Light charged clusters emitted in 32 MeV/nucleon Xe136,124+Sn124,112 reactions: Chemical equilibrium and production of He3 and He6

Background: The isovector part of the nuclear equation of state remains partly unknown and is the subject of many studies. The degree of equilibration between the two main collision partners in heavy ion reactions may be used to study the equation of state since it is connected to isospin (N/Z) transport properties of nuclear matter. Purpose: We aim to test chemical equilibrium attainment by measuring isotopic characteristics of emitted elements as a function of impact parameter. Method: We study four Xe136,124+Sn124,112 reactions at 32 MeV/nucleon. The data were acquired with the INDRA detector at the GANIL (Caen, France) facility. Combined (projectile+target) systems are identical for two studied reactions, therefore it is possible to study the path towards chemical equilibrium from different neutron to proton ratio (N/Z) entrance channels. The study is limited to identified isotopes detected in the forward part of the center of mass in order to focus on the evolution of projectile-like fragment isotopic content and the benefit of excellent detection performances of the forward part of the apparatus. Results: Light charged particle productions, multiplicities, and abundance ratios dependence against impact parameter are studied. It is measured to almost identical mean characteristics for the two Xe124+Sn124 and Xe136+Sn112 systems for central collisions. Comparing all four studied systems it is shown that mean values evolve from projectile N/Z to projectile+target N/Z dependence. Those identical mean characteristics concern all light charged particles except He3 whose mean behavior is strongly different. Conclusions: Our inclusive analysis (no event selection) shows that N/Z equilibration between the projectile-like and the target-like is realized to a high degree for central collisions. The light charged particle production mean value difference between Xe124+Sn124 and Xe136+Sn112 systems for central collisions is of the order of a few %. This slight difference could be explained by pre-equilibrium particle emission whose intensity may differ for the two reactions. This point is demonstrated using He3 mean characteristics whose production takes place before chemical equilibrium attainment. The realized N/Z balance between projectile-like and target-like does not imply a pure two-body mechanism. Indeed a midrapidity production of light charged particle does exist and its N/Z is different as compared to the projectile-like one: it is n enriched. This point is touched using He6 midrapidity production which is favored by the drift phenomenon.