Ultra-trace analysis of U and Th in organic liquid scintillators with high sensitivity

Abstract Rare event searches demand extremely low background levels, necessitating ever-advancing screening techniques to enhance sensitivity. Liquid scintillators are highly attractive as detector media due to their inherent radiopurity and scalability in mass. In this work, we present a screening procedure to measure ultra-trace concentrations of natural contaminants – $$^{238}$$ 238 U and $$^{232}$$ 232 Th – with sensitivities at the $${10}^{-15}$$ 10 - 15 g/g level. Our method combines neutron activation analysis with radiochemical techniques, followed by $$\beta $$ β / $$\gamma $$ γ coincidence spectroscopy to minimize interference backgrounds. This approach achieves sensitivities of $$0.65 \cdot 10^{-15}$$ 0.65 · 10 - 15 g/g for $$^{238}$$ 238 U and $$1.9 \cdot 10^{-15}$$ 1.9 · 10 - 15 g/g for $$^{232}$$ 232 Th, among the best reported worldwide. Potential pathways for further sensitivity improvements are outlined in the conclusions.