Low-energy dipole strength and the critical case of 48Ca

Abstract

Recent theoretical work has not led to a consensus regarding the nature of the low-energy E1 strength in the 40,44,48Ca isotopes, for which high-resolution (gamma,gamma') data exist. Here we revisit this problem using the first-order quasi-particle random-phase approximation (QRPA) and different interactions. First we examine all even Ca isotopes with N=14-40. All isotopes are predicted to undergo dipole transitions at low energy, of large and comparable isoscalar strength but of varying E1 strength. Provided a moderate and uniform energetic shift is introduced to the results, QRPA with the Gogny D1S interaction is able to account for the (gamma,gamma') data, because, up to N=28, it yields a rather pure isoscalar oscillation. A neutron-skin oscillation is anticipated for N larger or equal to 30. This contradicts existing predictions that 44,48Ca develop a neutron-skin mode. Which theoretical result is correct cannot be resolved conclusively using the available data. We propose that alpha-scattering, possibly followed by an electroexcitation experiment, could resolve the situation and thereby help to improve the different models aspiring to describe reliably the low-energy dipole strength of nuclei.

arXiv comment7 pages, incl. 3 figures; PLB submitted

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Cited by in corpus (8)

• Reference Database for Photon Strength Functions
• Information content of the low-energy electric dipole strength: correlation analysis
• Isospin properties of electric dipole excitations in 48Ca
• Low-energy electric dipole response of Sn isotopes
• Isoscalar and neutron modes in the E1 spectra of Ni isotopes and the relevance of shell effects and the continuum
• The decay of quadrupole-octupole $1^{-}$ states in ${}^{40}$Ca and ${}^{140}$Ce
• Excitation of the electric pygmy dipole resonance by inelastic electron scattering
• Proton pygmy resonances: predictions for N=20 isotones