Structure of the isovector dipole resonance in neutron-rich 60 Ca nuclei and direct decay from pygmy resonance

Abstract

The structure of the isovector dipole resonance in neutron-rich calcium isotope, 60 Ca, has been investigated by implementing a careful treatment of the differences of neutron and proton radii in the continuum random phase approximation (RP A).The calculations have taken into account the current estimates of the neutron skin.The estimates of the escape widths for direct neutron decay from the pygmy dipole resonance (P DR) were shown rather wide, implicating a strong coupling to the continuum. The width of the giant dipole resonance (GDR) was evaluated, bringing on a detailed discussion about its microscopic structure.PACS numbers: 21.10. Pc, 24.30.Cz, 24.30.Gd I. INTRODUCTIONThe investigation of the nuclei lying far from the β − stability line has been an interesting and active field of the nuclear physics in the last two decades. In that region, a great number of exotic features are observed like halo/skin formation, intruders levels, new magic numbers and new kinds of collective excitation, the so-called soft and pygmy resonances.These observations have forced the review of successful theoretical tools in nuclei around the β − stability valley [1,2,3,4].The description of the microscopic structure of the exotic nuclei is a current topic of study and several recent works had the concern of describing the giant resonance (GR) in neutronrich nuclei. The main questions treated are the giant dipole resonance (GDR) behavior and the appearance of the pygmy dipole resonance (P DR) in a nucleus with large ratio of neutron to proton number (N/Z). The P DR appears in medium and heavy neutron-rich nuclei, and, within the hydrodynamic sense, they are probably due to oscillation of the

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.…”

“…The RPA calculation is done by utilizing the Landau-Migdal residual interaction with density dependent parameters. Since the neutron and proton densities are too different in exotic nuclei, it is more appropriate to separate the nucleon density into neutron and proton parts, ξ(r) = N A ξ ν (r) + Z A ξ π (r), where each part is given by ξ k (r) = 1 1+e (r−R k )/a k , with k = ν(π) for neutron (proton) [2]. In this same sense, the radial single-particle orbits are represented by harmonic oscillator radial wave functions with different size parameters for neutrons and protons, …”

“…[1] with a careful treatment on the differences between the densities of neutrons and protons [2]. The calculation of partial escape width for each single-hole gives a good estimate of the escape width composition for the population of several single-holes in the residual nucleus.…”

Description of decay mechanisms of the giant dipole resonances with a RPA + FKK approach

“…A recent version [7] of the continuum Random Phase Approximation (RPA) that accounts for a careful treatment on the differences between the neutron and proton densities was used to calculate the escape width. The details of this calculation may be seen in the contribution for this proceeding: "Partial Escape Width for Nuclei with Neutron Excess" [8].…”

“…The method is applied for isoscalar giant dipole resonances (ISGDR) in 208 Pb, since new results are indicating the necessity of improvements in the structure calculations around the ISGDR [1][2][3][4][5]. The escape width was calculated using a previous version of the continuum RPA approach [6], which was modified to take into account the differences among the neutron and proton radii in nuclei with neutron excess [7,8]. To calculate the spreading width we present a new formulation of an approximated semi-microscopic method, based in the statistical Multi-Step Compound Theory (MSC) of Feshbach, Kerman and Koonin (FKK) [9] in connection with the RPA calculations.…”