Physics – Nuclear Physics
Scientific paper
Oct 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aps..dnp.ba002j&link_type=abstract
American Physical Society, 2002 Fall Meeting of the Division of Nuclear , abstract #BA.002
Physics
Nuclear Physics
Scientific paper
The ^7Be(p,γ)^8B reaction rate in the sun is a key nuclear physics input in solar models that determine the flux of high-energy neutrinos detected by SNO and Super Kamiokande. Recent results from these neutrino experiments have shown that the solar neutrino problem can be explained by neutrino oscillations, where electron neutrinos produced in the sun change their flavor on the way to earth to muon and/or tauon neutrinos with a possible admixture of sterile neutrinos. To improve the accuracy of the input of solar models and, as a result, to constrain better the neutrino oscillation parameters, it is of great importance to have an improved experimental determination of the astrophysical S-factor for this reaction. We have made new, direct measurements of the ^7Be(p,γ)^8B cross section from E_cm=0.19 to 1.20 MeV using the van de Graaff accelerator at the University of Washington and radioactive ^7Be targets produced at TRIUMF. Our experiment, designed to reach a precision in the astrophysical S-factor of better than 5%, involves measurement and minimization of all important systematic errors. A metallic ^7Be target evaporated on a Mo backing was mounted on one end of a rotating, water-cooled arm, irradiated in the proton beam and rotated in front of a Si-detector where the α particles emitted following the β decay of ^8B are counted. We have made several improvements over previous works: by rastering, we produce a nearly homogeneous large area beam flux over a small area target, which avoids systematic errors related to the inhomogeneity of the target and/or beam. The energy thickness of the target was determined from the narrow ^7Be(α ,γ)^11C reaction at E_α=1.38 MeV. We have also made the first measurement of ^8B backscattering losses. Results will be presented based on 3 separate cross section measurements using 3 different ^7Be targets, including a renormalization of our published result (1). Comparisons will be made to indirect S-factor determinations from Coulomb dissociation and from peripheral transfer reactions. 1) A. R. Junghans et al., Phys. Rev. Lett. 88, 041101-1 (2002).
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