Scintillation properties of lithium-6 salicylate-loaded liquid scintillators

Abstract

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.