Clustering in low density nuclear matter has been investigated using the NIMROD multidetector at Texas A&M University. Thermal coalescence modes were employed to extract densities, $\rho$, and temperatures, $T$, for evolving systems formed in collisions of $47A$ MeV ${}^{40}\mathrm{Ar}+{}^{112}\mathrm{Sn}$, ${}^{124}\mathrm{Sn}$ and ${}^{64}\mathrm{Zn}+{}^{112}\mathrm{Sn}$, ${}^{124}\mathrm{Sn}$. The yields of $d$, $t$, ${}^3\mathrm{He}$, and ${}^4\mathrm{He}$ have been determined at $\rho =0.002$ to 0.03 $\mathrm{nucleons}/{\mathrm{fm}}^3$ and $T=5$ to 11 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.

• Received 22 October 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.172701