Freezeout concept and dynamical transport model in intermediate-energy heavy-ion reactions

Abstract

For the central collision events of ${}^{40}\mathrm{Ca}\phantom{\rule{4pt}{0ex}}+{\phantom{\rule{0.16em}{0ex}}}^{40}\mathrm{Ca}$, generated by an antisymmetrized molecular dynamics model in the intermediate energy range of 35 to 300 MeV/nucleon, the density and temperature of a fragmenting source have been extracted using a self-consistent method with the modified Fisher model. Rather flat density values, $\ensuremath{\rho}/{\ensuremath{\rho}}_{0}\ensuremath{\sim}0.65$ to 0.7, are evaluated at the incident energies studied, even though the maximum density of the system achieved during the collisions increases monotonically from $\ensuremath{\rho}/{\ensuremath{\rho}}_{0}\ensuremath{\sim}1.3$ at 35 MeV/nucleon to 1.8 at 300 MeV/nucleon. Flat temperature values of $T\ensuremath{\sim}5.9$ to 6.5 MeV are also extracted in this incident energy range. These extracted values indicate that, in average, intermediate mass fragments are formed at a later stage when the hot nuclear matter reaches at a ``freezeout'' volume by the expansion, which is not assumed in any transport models, but assumed generally in statistical multifragmentation models. This is the first time to demonstrate quantitatively a direct connection between the freezeout concept and transport model simulations in a multifragmenting regime of heavy ion collisions.