  The yields of light elements ($Z=1,2$) obtained from spontaneous ternary
fission of $^{252}$Cf are treated within a nonequilibrium approach, and the
contribution of unstable nuclei and excited bound states is taken into account.
These light cluster yields may be used to probe dense matter, and to infer
in-medium corrections. Continuum correlations are calculated from scattering
phase shifts using the Beth-Uhlenbeck formula, and the effect of medium
modification is estimated. The relevant distribution is reconstructed from the
measured yields of isotopes. This describes the state of the nucleon system at
scission and cluster formation, using only three Lagrange parameters which are
the nonequilibrium counterparts of the temperature and chemical potentials, as
defined in thermodynamic equilibrium. We concluded that a simple nuclear
statistical equilibrium model neglecting continuum correlations and medium
effects is not able to describe the measured distribution of H and He isotopes.
Moreover, the freeze-out concept may serve as an important ingredient to the
nonequilibrium approach using the relevant statistical operator concept.
