B(E2) measurements in the yrast band of Mg28: Implications for the N=20 island of inversion

High-precision lifetime measurements in ${}^{28}\mathrm{Mg}$ were performed to study neutron shell evolution in Mg isotopes and the onset of the $N=20$ island of inversion. Using both the recoil distance and Doppler shift attenuation methods, five lifetimes were measured in addition to six upper limits. The observation of two long-lived, negative-parity states demonstrate the importance of studying Mg isotopes for the contribution of intruder configurations to $sd$-shell nuclei. Lifetimes of the $2_1^{+}$ and $4_1^{+}$ states of 1.81(5) ps and $172{{(}_{-10}^{+11})}_{\mathrm{stat}.}{\left(4\right)}_{\mathrm{stop}.}{\left(8\right)}_{\mathrm{feed}.}{\left(4\right)}_{\mathrm{targ}.}\mathrm{fs}$, respectively, demonstrate a loss of collectivity with increasing spin in the yrast band, permitting for distinguishing between current theoretical models. These measurements also highlight the progression of yrast structure across the Mg isotopic chain from rotational at $N=12$ to large shape mixing at $N=16$ and back to collective behavior at $N=20$ but with dominating intruder configurations.