Shell evolution approaching the N = 20 island of inversion: structure of [superscript 26]Na

Abstract

The levels in [superscript 26]Na with single particle character have been observed for the first time using the d([superscript 25]Na,p[gamma]) reaction at 5MeV/nucleon. The measured excitation energies and the deduced spectroscopic factors are in good overall agreement with (0 +1)[h bar][omega] shell model calculations performed in a complete spsdf p basis and incorporating a reduction in the N = 20 gap. Notably, the 1p[subscript 3/2] neutron configuration was found to play an enhanced role in the structure of the low-lying negative parity states in [superscript 26]Na, compared to the isotone [superscript 28]Al. Thus, the lowering of the 1p[subscript 3/2] orbital relative to the 0f[subscript 7/2] occurring in the neighbouring Z = 10 and 12 nuclei – [superscript 25,27]Ne and [superscript 27,29]Mg – is seen also to occur at Z = 11 and further strengthens the constraints on the modelling of the transition into the island of inversion.