Inside view of the Borexino stainless steel sphere before nylon vessel installation and fluid filling. The sphere is 13.7 meters in diameter. The curvature and the tightly packed photomultiplier tubes (PMTs) can be seen. There are 2212 installed PMTs, most of which equipped with aluminium light concentrators to focus their view on the central volume of the detector. Optical fibers used for PMT timing calibration can be seen. Nylon vessel installation was in 2004, final fluid filling in 2007.
Credit: Borexino Collaboration