Title: The Borexino Experiment: Test of the Purification Systems and Data Analysis in the Counting Test Facility


Borexino is a real-time detector for low-energy solar neutrinos installed at the Gran Sasso Underground Laboratory. The goal of the experiment is to measure the flux of monoenergetic Be-7 solar neutrinos via neutrino-electron scattering in 100 tons of high purity liquid scintillator. The low background levels, needed to observe solar neutrinos below 1 MeV, place very stringent requirements in the radiopurity of the scintillator and other detector materials. New technologies in low counting Nuclear Physics were developed through the test experiment Counting Test Facility (CTF) installed in Gran Sasso, that reached and measured some of the highest radiopurity levels ever achieved. The goals of the last CTF data campaign were two-fold: to identify each contamination source and to study the efficiency of the purification system. Borexino has also the potential to probe solar neutrinos from the pep fusion process and from the CNO cycle. The detectability of pep and CNO neutrinos in Borexino strongly depends on the cosmogenic 11C background reduction. 11C is produced underground in reactions induced by the residual cosmic muon flux. We investigated on the efficiency of the three-fold coincidence with the parent muon track and the subsequent neutron capture on protons in tagging 11C on a one-by-one basis. Moreover, we tested the three-fold coincidence method in the Counting Test Facility data. First in-situ production of muon induced 11C production underground could be observed.