Analysis of the 11B( 7Li, 7Be) 11Be reaction at 57 MeV in a microscopic approach

Abstract

The 11B( 7Li, 7Be) 11Be charge exchange reaction has been studied at an incident energy of 57 MeV. Spectra were measured at forward angles, θ cm⩽35°. The good energy resolution (∼50 keV) allowed the identification of transitions both to the 7Be (3/2 −, gs) and 7Be(1/2 −, 429 keV) exit channels and hence the direct measurement of the ratio of the respective cross sections and angular distributions. Besides the bound ground and first excited state of 11Be several low lying excitations just above the neutron threshold are observed. A structure seen at E ∗=9.4 MeV with FWHM ∼7 MeV is compatible with the spin dipole resonance (SDR). The data are analysed in a many-body approach. For the projectile transitions shell model results are used. In order to account properly for the special features of the weakly bound 11Be system the target transitions are described microscopically by Hartree–Fock–Bogoliubov (HFB) and quasi-particle random phase approximation (QRPA) theory. The HFB ground state densities and the QRPA transition densities, respectively, are used in folding calculations for the optical potentials and transition form factors. Spectra and β-decay transitions strengths are reasonably well described. The measured cross section are well reproduced by one-step direct charge exchange distorted wave born approximation (DWBA) calculations. A dominance of unnatural parity transitions is found. This is explained in terms of the spin transfer behaviour of the nucleon–nucleon isovector interaction at low bombarding energy.