Improved measurement of B-8 solar neutrinos with 1.5 kt . y of Borexino exposure

We report on an improved measurement of the B-8 solar neutrino interaction rate with the Borexino experiment at the Laboratori Nazionali del Gran Sasso. Neutrinos are detected via their elastic scattering on electrons in a large volume of liquid scintillator. The measured rate of scattered electrons above 3 MeV of energy is 0.223(-0.016)(+0.015) (stat) (+0.006)(-0.006) (syst) cpd/100 t, which corresponds to an observed solar neutrino flux assuming no neutrino flavor conversion of Phi(ES)(8B) = 2.57(-0.18)(+0.17) (stat) (+0.07)(-0.07) (syst) x 10(6) cm(-2) s(-1). This measurement exploits the active volume of the detector in almost its entirety for the first time, and takes advantage of a reduced radioactive background following the 2011 scintillator purification campaign and of novel analysis tools providing a more precise modeling of the background. Additionally, we set a new limit on the interaction rate of solar hep neutrinos, searched via their elastic scattering on electrons as well as their neutral current-mediated inelastic scattering on carbon, C-12(nu, nu')C-12* (E-gamma = 15.1 MeV).