The optical system in Borexino

Borexino detector is composed of 2214 photomultipliers (PMTs) collecting the scintillation light produced by the liquid scintillator. Phototubes are partially immersed in the scintillator and partially in high purity water. A proper electrical and mechanical selection of the devices as well as a custom-made encapsulation was mandatory for a long-term immersion in these two aggressive liquid. Underwater cables and connectors and optical fibers are also described in detail.