Isospin Fractionation in Nuclear Multifragmentation

Isotopic distributions for light particles and intermediate mass fragments have been measured for ${}^{112}\mathrm{Sn}{+}^{112}\mathrm{Sn}$, ${}^{112}\mathrm{Sn}{+}^{124}\mathrm{Sn}$, ${}^{124}\mathrm{Sn}{+}^{112}\mathrm{Sn}$, and ${}^{124}\mathrm{Sn}{+}^{124}\mathrm{Sn}$ collisions at $E/A=50\mathrm{}\mathrm{MeV}$. Isotope, isotone, and isobar yield ratios are utilized to estimate the isotopic composition of the gas phase at freeze-out. Analyses within the equilibrium limit imply that the gas phase is enriched in neutrons relative to the liquid phase represented by bound nuclei. These observations suggest that neutron diffusion is commensurate with or more rapid than fragment production.