Electric dipole response of low-lying excitations in the two-neutron halo nucleus 29F
Abstract
The neutron-rich 28,29F isotopes have been recently studied via knockout and interaction cross-section measurements. The 2n halo in 29F has been linked to the occupancy of pf intruder configurations. We investigate bound and continuum states in 29F, focusing on the E1 response of low-lying excitations and the effect of dipole couplings on nuclear reactions. 29F (27F+n+n) wave functions are built within the hyperspherical harmonics formalism, and 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 THO basis. The corresponding structure form factors are used in CDCC calculations to describe low-energy scattering. Parity inversion in 28F leads to a 29F 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 R=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 e2fm2 up to 6 MeV, and the E1 distribution exhibits a resonance at ≈ 0.85 MeV. Results using a standard shell-model order for 28F lead to a considerable reduction of the B(E1) distribution. The four-body CDCC calculations for 29F+120Sn around the Coulomb barrier are dominated by dipole couplings, which totally cancel the Fresnel peak in the elastic cross section. These results are consistent with a two-neutron halo and may guide future experimental campaigns.