First measurement of the He-3(He-3, 2p)He-4 cross section down to the lower edge of the solar Gamow peak

We give the LUNA results on the sigma(E) cross section measurement of a key reaction of the proton-proton chain strongly affecting the calculated neut rino luminosity from the Sun: He-3(He-3, 2p)He-4. Because of the cosmic ray suppression provided by the Gran Sasso underground laboratory, it has been possible to measure sigma(E) throughout the energy range in which this rea ction occurs in the Sun, i.e., down to 16.5 keV center of mass energy. The data clearly show the cross section increase due to the electron screening effect but they do not exhibit any evidence for a narrow resonance suggeste d to explain the B-8 and Be-7 solar neutrino flux reduction.