  The discrepancy between experimental data and theoretical calculations in
one-nucleon removal reactions at intermediate energies (quantified by the
so-called "quenching factors") and its dependence on the isospin asymmetry of
the nuclei has been an open problem in nuclear physics for the last fifteen
years. In this work, we propose an explanation for this long-standing problem,
which relies on the inclusion of the process of core destruction due to its
interaction with the removed nucleon. To include this effect, we extend the
commonly used eikonal formalism via an effective nucleon density, and apply it
to a series of nucleon knockout reactions. The effect of core destruction is
found to depend strongly on the binding energy of the removed nucleon, leading
to a significant reduction of the cross section for deeply bound nucleons,
which reduces the isospin dependence of the "quenching factors", making them
more consistent with the trends found in transfer and (p,pN) reactions.
