The Borexino prototype detector (CTF) shown here, a 4-ton spherical scintillator target surrounded by ultra-pure water and 100 photomultiplier tubes, operated between 1994 and 2003. The tubes detect flashes of light from ionizing radiation (including neutrinos) occurring in the scintillating volume. A thin nylon "shroud" prevents radioactive contamination from entering the center-most volume of the detector. The main Borexino experiment has been taking data since 2007.
Credit: Borexino Collaboration