Borexino: The Detector and the Physics Program

The exciting arena of the solar neutrino investigation will see in the near future a new actor coming in the game: Borexino. This massive, calorimetric, liquid scintillation detector, under installation in the underground Gran Sasso Laboratory, will address one of the fundamental issue of this field, the direct determination of the flux of the neutrinos produced in the 7 Be electron capture reaction in the Sun. Forerunner of Borexino, the Counting Test Facility operated for two years at Gran Sasso, provided the convincing evidence that the major technological challenge of the experiment, the production of scintillator at unprecedented radiopurity levels, can be accomplished successfully, thus opening the way to the realization of the experiment. 2 1. Introduction A long history of 30 years of measurements provides the foundation of the well known solar neutrino problem, which from a pure astrophysics puzzle has evolved to a profound particle physics issue, with strong hints for s..