Isospin Dependence of the Balance Energy

The energy at which collective transverse flow in the reaction plane disappears, the balance energy $E_{\mathrm{bal}}$, is found to depend on the isospin of the system using the reactions ${}^{58}\mathrm{Fe}{+}^{58}\mathrm{Fe}$ and ${}^{58}\mathrm{Ni}{+}^{58}\mathrm{Ni}$. The more neutron-rich system exhibits higher balance energies for all measured impact parameters, in agreement with the predictions of a transport model which incorporates an isospin dependent mean field and isospin dependent in-medium nucleon-nucleon cross sections.