  Clustering in low density nuclear matter has been investigated using the
NIMROD multi-detector at Texas A&M University. Thermal coalescence modes were
employed to extract densities, $\rho$, and temperatures, $T$, for evolving
systems formed in collisions of 47 $A$ MeV $^{40}$Ar + $^{112}$Sn,$^{124}$Sn
and $^{64}$Zn + $^{112}$Sn, $^{124}$Sn. The yields of $d$, $t$, $^{3}$He, and
$^{4}$He have been determined at $\rho$ = 0.002 to 0.032 nucleons/fm$^{3}$ and
$T$= 5 to 10 MeV. The experimentally derived equilibrium constants for $\alpha$
particle production are compared with those predicted by a number of
astrophysical equations of state. The data provide important new constraints on
the model calculations.
