Rapid structural evolution of neutron-rich silicon isotopes toward N = 28

Abstract

Neutron-rich Si isotopes represent a unique case of shell evolution, exhibiting a robust shell closure at $N=20$ and pronounced quadrupole collectivity at $N=28$. We report lifetime measurements of excited states in ${}^{40}$Si and the first simultaneous lifetime and heavy-ion inelastic-scattering measurements in ${}^{41}$Si. In ${}^{40}$Si, the extracted lifetimes for the $2_1^{+}$ and $(2_2^{+})$ states indicate moderate quadrupole collectivity at $N=26$, together with signatures of triaxiality. In ${}^{41}$Si, two near-degenerate states at 570 and 658~keV exhibit comparable $B(E2)$ strengths as extracted from inelastic scattering, while the measured lifetimes indicate dominant $M1$ decays. The combined lifetime and inelastic-scattering results suggest an evolution toward oblate shape, consistent with large-scale shell-model predictions.