Coulomb dissociation of ⁸B at E(⁸B) = 254 MeV/nucleon

The precise knowledge of the solar thermonuclear fusion reaction of ⁷Be(p, γ)⁸B is crucial for estimating the ⁸B solar neutrino flux and the predicted neutrino rates in terrestrial neutrino detectors. The flux of ⁸B solar neutrinos is particularly important for the results of the Homestake, Super-Kamiokande, and SNO experiments, because they measure high-energy solar neutrinos mainly or solely from the ⁸B decay.

We have measured the Coulomb dissociation of ⁸B into ⁷Be and proton on a ²⁰⁸Pb target at 254 MeV/nucleon using the large acceptance focusing spectrometer KaoS at GSI. Since the dissociation process can be regarded as absorption of a virtual photon, the Coulomb dissociation cross section is converted to the photo-absorption ⁸B( γ,p)⁷Be cross section which is directly related to the ⁷Be(p, γ)⁸B cross section.

In a first experiment the astrophysical S factor for the ⁷Be(p,γ)⁸B reaction is extracted to be S17(0)=20.6 ±1.2(exp) ±1.0(theo) eV-b for 0.25-2.78 MeV (1). This result agrees with the presently adopted zero-energy S17 factor obtained in direct-reaction measurements and with the results of other Coulomb dissociation studies performed at 46.5 and 51.2 A MeV. A second run allowed to also obtain information on the E1/E2 ratio by analysing the angular correlation of ⁷Be and the proton.