Nuclear interference effects in 8B sub-Coulomb breakup

The breakup of B on Ni below the Coulomb barrier was measured recently with the aim of determining the Coulomb breakup components. We reexamine this reaction, and perform one step quantum-mechanical calculations that include E1, E2 and nuclear contributions. We show that the nuclear contribution is by no means negligible at the intermediate angular range where data was taken. Our results indicate that, for an accurate description of this reaction, Coulomb E1, E2 and nuclear processes all have to be taken into account. In order to understand the details of Coulomb breakup experiments of 8B on heavy targets [1, 2], and to extract an S17 for astrophysical relative p+7Be energies from the measured Coulomb Dissociation (CD) cross sections, it is necessary to know the relative importance of the E1 and E2 Coulomb contributions to breakup. The Notre Dame experiment on the Coulomb dissociation of 8B on 58Ni at 26 MeV [3] therefore sought to determine these contributions by measuring the integrated cross section of the 7Be fragment between 39◦ and 51◦. At these much lower beam energies the E2 should be as large (or larger) than the E1 contribution, and so a measurement of the total breakup probability should give strong constraints on both the integrated B(E1) and B(E2) transition strengths. Since the closest distance of approach at 26 MeV for the above angles is 15 fm, and plausible optical potentials give elastic σ/σR < 0.5 only beyond 90◦, it was believed that the E1 and E2 contributions could be measured free from nuclear effects with the Notre Dame setup. However, the results of the Notre Dame experiment [3] disagreed [4, 5] with the theoretical predictions for a variety of structure models of 8B ([6, 7]), when using the standard semiclassical theory of Coulomb excitation [8]. The predictions of the semiclassical breakup theory were twice or three times the measured cross section. This discrepancy raises some fundamental questions: How strong is the dependence of the CD cross section on structure model of 8B? Should nuclear effects be taken into account? How important are interference terms: Coulomb-Coulomb, nuclear-nuclear and Coulomb-nuclear? In ref. [5] it is shown that no remotely-reasonable structure model could give a breakup probability as small as that measured in [3]. This work answers the second question on nuclear contributions, and hints on a possible answer to the third question on multistep effects. Email address: [email protected]