New Measurement Of The Proton Capture Rate On Be-7 And The S-17(0) Factor

The Be-7(p,gamma)B-8 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 Be-7 target, a raster-scanned beam, and the elimination of backscattering losses. The new measurement incorporates a higher activity Be-7 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 S-17(E-c.m.=850 keV)=24.0+/-60.5 eV b, to serve as a benchmark. Measurements were carried out at lower energies as well. Due to the precise knowledge of the implanted Be-7 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+/-0.7 eV b, using the cluster model of Descouvemont and Baye.