  We study the associated production of $ J/\psi\ + \gamma $ in fixed-target
experiments with nuclear targets as well as at the relativistic heavy-ion
collider (RHIC). We find that this process affords a very clean probe of
$\rho_g$, the ratio of gluon density in a heavy nucleus to that of a proton.
The combined $x$ range thus available can be used to discriminate between the
predictions of different models of the EMC effect for $\rho_g$.
