A scintillator purification system for a large scale solar neutrino experiment

An on-line purification system for a large-scale low-background liquid scin tillation detector has been tested with the 5-ton liquid scintillation dete ctor, the Counting Test Facility at Gran Sasso. Counter-current water extra ction removes polar and charged impurities such as ions and metal salts. Va cuum distillation of the entire scintillator has been applied to remove bot h radioactive impurities and chemical impurities that affect the optical pr operties of the scintillator. Filtration with a 0.05 mu m particulate filte r removed suspended dust particles that may contain U, Th and K. Nitrogen s tripping was employed to remove dissolved water and noble gases (e.g. Kr-85 ) from the scintillator. The system has effectively reduced the internal ba ck in the 5 ton liquid scintillation detector from 470+/-90 events/d to the detection limit of the detector (+/-45 events/d) in the energy window 250< E<800 keV.