Abstract
Results are presented from a one-neutron knockout experiment at relativistic energies of $ ____approx 420 A$ MeV on 51-55Sc using the GSI Fragment Separator as a two-stage magnetic spectrometer and the MINIBALL array for gamma-ray detection. Inclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the contributions corresponding to knockout from the $ ____nu p_{1/2}$ , $ ____nu p_{3/2}$ , (L = 1 and $ ____nu f_{7/2}$ , $ ____nu f_{5/2}$ (L = 3 neutron orbitals. The observed L = 1 and L = 3 contributions are compared with theoretical cross-sections using eikonal knockout theory and spectroscopic factors from shell model calculations using the GXPF1A interaction. The measured inclusive knockout cross-sections generally follow the trends expected theoretically and given by the spectroscopic strength predicted from the shell model calculations. However, the deduced L = 1 cross-sections are generally 30-40% higher while the L = 3 contributions are about a factor of two smaller than predicted. This points to a promotion of neutrons from the $ ____nu f_{7/2}$ to the $ ____nu p_{3/2}$ orbital indicating a weakening of the N = 28 shell gap in these nuclei. While this is not predicted for the phenomenological GXPF1A interaction such a weakening is predicted by recent calculations using realistic low-momentum interactions $ V_{low k}$ obtained by evolving a chiral N3LO nucleon-nucleon potential.