  Solar neutrino fluxes depend both on astrophysical and on nuclear physics
inputs, namely on the cross sections of the reactions responsible for neutrino
production inside the Solar core. While the flux of solar 8B neutrinos has been
recently measured at Superkamiokande with a 3.5% uncertainty and a precise
measurement of 7Be neutrino flux is foreseen in the next future, the predicted
fluxes are still affected by larger errors. The largest nuclear physics
uncertainty to determine the fluxes of 8B and 7Be neutrinos comes from the
3He(alpha,gamma)7Be reaction. The uncertainty on its S-factor is due to an
average discrepancy in results obtained using two different experimental
approaches: the detection of the delayed gamma rays from 7Be decay and the
measurement of the prompt gamma emission. Here we report on a new high
precision experiment performed with both techniques at the same time. Thanks to
the low background conditions of the Gran Sasso LUNA accelerator facility, the
cross section has been measured at Ecm = 170, 106 and 93 keV, the latter being
the lowest interaction energy ever reached.
  The S-factors from the two methods do not show any discrepancy within the
experimental errors. An extrapolated S(0)= 0.560+/-0.017 keV barn is obtained.
Moreover, branching ratios between the two prompt gamma-transitions have been
measured with 5-8% accuracy.
