Important progresses in Heavy Ion (HI) physics at medium energies have been achieved in the field of nuclear structure and reaction mechanism over the last three decades. The ultimate goal of such studies was to pin down basic properties of the effective in-medium interaction. In particular, large efforts have been devoted to understand dynamical and thermal instabilities versus isospin asymmetries of nuclei. In this scenario, a prominent role has been played by the design of innovative devices covering a large part of the available phase space. In this lecture, the contribution of the 4π detector CHIMERA is emphasized with respect to the relevant case of the time scale in neck fragmentation around 40 MeV/nucleon of beam energy.