Shell evolution approaching the N = 20 island of inversion: Structure of 26Na

The levels in 26Na with single particle character have been observed for the first time using the d(25Na, pγ  ) reaction at 5 MeV/nucleon. The measured excitation energies and the deduced spectroscopic factors are in good overall agreement with (0+1)ħω(0+1)ħω shell model calculations performed in a complete spsdfpspsdfp basis and incorporating a reduction in the N=20 gapN=20 gap. Notably, the 1p3/21p3/2 neutron configuration was found to play an enhanced role in the structure of the low-lying negative parity states in 26Na, compared to the isotone 28Al. Thus, the lowering of the 1p3/21p3/2 orbital relative to the 0f7/20f7/2 occurring in the neighbouring Z=10Z=10 and 12 nuclei – 25,27Ne and 27,29Mg – is seen also to occur at Z=11Z=11 and further strengthens the constraints on the modelling of the transition into the island of inversion.