The Borexino thin nylon spherical "inner vessel" (8.5 meters diameter) is surrounded by a second nylon vessel and by ~2,000 photomultiplier tubes. A second, concentrical nylon vessel (11.5 meters diameter) prevents radioactive contamination from reaching the innermost volume of the detector. Both nylon membranes are 125 micron thick, were assembled as a nested package in a radon-suppressed clean room, and installed inside the detector in 2004. The nylon vessels are restrained with ultra-high molecular polyethylene ropes. The water was later displaced by the organic liquid scintillator, a benzene-like transparent liquid that produces flashes of light when neutrinos (and other ionising radiation) interacts with its electrons and nuclei.
Credit: Borexino Collaboration