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Abstract

The energy spectrum of solar neutrinos produced in radioactive decays of Be nuclei was directly measured for the first time in the Borexino experiment (Gran Sasso National Laboratory near L'Aquila, Italy) by the team composed of nearly 100 researchers from Italy, the United States, Russia, Germany, France, and Poland. Be nuclei are created in the 3He‡ He synthesis; their decay produces emission of monoenergetic electron neutrinos ne with an energy of 0.862 MeV. In the past, this neutrino component was detected only integrally by using radiochemical separation. The existing detectors that respond to the Vavilov ±Cherenkov radiation due to neutrino interaction with water can only reveal neutrinos with energy levels above 5 MeV. The new Borexino experiment utilizes a liquid organic scintillator. Excitation of scintillator molecules by elastic scattering of neutrinos on electrons results in a signal that is sufficiently strong for real-time spectral measurement of sub-MeV neutrinos, beginning with about 200 keV. High background noise of b-decays of C nuclei dominates at still lower energies. The experimental facility included structural elements and materials with a very low level of natural radioactivity, fabricated using specially dedicated technologies. The scintillator was poured into a spherical nylon shell and placed inside a steel sphere surrounded with a layer of water, while the experiment as a whole was run inside a mountain tunnel. After subtracting the background events from all known processes and taking into account the effect of neutrino oscillations via the Mikheev ± Smirnov ±Wolfenstein mechanism, the neutrino spectrum was found to be consistent, with high accuracy, with the beryllium solar neutrino predicted in the Standard Solar Models. Source: http://arXiv.org/abs/0708.2251