Calaprice In 1938, Hans Bethe provided the first explanation of the energy production inside the Sun that was able to account for the energy flux we have observed over the lifetime of the Earth. This theory is based on thermonuclear reaction processes occurring in the Sun's core. In Bethe's seminal work, two energy production mechanisms were discussed: the so-called carbon-nitrogen-oxygen (CNO) and the proton-proton (pp) cycles. Today, it is the latter process that is thought to be responsible for the production of more than 98% of the energy flowing from the Sun. The pp cycle consists of a series of nuclear reactions that convert four hydrogen atoms to a helium atom, releasing an energy of 26.7 Mev for each conversion. The energy is released in the form of charged particle kinetic energies, gamma rays, and neutrinos. Because of their weak interaction, the neutrinos leave the sun with very few interactions. The "solar neutrino problem" came in existence more than 35 years ago, when the pioneering "Chlorine" experiment of Ray Davis and collaborators showed that the actual number of neutrinos measured on Earth was only about 1/3 of the number predicted by the "standard solar model". Since then, other experiments have been performed and all find a deficit of observed neutrinos, compared to the prediction. The present body of experimental evidence indicates that the deficit is not due to a flaw in our understanding of the Sun, but rather, is caused by fundamental neutrino processes that convert the normal electron-type neutrinos produced in the sun to neutrinos not readily detected in the current detectors. The neutrino conversions or "neutrino oscillations" can occur if the neutrino has a non-zero mass. But a non-zero neutrino mass is contrary to standard particle physics theory. Thus, the "solar neutrino problem" has become an issue of fundamental importance to elementary particle physics. If the neutrino does have mass, as suggested by the current experiments, new fundamental physics theories will be required. Borexino is a new solar neutrino detector designed to detect low energy solar neutrinos by use of a large 300 ton liquid scintillator. The detector will be located in the Gran Sasso underground laboratory in Italy. It's principle objective is to detect the mono-energetic Be-7 neutrino at an energy of 0.86 MeV. According to current analysis of solar neutrino data, the Be-7 neutrino is expected to exhibit maximal neutrino oscillation effects. Borexino will attempt to observe direct evidence of neutrino oscillations and confirm the present hypothesis of neutrino mass. The signals to be searched for include very large annual and day/night variations in count rate that are uniquely caused by neutrino oscillations.
