Abstract
Shell evolution is studied in the neutron-rich silicon isotopes Si36,38,40 using neutron single-particle strengths deduced from one-neutron knockout reactions. Configurations involving neutron excitations across the N=20 and N=28 shell gaps are quantified experimentally in these rare isotopes. Comparisons with shell model calculations show that the tensor force, understood to drive the collective behavior in Si42 with N=28, is already important in determining the structure of Si40 with N=26. New data relating to cross-shell excitations provide the first quantitative support for repulsive contributions to the cross-shell T=1 interaction arising from three-nucleon forces.