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

TL;DR

This study investigates the structure of 26Na near the N=20 island of inversion, revealing the significant role of the 1p3/2 neutron orbital and confirming shell model predictions about the evolving nuclear shell structure.

Contribution

First observation of levels in 26Na with single particle character, providing experimental data that supports shell model calculations and insights into shell evolution near the island of inversion.

Findings

Measured excitation energies and spectroscopic factors agree with shell model calculations.

Enhanced role of 1p3/2 neutron configuration in low-lying states.

Evidence of the lowering of the 1p3/2 orbital at Z=11.

Abstract

The levels in 26Na with single particle character have been observed for the first time using the d(25Na,p gamma) reaction at 5 MeV/nucleon. The measured excitation energies and the deduced spectroscopic factors are in good overall agreement with (0+1) hbar-omega shell model calculations performed in a complete spsdfp basis and incorporating a reduction in the N=20 gap. Notably, the 1p3/2 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 1p3/2 orbital relative to the 0f7/2 occurring in the neighbouring Z=10 and 12 nuclei -- 25,27Ne and 27,29Mg -- is seen also to occur at Z=11 and further strengthens the constraints on the modelling of the transition into the island of inversion.