Properties and Significance of the Surface Dipole Mode

Abstract

A strong isoscalar dipole resonance is known to be excited in a variety of nuclei, including isospin symmetric ones, at approximately 6 − 7 MeV. A series of theoretical studies summarized here and accumulating experimental evidence support an interpretation of the above dipole resonance as an elementary surface vibration. Obviously, such a mode is potentially as interesting as any collective excitation for a variety of reasons. In addition, though, it was found to account for the observed isoscalar segment of pygmy dipole strength. As discussed in this brief survey, this has important implications for pygmy-strength interpretations and searches for genuine neutron-skin oscillations.

Isoscalar and neutron modes in theE1spectra of Ni isotopes and the relevance of shell effects and the continuum

“…The corresponding proton and neutron transition densities are very similar to each other and have a node, fitting well in the picture of the surface dipole mode of ordinary nuclei discussed in Ref. [15]. Let us now see to what extent such a picture persists in the other Ni isotopes.…”

“…First we observe, in the compilation of Ref. [15], that the energy of the isoscalar segment, typically 6 − 7 MeV, varies weakly with mass number in comparison, e.g., to the shell energy 1 ω = 41A −1/3 MeV. Pygmy dipole strength, according to non-relativistic models, is expected at around 1 ω [12] and can be attributed to fragments of 1 ω transitions, rather than a collective mechanism [3,[9][10][11].…”

“…A variety of microscopic models have been employed in related studies, mainly the quasi-particle or plain randomphase approximation (QRPA or RPA) and models considering phonon coupling. The presence of strength has been attributed to the concentration of single-particle transitions in the 1 ω energy region [3,[9][10][11][12], an alphacluster mode [13,14], a surface-dipole vibration, akin to the other known quadrupole and octupole vibrations [15][16][17][18][19] and of toroidal structure [20], or, in the case of neutron(proton)-rich nuclei, an exotic neutron(proton)skin oscillation [2]. The fragmentation patterns and precise strength can be explained by the coupling of the dipole transitions to collective phonons [21][22][23][24] or collisional damping [25].…”

“…That is the conclusion we have reached in Refs. [15][16][17][18][19] for the dipole strength below particleemission threshold and in moderately neutron-rich nuclei (below critical shell closures). The IS strength is attributed to a collective mode of vibration, which is excited in N = Z nuclei too; the IV strength, largely to single-particle transitions.…”

“…One is an ordinary surface dipole vibration, which corresponds to the isoscalar low-energy dipole (IS-LED) mode studied in Refs. [15][16][17][18][19]. We will therefore denote it as IS-LED.…”