Coulomb excitation of Na-29,Na-30: Mapping the borders of the island of inversion

Abstract: Nuclear shell evolution in neutron-rich Na nuclei around N = 20 was studied by determining reduced transition probabilities, i.e., B ( E 2) and B ( M 1) values, in order to map the border of the island of inversion. To this end Coulomb-excitation experiments, employing radioactive 29 , 30 Na beams with a final beam energy of 2.85 MeV / nucleon, were performed at REX-ISOLDE, CERN. De-excitation γ rays were detected by the MINIBALL γ -ray spectrometer in coincidence with scattered particles in a segmented Si detector. Transition probabilities to excited states were deduced. The measured B ( E 2) values agree well with shell-model predictions, supporting the idea that in the Na isotopic chain the ground-state wave function contains significant intruder admixture already at N = 18, with N = 19 having an almost pure two-particle–two-hole deformed ground-state configuration.