Inside view of the Borexino stainless steel sphere, prior to nylon vessel installation and fluid filling. The sphere is 13.7 meters in diameter; the picture was taken from the bottom looking upwards. There are 2212 installed photomultipliers (PMTs) to detect light from neutrino interactions, most of which equipped with aluminium light concentrators to focus their view on the central volume of the detector. The central flange is where the nylon vessels and fluid piping was anchored. Nylon vessel installation was in 2004, final fluid filling in 2007.
Credit: Borexino Collaboration