Energy dependence of the isotopic composition in nuclear multifragmentation

Isotopic composition of the fragments emitted during the multifragmentation of ${}^{58}\mathrm{Ni}{+}^{58}\mathrm{Ni},$ ${}^{58}\mathrm{Fe}{+}^{58}\mathrm{Ni},$ ${}^{58}\mathrm{Ni}{+}^{58}\mathrm{Fe},$ and ${}^{58}\mathrm{Fe}{+}^{58}\mathrm{Fe}$ was studied at beam energies of 30, 40, and 47 MeV/nucleon. Within the context of a grand-canonical approach, relative free neutron and proton densities, extracted from the measured isotope and isotone yield ratios, suggest that the neutron content of the vapor phase increases with the isospin $\mathrm{}(N/Z)\mathrm{}$ of the initial colliding nuclei and decreases with the beam energy.