Purification of lanthanides for large neutrino detectors: Thorium removal from gadolinium chloride

Metal-loaded liquid scintillators are the detectors of choice for various neutrino experiments. Procedures have been developed to transfer metals into organic liquids by solvent extraction or direct dissolution of a metallic compound. Traces of natural radioactivity introduced into the scintillator with the metal may produce undesirable backgrounds. Measurements using a 229Th tracer indicate that the inclusion of a pH-controlled partial hydrolysis and filtration prior to the preparation of a gadolinium-loading compound can reduce thorium by a factor of ∼100. This “self-scavenging” procedure has the advantage that it uses only reagents encountered in the production process. Addition of non-elemental scavengers such as iron, or the use of solvent extraction or ion exchange procedures can be avoided. It also improves the optical transmission in the blue region by removing traces of iron. This purification method has potential applications to the large-scale production of other metal-loaded liquid scintillators and for the removal of traces of thorium in the industrial production of lanthanides.