Soft Dipole Resonance in the Neutron-Skin NucleusH6e

Abstract

A candidate for a soft dipole resonance, a dipole oscillation mode between a core cluster and a neutron skin, was observed at Ex = 4+/-1 MeV and with a width of 4+/-1 MeV in 6He via the 6Li( 7Li, 7Be) reaction at an incident energy of 65A MeV and forward scattering angles including 0 degrees. Its cross section is deduced to be sigma(0 degrees ) = 0.9+/-0.2 mb/sr. This value is comparable to that of the giant dipole resonance simultaneously measured.

Excitation modes ofHe6from proton collisions

“…These very difficult studies have also often led to some contradictory results in particular with respect to existence of a dipole excitation [6]. It is thus fair to say that our theoretical and experimental knowledge of the Borromean resonant continuum sea is today still very inconclusive.…”

“…To illustrate the puzzling features half-unveiled in this field, various theories [1][2][3] predict a variety of excitation/resonance modes at low energies in the continuum of the above Borromean nuclei, but up to now there is only a clear evidence of a J π = 2 + resonance for 6 He [4]. Thus, for more than a decade the existence and the understanding of the excitation modes is far from being achieved, now a detailed study of the resonances in the continuum sea is one of the priority tasks of radioactive beam research.…”

“…The Borromean two-neutron halo nuclei 11 Li and 6 He have been attracting many theoretical and experimental studies since the ground state and the excited modes are not both well understood yet, and knowledge of the continuum properties will contribute to our understanding of the properties of these nuclei. To illustrate the puzzling features half-unveiled in this field, various theories [1][2][3] predict a variety of excitation/resonance modes at low energies in the continuum of the above Borromean nuclei, but up to now there is only a clear evidence of a J π = 2 + resonance for 6 He [4].…”

Four-body DWBA calculations of the Coulomb breakup of 6He

“…Although the SDR was predicted long ago [4,9], properties of this resonance measured in Ref. [25] are at variance with those predicted by the detailed three-body calculations [16]. The breakup reaction of this nucleus gives access to the study of the neutron-neutron (n-n) and neutron-alpha particle (n-4 He) correlations, which are important because they provide a critical test of the dineutron model, in which the halo neutrons were treated as a point like "dineutron" cluster orbiting the core.…”

“…The study of the breakup reaction of 6 He is interesting due to several reasons. It leads to the excitation of the continuum of this nucleus, which has received much attention recently due to the experimental discovery [25] of the soft dipole resonance (SDR), signifying the dipole oscillation between the core cluster and the valence neutrons. Although the SDR was predicted long ago [4,9], properties of this resonance measured in Ref.…”