Exploring the ``island of inversion'' by in-beam γ-ray spectroscopy of the neutron-rich sodium isotopes Na31,32,33

The structure of the neutron-rich sodium isotopes Na31,32,33 was investigated by means of in-beam γ-ray spectroscopy following one-neutron knockout and inelastic scattering of radioactive beams provided by the RIKEN Radioactive Ion Beam Factory. The secondary beams were selected and separated by the fragment separator BigRIPS and incident at ≈240 MeV/nucleon on a natural carbon (secondary) target, which was surrounded by the DALI2 array to detect coincident de-excitation γ rays. Scattered particles were identified by the spectrometer ZeroDegree. In Na31, a new decay γ ray was observed in coincidence with the known (5/2₁⁺)→3/2_g.s.⁽⁺⁾ transition, while for Na32,33 excited states are reported for the first time. From a comparison to state-of-the-art shell-model calculations it is concluded that the newly observed excited state in Na31 belongs to a rotational band formed by a 2p2h intruder configuration within the “Island of Inversion.”