Application of the transport model to study the ternary breakup in heavy-ion collisions

Abstract

<p indent="0mm">The phenomenon of “fast ternary breakup” is reported in experiments as a novel type of nuclear reaction mechanism. Its characteristics involve non-equilibrium fission with a high probability of ternary breakup, a short time-scale, and a near collinear orientation. It has become a challenging issue because the physical mechanism of the fast ternary breakup is not yet well understood in both experiments and theory at present. The systematic exploration of fast ternary breakup will significantly enhance our knowledge about the nuclear reaction process. In this paper, the fast ternary breakup of ¹⁹⁷Au + ¹⁹⁷Au at the energy range of <sc>5–30 MeV/u</sc>, has been investigated by the improved quantum molecular dynamics (ImQMD) model. The modes and mechanisms of ternary breakup are studied by time-dependent snapshots of ternary events. Three distinct ternary breakup modes, namely direct prolate, direct oblate and cascade ternary breakup, are clearly demonstrated and their production probabilities are obtained. In a direct prolate ternary event, two necks are formed and ruptured almost simultaneously, and the centers of three fragments are almost in a straight line. The physical mechanism of the fast ternary breakup is that the composite system has pre-formed two neck configurations before the primary fission. The direct oblate ternary breakup is an extremely rare ternary breakup event, in which three necks are formed and rupture simultaneously, forming equally sized three fragments along space-symmetric directions in the reaction plane. The cascade ternary breakup is a two-step fission process, the time interval between the first fission and the second fission of the system reaches hundreds and thousands of fm/c. It is found that the probability of ternary breakup depends on the incident energy and the impact parameter. The optimum conditions for the fast ternary breakup reaction of ¹⁹⁷Au + ¹⁹⁷Au are given as the incident energy of approximately <sc>24 MeV/u</sc> and the impact parameters within the range of 3–7 fm. The simulation results reproduce the experimental data of the mass distributions and angular distributions of the three fragments, and clarify that the energy dissipation mechanism of the fast ternary breakup reaction is dominated by the two-body dissipation, and the viscosity coefficient of the two-body dissipation is estimated to be about <sc>10⁻²¹ MeV s fm⁻²</sc>. The results indicate that the new ternary breakup undergoes a strong damping process.</p>

References