Isospin effects on rotational flow in intermediate energy heavy ion collisions

Within the framework of an isospin-dependent quantum molecular-dynamics model, the rotational flow in reactions of Fe-58+Fe-58 and Ni-58+Ni-58 at 40 MeV/nucleon for different impact parameters is investigated by analyzing the midrapidity azimuthal distribution. The rotational observables are also calculated semiquantitatively. For the first time, it is found that the more neutron-rich system (Fe-58+Fe-58) exhibits stronger rotational collective flow. This isospin dependence of rotational collective flow is more appreciable in semiperipheral collisions and it is shown to mainly result from the isospin dependence of nucleon-nucleon cross section rather than the symmetry energy. Meanwhile, it is indicated that the rotational flow depends strongly on the impact parameter.