Precision Measurement of theBe7(p,γ)B8Cross Section with an ImplantedBe7Target

The ${}^7\mathrm{B}\mathrm{e}(p,\gamma ){}^8\mathrm{B}$ 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 ${}^7\mathrm{B}\mathrm{e}$ target, a raster-scanned beam, and the elimination of the backscattering loss. The new measurement incorporates a more abundant ${}^7\mathrm{B}\mathrm{e}$ target and a number of improvements in design and procedure. The point at $E_{\mathrm{l}\mathrm{a}\mathrm{b}}=991\mathrm{k}\mathrm{e}\mathrm{V}$ was measured several times under varying experimental conditions, yielding a value of $S_{17}(E_{\mathrm{c}\mathrm{.}\mathrm{m}\mathrm{.}}=850\mathrm{k}\mathrm{e}\mathrm{V})=24.0\pm 0.5\mathrm{e}\mathrm{V}\mathrm{b}$. Measurements were carried out at lower energies as well. Because of the precise knowledge of the implanted ${}^7\mathrm{B}\mathrm{e}$ 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 $S_{17}(0)=21.2\pm 0.7\mathrm{e}\mathrm{V}\mathrm{b}$.