Collectivity of the low-lying dipole strength in relativistic random phase approximation

Abstract

The relativistic random phase approximation is applied in the analysis of the evolution of the isovector dipole response in nuclei with a large neutron excess. The self-consistent framework of relativistic mean-field theory, which has been very successfully applied in the description of ground-state properties of nuclei far from the valley of β-stability, is extended to study the possible onset of low-energy collective isovector dipole modes in nuclei with extreme isospin values.

Partial escape width for nuclei with neutron excess

“…According to previous RPA calculations for 60 Ca [5][6][7][8], our calculations [2] also predict a considerable strength in the energy region around ∼ 8.6 MeV with a broad neutron peak. The broad width is due to the fact of the energy of the resonance to be above of the small neutron separation energy, implicating a strong coupling of the external neutrons to the continuum region.…”

Astrophysical relevance ofγtransition energies

“…Thus, it does not affect the neutron capture cross section. Comparing to an available prediction of the pygmy resonance energy within an isotopic chain, it can be seen that it is predicted to always lie well above S n [14,15,16]. If this is confirmed, it would mean that the pygmy resonance does not play a role in astrophysical neutron capture unless it is sufficiently wide to bring some additional strength below S n .…”

“…Recently, a number of experimental investigations indicated additional strength confined to a small energy range in the low-energy tail (see, e.g., [5,6,7,8,9]). Theory provides different possibilities to explain the additional strength, such as collective vibration of a neutron skin against an inert core composed of protons and neutrons (pygmy resonance) or other collective modes (see, e.g., [10,11,12,13,14,15,16]). Accordingly, the predictions regarding location and width of this additional E1 resonance vary.…”

Finite amplitude method for the solution of the random-phase approximation

“…The RPA calculations with full self-consistency are becoming a current trend in nuclear structure studies, however, they are essentially only for spherical nuclei at present [4,5,6,7]. The applications to deformed nuclei are very few, but have been done for the Skyrme energy functional using the three-dimensional mesh-space representation [8,9,10,11].…”