Structure of Mg28 and influence of the neutron pf shell

Abstract

Gamma-ray spectroscopy and lifetime measurements using the Doppler shift attenuation method (DSAM) were performed on the nucleus $^{28}\mathrm{Mg}$ near the $N=20$ ``island of inversion,'' which was populated using a $^{12}\mathrm{C}(^{18}\mathrm{O},2\mathrm{p})^{28}\mathrm{Mg}$ fusion-evaporation reaction to investigate the impact of shell evolution on its high-lying structure. Three new levels were identified at 7203(3), 7747(2), and 7929.3(12) keV along with several new gamma rays. A newly extracted $B(E2;{4}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ of 42(7) ${e}^{2}{\mathrm{fm}}^{4}$ indicates reduced collectivity in the yrast band at high spin, consistent with ab initio symmetry adapted no-core shell model (SA-NCSM) calculations. At high excitation energy, evidence for the population of intruder orbitals was obtained through identification of negative parity levels $[{I}^{\ensuremath{\pi}}={(0,4)}^{\ensuremath{-}}$, ${(4,5)}^{\ensuremath{-}}]$. Calculations using the SDPF-MU interaction indicate that these levels arise from single neutron excitation to the $pf$ shell and provides evidence for the lowering of these intruder orbitals approaching the island of inversion.