Precision measurement of the 7Be(p,gamma)8B cross section with an implanted 7Be target.

The 7 Be(p, γ) 8 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 Be target, a raster-scanned beam, and the elimination of the backscattering loss. The new measurement incorporates a more abundant 7 Be target and a number of improvements in design and procedure. The point at E l a b = 991 keV was measured several times under varying experimental conditions, yielding a value of S 1 7 (E c . m . = 850 keV) = 24.0 ′ 0.5 eVb. Measurements were carried out at lower energies as well. Because of the precise knowledge of the implanted 7 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 1 7 (0) = 21.2 ′ 0.7 eVb.