  Single neutron- and proton-removal cross sections have been systematically
measured for 72 medium-mass neutron-rich nuclei around Z=50 and energies around
900A MeV using the FRagment Separator (FRS) at GSI. Neutron-removal cross
sections are described by considering the knock-out process together with
initial- and final-state interactions. Proton-removal cross sections are,
however, significantly smaller than predicted by the same calculations. The
observed difference can be explained as due to the knockout of short-correlated
protons in neutron-proton dominating pairs.
