Borexino is a new solar neutrino experiment that will use 300 tons of high purity liquid scintillator to measure the low energy part of the solar neutrino spectrum (E < 1MeV). More than 30 years of experiments have shown that fewer solar neutrinos are detected than expected from the nuclear fusion reactions that power the sun. Recent experiments indicate that the discrepancy is due to "neutrino oscillations", a process in which neutrinos produced in the sun as "electron-neutrinos", oscillate to a non-electron-neutrino state. The oscillation process is possible if neutrinos do indeed have a finite mass, rather than a zero mass, as is the case in the present standard model of elementary particles. If oscillations are confirmed, and neutrinos have mass, the result would be a major departure from our current understanding of elementary particle, with important implications in physics and astronomy. The latest experimental data of the Sudbury Neutrino Observatory, together with the SuperK experiment, provide the most convincing evidence that neutrinos are oscillating. The interpretation of these and other data in the Mikheyev-Smirnov-Wolfenstein theoretical framework indicates that oscillation effects for neutrinos below 1 MeV could be dramatic. One observational possibility is a large day/night asymmetry in count rate for low energy neutrinos. Borexino is designed to measure the 0.86 MeV 7Be neutrinos and is thus well positioned to explore this unique low energy neutrino oscillation effect. The Borexino detector is located in the Gran Sasso underground laboratory in Italy. The low background requirements necessary for detecting 7Be neutrinos pose formidable technical challenges. After several years of R&D with a prototype detector, and four years of construction, Borexino is expected to become operational in 2002. The objective of this proposal in the two-year period 2002-2004 is to complete the commissioning and start-up phase of Borexino and begin taking solar neutrino data. This proposal will support the Princeton University Group in completing and operating the muon detector and the scintillator purification, filling, and containment subsystems of the detector. It will also support the early stages of data acquisition and data analysis.
