Measurement of the Dipole Polarizability of the Unstable Neutron-Rich Nucleus Ni-68
The E1 strength distribution in Ni-68 has been investigated using Coulomb excitation in inverse kinematics at the (RB)-B-3-LAND setup and by measuring the invariant mass in the one-and two-neutron decay channels. The giant dipole resonance and a low-lying peak (pygmy dipole resonance) have been observed at 17.1(2) and 9.55(17) MeV, respectively. The measured dipole polarizability is compared to relativistic random phase approximation calculations yielding a neutron-skin thickness of 0.17(2) fm. A method and analysis applicable to neutron-rich nuclei has been developed, allowing for a precise determination of neutron skins in nuclei as a function of neutron excess.