Mass dependence of transverse flow in heavy ion collisions at intermediate energies

The mass dependence of the transverse flow in the reactions of ${}^{40}\mathrm{Ca}+{}^{40}\mathrm{Ca}$ at 35 MeV/nucleon has been determined for emitted isotopes with $Z=1$ to 9. The observed flow is compared with that calculated using a constrained molecular dynamics (CoMD) simulation. With the application of the appropriate experimental filter, the general trend of the experimental mass-dependent flow is well reproduced by the simulation employing an effective interaction corresponding to a soft equation of state $(K=200$ MeV). The CoMD events are further utilized to study the mechanism of generation of the mass-dependent flow. It is found that the mass-dependent flow is generated by the interplay between the thermal and collective motions under a momentum conservation in the fragmenting system. With the help of the collective-thermal-interplay model, the mass-dependent flow scaled by the reduced mass of fragments $A/{A}_{\mathrm{sys}}$ is found to be almost independent of the size of the system.