Quadrupole collectivity in silicon isotopes approaching neutron number N=28N=28

Quadrupole deformation parameters, |β2,(p,p′)||β2,(p,p′)|, have been deduced for 36,38,40Si from measured inelastic proton-scattering cross sections. Due to the strong Z=14Z=14 subshell gap, low-lying quadrupole collectivity in these nuclei is attributed to the excitation of valence neutrons. Enhanced collectivity at N=26N=26 indicates a reduced N=28N=28 shell gap at large neutron excess in this chain of isotopes. Data are compared to large-scale shell-model calculations and prior Coulomb excitation measurements on 36,38Si.