Development of liquid scintillator containing a zirconium complex for neutrinoless double beta decay experiment

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Abstract

An organic liquid scintillator containing a zirconium complex has been developed for a new neutrinoless double beta decay experiment. In order to produce a detector that has good energy resolution (4% at 2.5 MeV) and low background (0.1counts/(t·year)) and that can monitor tons of target isotope, we chose a zirconium β-diketone complex having high solubility (over 10 wt%) in anisole. However, the absorption peak of the diketone ligand overlaps with the luminescence of anisole. Therefore, the light yield of the liquid scintillator decreases in proportion to the concentration of the complex. To avoid this problem, we synthesized a β-keto ester complex introducing –OC3H7 or –OC2H5 substituent groups in the β-diketone ligand, which shifted the absorption peak to around 245 nm, which is shorter than the emission peak of anisole (275 nm). However, the shift of the absorption peak depends on the polarity of the scintillation solvent. Therefore we must choose a low polarity solvent for the liquid scintillator. We also synthesized a Zr–ODZ complex, which has a high quantum yield (30%) and good emission wavelength (425 nm) with a solubility 5 wt% in benzonitrile. However, the absorption peak of the Zr–ODZ complex was around 240 nm. Therefore, it is better to use the scintillation solvent which has shorter luminescence wavelength than that of the aromatic solvent.

Introduction

In 1998, Super-Kamiokande discovered atmospheric νμ oscillation in its zenith angle measurement [1]. This was the first evidence of a non-zero neutrino mass which indicates the existence of physics beyond the standard model. Recent leptogenesis models postulate the existence of heavy right-handed neutrinos, which are also generally present in the See–Saw model, and strongly favor the existence of Majorana neutrinos. The observation of neutrinoless double beta decay would confirm the Majorana nature of the neutrino and would also provide more information about the neutrino mass scale and hierarchy. Therefore, it is important to try to detect a real signal from neutrinoless double beta decay (0νββ).

The half-life of 0νββ is given by[T120ν(0+0+)]1=G0νM0ν2mν2me2where G0ν is the kinematic phase space factor, M0ν is the matrix element including Fermi, Gamow–Teller and tensor contributions, me is the electron mass, and mν is the effective neutrino mass. According to Eq. (1), we have to be able to measure a half-life of the order of 1025 years assuming the neutrino mass to be 100 meV. On the other hand, the half-life can also be expressed experimentally asT120νaMTΔEBwhere a is the abundance of the target isotope, M is the target mass, T is the measurement time, ΔE is the energy resolution, and B is the background rate. For next-generation 0νββ experiments, the target isotope mass should reach the order of 1000 kg and the background rate should stay around 0.11counts/(t·year) with an energy resolution of 4% at 2.5 MeV (alternatively we could combine a relatively low target mass target with very high energy resolution).

Many 0νββ experiments are now ongoing and more are planned as future experiments with several target isotopes. Table 1 shows a summary of future 0νββ experiments. According to Table 1, there is no experiment planning to use 96Zr (Q-value=3350 keV) as a target isotope. Here we report new liquid scintillator containing a zirconium complex that could be used in a future 0νββ experiment.

Section snippets

Liquid scintillator containing a zirconium complex

To use 96Zr for a 0νββ experiment, we have developed a liquid scintillator containing a zirconium complex. A liquid scintillator was used for neutrino experiments such as KamLAND and SNO, because of their large target masses. As described in the previous section, a next-generation 0νββ experiment should also have a target isotope mass of about a tonne and a good energy resolution in order to detect a neutrino mass below 100 meV. However, a large volume detector generally worsens both the energy

Zirconium complex with substituent groups

There are substituent groups that can be used to shorten the absorption wavelength. We chose a β-keto ester complex with –OC3H7 or –OC2H5, instead of the β-diketone complex. We and Prof. Takahiro Gunji (Tokyo University of Science) synthesized the zirconium β-keto ester complex shown in Fig. 4. The molecular masses of a tetrakis(isopropyl acetoacetato) zirconium complex (Zr(CH3CCOCHCOOCH(CH3)2)4 Zr(iprac)4) and a tetrakis(ethyl acetoacetato) zirconium complex (Zr(CH3CCOCHCOOCH2CH3)4 Zr(etac)4)

Light yield of the liquid scintillator

To transfer the energy in the solvent to the photomultiplier, we dissolved 2,5-diphenyloxazole (PPO), which has an absorption peak at 310 nm and an emission peak at 368 nm, as a secondary scintillator. The addition of 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP), which has an absorption peak at 364 nm and an emission peak at 427 nm, improves the quantum efficiency of the energy transfer to the photomultiplier. We used 100 mg PPO and 10 mg POPOP in 20 ml of anisole as the typical scintillator cocktail.

Zirconium complex with photoluminescence

Another possibility for the use of a zirconium complex is to utilize photoluminescence. We synthesized a tetrakis(8-quinolinolate) zirconium (ZrQ4, C36H24N4O4Zr) complex, and found its solubility to be 2 wt% in benzonitrile. The photoluminescence spectrum of ZrQ4 in benzonitrile was measured, and the maximum emission peak was found to be around 548 nm. We made a liquid scintillator cocktail using ZrQ4 (50 mg) in benzonitrile solutions (20 ml) with both PPO (100 mg) and POPOP (10 mg), and measured the

Acknowledgments

This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (No. 24104501) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and a Grant-in-Aid for Scientific Research (C) (No. 24540295) from the Japanese Society for the Promotion of Science (JSPS).

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