New challenging questions in astro-particlephysics are opened with the discovery of neutrino mass and mixing. Low energy solar neutrino spectroscopy plays now a fundamental role in understanding the new physics and the nuclear fusion processes in the Sun. A large scale scintillation detector Borexino at the Gran Sasso Laboratories (Italy) will soon pursue this goal in real time. In this work a custom data acquisition system (electronics and software) has been developed together with the offline data processing algorithms. A water Cherenkov detector has been installed for cosmogenic background identification, including the tagging of 11C in the energy window of pep neutrinos. With a prototype detector, the in-situ 11C production rate was measured for the first time, resulting in 0.135±0.024[stat]±0.014[syst]/d/ton. The detection of the weak spectral contribution of pep neutrinos in Borexino was shown to be realistic and will determine uniquely the fundamental solar pp fusion rate.
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