Scintillation properties of lithium-6 salicylate-loaded liquid scintillators

The limited availability of conventional ³He proportional counters provides impetus for developing novel neutron detectors. As a candidate, lithium-6-loaded liquid scintillators with neutron/gamma pulse shape discrimination (n-γ PSD) capabilities have been developed. However, the trade-off relationship between the ⁶Li-loading amount and scintillation light yield is a significant problem. This is because ⁶Li-loading involves the addition of non-luminescent materials, which cause non-radiative relaxation of the excited states. Therefore, aiming to reduce non-radiative relaxation, we chose lithium-6 salicylate (⁶LiSal), which shows fluorescence in the visible light region, as a chemical for ⁶Li-loading. In this study, we analyzed the photoluminescence/scintillation properties based on the Förster resonance energy transfer and investigated the optimal content for obtaining a high light yield. By maximizing the sequential energy transfer from the solvent (toluene) to the phosphor (POPOP), a high light yield ⁶Li-loaded liquid scintillator (4220 photons per MeV under gamma-ray irradiation) with a ⁶Li concentration of approximately 0.1 wt% was developed. Thermal neutron events were successfully detected with a light yield of 3970 photons per neutron, which is more than three times higher than those of other organic scintillators. In addition, focusing on the triplet-triplet annihilation process and further optimizing the component for the n-γ PSD, the thermal neutron and gamma-ray events were successfully separated. The developed high light yield ⁶Li-loaded liquid scintillators show n-γ PSD capabilities and can be promising candidates as alternative detectors to the ³He proportional counter.