Low Energy Nuclear Physics at Princeton - The Borexino Solar Neutrino Experiment

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Borexino is a new solar neutrino experiment that will use 300 tons ofhigh purity liquid scintillator to measure the low energy part of thesolar neutrino spectrum (E 1MeV). More than 30 years of experimentshave shown that fewer solar neutrinos are detected than expected fromthe nuclear fusion reactions that power the sun. Recent experimentsindicate that the discrepancy is due to "neutrino oscillations", aprocess in which neutrinos produced in the sun as "electron-neutrinos",oscillate to a non-electron-neutrino state. The oscillation process ispossible if neutrinos do indeed have a finite mass, rather than a zeromass, as is the case in the present standard model of elementaryparticles. If oscillations are confirmed, and neutrinos have mass, theresult would be a major departure from our current understanding ofelementary particle, with important implications in physics andastronomy. The latest experimental data of the Sudbury Neutrino Observatory,together with the SuperK experiment, provide the most convincingevidence that neutrinos are oscillating. The interpretation of theseand other data in the Mikheyev-Smirnov-Wolfenstein theoretical frameworkindicates that oscillation effects for neutrinos below 1 MeV could bedramatic. One observational possibility is a large day/night asymmetryin count rate for low energy neutrinos. Borexino is designed to measurethe 0.86 MeV 7Be neutrinos and is thus well positioned to explore thisunique low energy neutrino oscillation effect. The Borexino detector is located in the Gran Sasso undergroundlaboratory in Italy. The low background requirements necessary fordetecting 7Be neutrinos pose formidable technical challenges. Afterseveral years of R&D with a prototype detector, and four years ofconstruction, Borexino is expected to become operational in 2002. Theobjective of this proposal in the two-year period 2002-2004 is tocomplete the commissioning and start-up phase of Borexino and begintaking solar neutrino data. This proposal will support the PrincetonUniversity Group in completing and operating the muon detector and thescintillator purification, filling, and containment subsystems of thedetector. It will also support the early stages of data acquisition anddata analysis.