  Yields of equatorially emitted light isotopes, $1\le Z\le 14$, observed in
ternary fission in the reaction $^{241}$Pu($n_{\rm th}$,f) are employed to
determine apparent chemical equilibrium constants for low-temperature and
low-density nuclear matter. The degree of liberation and role of medium
modifications are probed through a comparison of experimentally derived
reaction quotients with equilibrium constants calculated using a relativistic
mean-field model employing a universal medium modification correction for the
attractive $\sigma$ meson coupling. The results of these comparisons indicate
that equilibrium is achieved for the lighter ternary fission isotopes. For the
heavier isotopes experimental reaction quotients are well below calculated
equilibrium constants. This is attributed to a dynamical limitation reflecting
insufficient time for full equilibrium to develop. The role of medium effects
leading to yield reductions is discussed as is the apparent enhancement of
yields for $^8$He and other very neutron rich exotic nuclei.
