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

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 – 238U and 232Th – with sensitivities at the 10-15 g/g level. Our method combines neutron activation analysis with radiochemical techniques, followed by β/γ coincidence spectroscopy to minimize interference backgrounds. This approach achieves sensitivities of 0.65·10-15 g/g for 238U and 1.9·10-15 g/g for 232Th, among the best reported worldwide. Potential pathways for further sensitivity improvements are outlined in the conclusions.