Borexino is a solar neutrino experiment at the Laboratori Nazionali del Gran Sasso, in Italy, designed to detect low-energy solar neutrinos, in real time, using 300 tons of liquid scintillator in an unsegmented detector. The detector is located in Hall C of the Gran Sasso underground laboratory. Neutrino-electron scattering in the scintillator produces flashes of scintillation light which are observed by 2000 photomultiplier tubes (20 cm diameter). With a fiducial volume of about 100 tons, the expected neutrino count rate is about 30 events per day above 250 keV (Standard Solar Model and Large Mixing Angle neutrino oscillation model, assuming vacuum oscillations), due mostly to 7Be solar neutrinos.

The emphasis in Borexino is on the ultimate in radiopurity, for the energies where the 7Be neutrino signals occur are filled with false positives from many different species of radioactive decay. In particular, the uranium and thorium chains are great problems, as well as the radioactive noble gas isotopes 39Ar, 85Kr, and 222Rn, all present in the Earth’s atmosphere. Numerous methods of getting rid of these isotopes have been used, ranging from careful materials selection, through construction of the most delicate parts of the detector in a radon-free clean room, up to a several month sequence of purging the entire detector with special nitrogen free of argon, krypton and radon.

Structure of Borexino

The concentric volumes of the detector are based on the principle of graded shielding: the cleanest parts are at the very center. First, of course, the underground location of the detector, under 3500 meters of water equivalent, reduces the cosmic ray flux to a fraction of its value at the surface. Next, a shield of ultra-pure water protects internal parts of the detector from neutrons and gamma rays, emitted by radioactive decays in the rock walls of the laboratory.

Within the ultra-pure water is a stainless steel sphere (SSS) that acts as a support structure for > 2000 PMTs. About 200 of these are located in the volume of water; they act as a muon detector by observing the tracks of Cherenkov light left by muons passing through. The remainder are on the inner surface of the SSS, pointed inward.