Determination of the Be(p7,γ)8B cross section at astrophysical energies using a radioactive 7Be ion beam

The reaction Be(p7,γ)8B plays an important role in the Sun, where it determines the high energy component of the solar neutrino spectrum. The importance of this reaction triggered several experiments over the last decades. A combined analysis of their results produces an overall consistent picture for the energy dependence of the cross section, while an inflation of the quoted uncertainties is needed to accommodate the observed discrepancy in the absolute scale of the different data sets. The origin of this discrepancy needs to be understood for a reliable estimate of the astrophysical rate of Be(p7,γ)8B and its uncertainty. In addition, there is a question about possible common systematic effects, considering that all measurements performed so far share the same experimental approach, i.e. an intense proton beam impinging on a Be7 radioactive target. A direct measurement using a radioactive Be7 ion beam on a pure hydrogen gas target has been since long envisioned as a way to improve the situation. First attempts showed the feasibility of an experiment based on the use of a recoil mass separator to collect reaction products with high efficiency, but failed to reach a useful statistical significance because of the low beam intensity. Here we present the results obtained using the intense Be7 beam available at the Tandem Accelerator Laboratory at CIRCE, University of Campania, Italy coupled to the recoil mass separator ERNA in the energy range Ecm=367 to 812 keV. Our results are compatible only with a part of previous measurements, in particular those indicating a low value of the astrophysical S-factor at zero energy S17, thus exacerbating the discrepancy between existing measurements. The analysis of our data together with the results of previous data provides an estimate S17(0)=20.0±0.8 eV⋅b, where systematic uncertainties are inflated to obtain a statistically compatible data set.