Performances of the CTF experiment in prospect of Borexino

The crucial issue of the direct determination of the 7Be solar neutrino flux will be addressed by the Borexino experiment, a real time, massive calorimetric liquid scintillation detector to be installed at the Gran Sasso Laboratory. The challenge of the extremely low radioactive level allowed in the scintillator for the feasibility of the measurement required the precise and sensitive evaluation of its radiopurity with a dedicated 5 tons prototype, the Counting Test Facility. The main experimental features of the CTF and its unprecedented results in the area of the low activity measurements are described in this paper, together with their implications in the design of the full scale experiment