5th International Symposium on Nuclear Symmetry Energy NuSYM15, June 29 - July 2, 2015 Kraków, POLAND

Dominic Rossi
NSCL, Michigan State University East Lansing, UNITED STATES

Dipole Polarizability of 68Ni

Abstract
The symmetry energy part of the nuclear equation-of-state describes properties of nuclei far from stability as well as astrophysical phenomena. The symmetry energy and in particular its density dependence have a strong influence on the electric dipole response of neutron-rich nuclei. Low-lying E1 strength has been observed in a couple of unstable nuclei, and has been used in some cases to constrain the parameters of the symmetry-energy term of the equation-of-state or the neutron-skin thickness. While the E1 strength values were used previously to constrain these properties, it has been shown more recently that the electric-dipole polarizability offers a stronger correlation between the experimental observable and equation-of-state parameters, as well as a smaller model-dependence. The first experimental measurement of the dipole polarizability of a radioactive nucleus was performed in 68Ni using Coulomb excitation in inverse kinematics at the R$^{3}$B-LAND setup at GSI in Darmstadt. The high beam energies allow the excitation-energy regions of low-lying E1 strength and of the Giant Dipole Resonance to be covered, enabling the extraction of the polarizability. Mean-field plus RPA calculations were used to correlate the experimental polarizability of $^{68}$Ni with the neutron-skin thicknesses of $^{68}$Ni and $^{208}$Pb, allowing for a comparison of the value obtained from the polarizability measurement of $^{208}$Pb of Tamii et al.