The crucial issue of the direct determination of the Be-7 solar neutri
no flux will be addressed by the Borexino experiment, a red time, mass
ive 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 measureme
nt required the precise and sensitive evaluation of its radiopurity wi
th 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, tog
ether with their implications in the design of the full scale experime
nt.