New measurement of the proton capture rate on7Beand theS17(0)factor

The ${}^7\mathrm{Be}(p,\gamma {)}^8\mathrm{B}$ 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 ${}^7\mathrm{Be}$ target, a raster-scanned beam, and the elimination of backscattering losses. The new measurement incorporates a higher activity ${}^7\mathrm{Be}$ target and a number of improvements in design and procedure. The cross section at a selected energy $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=850\mathrm{}\mathrm{keV}$ (where c.m. stands for center of mass) was measured several times under varying experimental conditions, yielding a value of $S_{17}{(E}_{\mathrm{c}.\mathrm{m}.\mathrm{}}=850\mathrm{}\mathrm{keV})=24.0\mathrm{}\pm 0.5\mathrm{eV}\mathrm{}\mathrm{b},$ to serve as a benchmark. Measurements were carried out at lower energies as well. Due to the precise knowledge of the implanted ${}^7\mathrm{Be}$ 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 $S_{17}\mathrm{}\left(0\right)=21.2\mathrm{}\pm 0.7\mathrm{eV}\mathrm{}\mathrm{b},$ using the cluster model of Descouvemont and Baye.