Development of Liquid Scintillator containing a Zirconium Complex for Neutrinoless Double Beta Decay Experiment
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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 (0.1 counts/(tonne・year) and that can monitor tonnes 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, and a diethyl malonate complex. Those shifted the absorption peak to around 245nm and 210nm, respectively, which are shorter than the emission peak of anisole (275nm). However, the shift of the absorption peak depends on the the scintillation solvent. Therefore we have to choose an adequate solvent for the liquid scintillator. The best performance will be obtained by pure anisole scintillator containing a tetrakis diethyl malonate zirconium. We also synthesized a Zr-ODZ complex, which has a high quantum yield (30%) and good emission wavelength (425nm) 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 benzonitrile.
宮城教育大学紀要 48, 125-132, 2013