Fabrication of liquid scintillators loaded with 6-phenylhexanoic acid-modified ZrO₂ nanoparticles for observation of neutrinoless double beta decay

The observation of neutrinoless double beta decay is an important issue in nuclear and particle physics. The development of organic liquid scintillators with high transparency and a high concentration of the target isotope would be very useful for neutrinoless double beta decay experiments. Therefore, we propose a liquid scintillator loaded with metal oxide nanoparticles containing the target isotope. In this work, 6-phenylhexanoic acid-modified ZrO₂ nanoparticles, which contain⁹⁶Zr as the target isotope, were synthesized under sub/supercritical hydrothermal conditions. The effects of the synthesis temperature on the formation and surface modification of the nanoparticles were investigated. Performing the synthesis at 250 and 300^◦C resulted in the formation of nanoparticles with smaller particle sizes and higher surface modification densities than those prepared at 350 and 400^◦C. The highest modification density (3.1 ± 0.2 molecules/nm²) and Zr concentration of (0.33 ± 0.04 wt.%) were obtained at 300^◦C. The surface-modified ZrO₂ nanoparticles were dispersed in a toluene-based liquid scintillator. The liquid scintillator was transparent to the scintillation wavelength, and a clear scintillation peak was confirmed by X-rayinduced radioluminescence spectroscopy. In conclusion, 6-phenylhexanoic acid-modified ZrO₂ nanoparticles synthesized at 300^◦C are suitable for loading in liquid scintillators.