Development of water scintillator for a future large-scale neutrino experiment

The water Cherenkov detectors, such as Kamiokande and Super-Kamiokande, have led the study of neutrinos so far, as seen in the observation of supernova neutrinos and discovery of neutrino oscillations. In the next-generation neutrino experiment, a large-scale scintillator experiment of the same size as Super-Kamiokande is necessary. Because organic scintillators are toxic and flammable, we are developing safe and clean water scintillators suitable for large-scale experiments. The light emission was confirmed with a water scintillator prepared by mixing pseudocumene with water, fluorescence and surfactant, and it was revealed that the amount of light emission depends on the amount of pseudocumene. However, the transmittance of this water scintillator was low. When a laser was injected, a trajectory of light was seen which could not be seen in water, so we judged that scattering occurred inside and the transmittance was lowered. Because of the difference between the refractive index of the surfactant and the refractive index of water, it is considered that reflection occurred on the micell surface. We are developing a fluorochemical surfactant with a refractive index close to that of water. In this report, we report the performance of the water scintillator using hydrocarbon surfactant, and transparency of five different surfactant including a fluorochemical surfactant.