (2016) 062019.
[3] Y.Fukuda, Y.Kamei, Narengerile, A.Obata, S.Moriyama, I.Ogawa, T.Gunji, R.Hayami, S.Tsukada,
Bulletin of Miyagi University of Education 52 (2017) 139-147.
[4] Y.Fukuda, Y.Kamei, Narengerile, A.Obata, S.Moriyama, K.HIraide, I.Ogawa, T.Gunji, R.Hayami,S.Tsukada, S.Kurosawa Bulletin of Miyagi University of Education 53 (2018) 183-192.

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