Low-lying neutron f p-shell intruder states in 27Ne

The quenching of the N = 20 shell gap in neutron-rich nuclei is investigated by studying the single-particle structure of 27Ne via neutron transfer using a 26Ne beam. Two low-lying negative-parity intruder states have been observed, the lowest of which is identified as J π = 3/2−, confirming earlier speculations. A level identified as 7/2− is observed higher in energy than the 3/2−, contrary to the ordering at β-stability and at an energy significantly different from the predictions of previous shell-model calculations. The measured energies and deduced spectroscopic factors are well reproduced in full (0,1)-¯hω 0s-0p-0d-1s-0f -1p calculations in which there is a significant ad hoc reduction (∼ 0.7 MeV) in the N = 20 shell gap.