A large-scale low-background liquid scintillation detector: The counting test facility at Gran Sasso

A 4.8 m³ unsegmented liquid scintillation detector at the underground Laboratori Nazionali del Gran Sasso has shown the feasibility of multi-ton low-background detectors operating to energies as low as 250 keV. Detector construction and the handling of large volumes of liquid scintillator to minimize the background are described. The scintillator, 1.5 g PPO/L-pseudocumene, is held in a flexible nylon vessel shielded by 1000 t of purified water. The active detector volume is viewed by 100 photomultipliers, which measure time and charge for each event, from which energy, position and pulse shape arc deduced. On-line purification of the scintillator by water extraction, vacuum distillation and nitrogen stripping removed radioactive impurities. Upper limits were established of < 10^-7 Bq/kg-scintillator for events with energies 250 keV < E < 800 keV, and < 10^-9 Bq/kg-scintillator due to the decay products of uranium and thorium. The isotopic abundance of ¹⁴C/¹²C in the scintillator was shown to be approximately 10^-18 by extending the energy window of the detector to 25-250 keV. The ¹⁴C abundance and uranium and thorium levels in the CTF are compatible with the Borexino Solar Neutrino Experiment.