Development of liquid scintillator containing a zirconium complex for neutrinoless double beta decay experiment
Introduction
In 1998, Super-Kamiokande discovered atmospheric 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 ().
The half-life of is given bywhere is the kinematic phase space factor, is the matrix element including Fermi, Gamow–Teller and tensor contributions, me is the electron mass, and is the effective neutrino mass. According to Eq. (1), we have to be able to measure a half-life of the order of years assuming the neutrino mass to be 100 meV. On the other hand, the half-life can also be expressed experimentally aswhere a is the abundance of the target isotope, M is the target mass, T is the measurement time, is the energy resolution, and B is the background rate. For next-generation experiments, the target isotope mass should reach the order of 1000 kg and the background rate should stay around 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 experiments are now ongoing and more are planned as future experiments with several target isotopes. Table 1 shows a summary of future 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 experiment.
Section snippets
Liquid scintillator containing a zirconium complex
To use 96Zr for a 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 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 ester complex with –OC3H7 or –OC2H5, instead of the complex. We and Prof. Takahiro Gunji (Tokyo University of Science) synthesized the zirconium 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).
References (6)
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