  Detailed assessments of the ability of recent theoretical approaches to
modeling existing experimental data for ternary fission confirm earlier
indications that the dominant mode of cluster formation in ternary fission is
clusterization in very neutron rich, very low density, essentially chemically
equilibrated, nucleonic matter. An extended study and comparison of these
approaches applied to ternary fission yields in the thermal neutron induced
reaction $^{241}$Pu($n_{\rm th}$,f) has been undertaken to refine the
characterization of the source matter. The resonance gas approximation has been
improved taking in-medium effects on the binding energies into account. A
temperature of 1.29 MeV, density of $6.7 \times 10^{-5}$ nucleons/fm$^3$ and
proton fraction $Y_p$ = 0.035 are found to provide a good representation of
yields of the ternary emitted light particles and clusters. In particular,
results for $Z= 1$ and 2 isotopes are presented. Isotopes with larger $Z$ are
discussed, and the roles of medium and continuum effects, even at very low
density are illustrated.
