  We perform realistic shell-model calculations for nuclei with valence
nucleons outside 48Ca, employing two different model spaces. The matrix
elements of the effective two-body interaction and electromagnetic multipole
operators have been calculated within the framework of the many-body
perturbation theory, starting from a low-momentum potential derived from the
high-precision CD-Bonn free nucleon-nucleon potential. The role played by the
neutron orbital 1d5/2 has been investigated by comparing experimental data on
yrast quadrupole excitations of isotopic chains north-east of 48Ca with the
results of calculations including or not including this single-particle state
in the model space.
