New measurement of the proton capture rate on 7Be and the S17(0) factor

The ⁷Be(p,γ)⁸B reaction plays a central role in the evaluation of solar neutrino fluxes. We report on a new precision measurement of the cross section of this reaction, following our previous experiment with an implanted ⁷Be target, a raster-scanned beam, and the elimination of backscattering losses. The new measurement incorporates a higher activity ⁷Be target and a number of improvements in design and procedure. The cross section at a selected energy E_c.m.=850 keV (where c.m. stands for center of mass) was measured several times under varying experimental conditions, yielding a value of S₁₇(E_c.m.=850 keV)=24.0 ±0.5 eV b, to serve as a benchmark. Measurements were carried out at lower energies as well. Due to the precise knowledge of the implanted ⁷Be density profile, it was possible to reconstitute both the off- and on-resonance parts of the cross section and to obtain from the entire set of measurements an extrapolated value of S₁₇(0)=21.2±0.7 eV b, using the cluster model of Descouvemont and Baye.