Photomultiplier tube (PMT) held by technician Giuseppe Dipietro of the Gran Sasso Laboratory. 2200 PMTs are incorporated into the Borexino neutrino detector (not seen). Neutrinos are elementary particles produced by nuclear reactions in stars and in supernova explosions. The PMT detects the flash of light when a neutrino hits the liquid core of the detector. The Borexino detector measures neutrino flux from the Sun. The Gran Sasso Laboratory is in Abruzzo, Italy. The neutrino detector is located 1400 metres underground and is accessed from a 10 kilometre long road tunnel. The rock overlay shields the laboratories from cosmic rays.