Counting test facility for the Borexino experiment

A fundamental breakthrough which opened the way to the realization of the Borexino detector was the demonstration of exceptionally low, unprecedented radioactive contaminations in the liquid scintillator, obtained with its pilot prototype Counting Test Facility. Though of limited dimension, with its 4.8 m(3) of active liquid core, CTF has however been a key milestone not only for Borexino, but also for the entire field of the ultra-low background searches. Here, we succinctly remind the motivations, which concurred to lay down the project, as well as the specific radiopurity challenge, which guided the design. After the description of the technical elements of the detector, the main outcomes are summarized, both regarding optical and purity scintillator properties, with special emphasis on the exceptional achievements in term of ultra-low traces of radioactive contaminants. The discussion is completed with the description of how CTF was employed for the pre-qualification of the entire inventory of the Borexino scintillator, confirming also in the final phase of its life its essential role for the success of the overall Borexino solar neutrino program.