A scintillator purification system for a large scale solar neutrino experiment

An on-line purification system for a large-scale low-background liquid scintillation detector has been tested with the 5-ton liquid scintillation detector, the Counting Test Facility at Gran Sasso. Counter-current water extraction removes polar and charged impurities such as ions and metal salts. Vacuum distillation of the entire scintillator has been applied to remove both radioactive impurities and chemical impurities that affect the optical properties of the scintillator. Filtration with a 0.05 μm particulate filter removed suspended dust particles that may contain U, Th and K. Nitrogen stripping was employed to remove dissolved water and noble gases (e.g. ⁸⁵Kr) from the scintillator. The system has effectively reduced the internal back 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.