Borexino

Borexino is a solar neutrino experiment hosted at the Gran Sasso National Laboratory in Italy. It is designed to detect low-energy solar neutrinos in real time, using a ultra-pure liquid scintillator to observe faint light produced when neutrinos scatter off electrons.

The detector contains about 278 tonnes of liquid scintillator based on pseudocumene with a fluor (PPO). This scintillator is housed in a spherical acrylic inner vessel of about 4.25 meters in diameter and is surrounded by a stainless-steel outer structure and buffer fluids. The assembly is contained within a large water-filled tank that provides shielding and acts as an active muon veto via Cherenkov light detected by an array of photomultiplier tubes (PMTs). An outer Cherenkov detector enhances the rejection of cosmic-ray backgrounds. The combination of radiopurity, shielding, and light collection enables measurements of solar neutrinos down to a few hundred keV.

Borexino began operations in the mid-2000s and achieved the first real-time measurement of 7Be solar neutrinos with high precision, an important milestone in solar-neutrino spectroscopy. It subsequently extended its program to other solar components, including pp, pep, and 8B neutrinos, and, in 2020, provided the first strong evidence for solar CNO neutrinos. These results have contributed to tests of neutrino oscillations, particularly the MSW effect, and to constraints on solar models and metallicity.

The collaboration continues data collection and analysis, maintaining Borexino as a reference for low-background, real-time solar-neutrino measurements.