The Borexino detector: Methodology for a large-scale low background liquid scintillator

Borexino, a real-time device for low energy neutrino spectroscopy, has completed the construction phase in the middle of 2006, in the underground laboratories at Gran Sasso, Italy (LNGS). The detector has been filled with about 300 tons of highly purified scintillator surrounded by an internal buffer of 1000 tons of a purified organic solvent buffer, 2400 tons of ultra-pure water as an external shield and located underground with the equivalent shielding of 3500 m of water. Since May 16, 2007, Borexino is under data acquisition. The experimental goal is the direct measurement of the flux of 7Be solar neutrinos of all flavours via neutrino-electron scattering in an ultra-pure scintillation liquid. The paper describes the design of the Borexino detector, the construction phases and the various facilities that play an essential role in its operation. This paper describes the design requirements and methods for reducing background impurities for the Borexino detector.