Coulomb dissociation of 8B: determination of the E2 component

Recently, the solar fusion reaction Be(p,γ)B has received renewed interest: on one hand, it represents the source of the high-energy neutrinos that have been measured in the SNO and Super-Kamiokande experiments to pin down neutrino oscillations; the zero-energy S17-factor is directly proportional to the predicted solar neutrino flux to be compared to the SNO and Super-K experiments. On the other hand, new discrepancies in the most recent direct capture experiments [1,2] have made it desirable to check their results by an independent measurement. One indirect method is Coulomb dissociation (CD) of B. We have previously performed such an experiment [3]. The analysis that allows to extract the zero-energy S-factor S17(0) from CD experiments requires a precise knowledge of the multipolarity contributions. Here, discrepancies among different CD experiments were observed: The GSI and a similar RIKEN experiment [4] concluded from their angular distributions that the multipolarity in CD was predominantly E1, whereas a MSU experiment [5] found evidence for a finite E2 contribution from an analysis of Be longitudinal momentum spectra.. We have repeated the CD experiment at GSI with an improved setup. A 254 AMeV B beam was produced and purified at the FRS; the breakup was measured in complete kinematics at KaoS. The new setup allowed to track the incident B beam at the Pb breakup target with good angular resolution (≈0.5 mrad). Downstream from the target, the breakup products, p and Be, were tracked with two pairs of small-pitch (100 μm) Si strip detectors. This allowed to measure the angular correlations of the breakup products both in the reaction plane and perpendicular to it. Fig.1 shows in the top panel the distribution of the breakup angles, Φ, of the p-Be system relative to the incoming B beam and transverse to the reaction plane. The bottom panel shows the transverse momenta in the reaction plane, p t . Both distributions can be well approximated with pure E1 multipolarity. The theoretical curves shown in Fig.1 have been calculated with E1 plus 10% of the theoretical E2 multipolarity, this gives roughly an upper limit to the E2 amplitude that is compatible with the data. The in-plane angles, θ, (not shown) are symmetric around 90 degrees and are also compatible with this theoretical description, even though the data show a somewhat larger anisotropy. These findings confirm the analyses of the earlier GSI and RIKEN experiments [3,4], which were done under the assumption of pure E1 multipolarity. The preliminary result for S17(0) from the present experiment is 19.7 ± 1.2 500 600 700 800 90