Electric dipole response of low-lying excitations in the two-neutron halo nucleus F 29

Background: The neutron-rich F28,29 isotopes have been recently studied via knockout and interaction cross-section measurements. The two-neutron halo in F29 has been linked to the occupancy of pf intruder configurations.Purpose: We investigate the bound spectrum and continuum states in F29, focusing on the electric dipole (E1) response of low-lying excitations and the effect of dipole couplings on nuclear reactions. Method: F29 (F27+n+n) wave functions are built within the hyperspherical harmonics expansion formalism, and total reaction cross sections are calculated using the Glauber theory. Continuum states and B(E1) transition probabilities are described in a pseudostate approach using the analytical transformed harmonic oscillator basis. The corresponding structure form factors are used in continuum-discretized coupled-channels (CDCC) calculations to describe low-energy scattering. Results: Parity inversion in F28 leads to a F29 ground state characterized by 57.5% of (p3/2)2 intruder components, a strong dineutron configuration, and an increase of the matter radius with respect to the core radius of ΔR=0.20 fm. Glauber-model calculations for a carbon target at 240 MeV/nucleon provide a total reaction cross section of 1370 mb, in agreement with recent data. The model produces also a barely bound excited state corresponding to a quadrupole excitation. B(E1) calculations into the continuum yield a total strength of 1.59 e2 fm2 up to 6 MeV, and the E1 distribution exhibits a resonance at ≈0.85 MeV. Results using a standard shell-model order for F28 lead to a considerable reduction of the B(E1) distribution. The four-body CDCC calculations for F29+Sn120 around the Coulomb barrier are dominated by dipole couplings, which totally cancel the Fresnel peak in the elastic-scattering cross section.Conclusions: Our three-body calculations for F29, using the most recent experimental information on F28, are consistent with a two-neutron halo. Our predictions show the low-lying enhancement of the E1 response expected for halo nuclei and the relevance of dipole couplings for low-energy reactions on heavy targets. These findings may guide future experimental campaigns.