18 September 1997 Physics Letters B 409 (1997) 58-63 Charge equilibration in heavy-ion reactions energies R. Donangelo ‘, S.R. Souza2 PHYSICS LETTERS B at intermediate zyxwvutsrqp Institute de Fisica. Vniversidade Federal do Rio de Janeiro, C.F! 68.528, 2I945-970 Rio de Janeiro, Brazil Received 16 December 1996; revised manuscript received 2 June 1997 Editor: W. Haxton Abstract We investigate the time scale for charge equilibration in heavy-ion collisions at intermediate energies. The dynamics is studied within the framework of the Boltzmann-Uehling-Uhlenbeck approach and two molecular dynamics models which differ on the treatment for the Pauli principle. The BUU description predicts a relatively short time scale for charge equilibration, whereas this process turns out to depend quite sensitively on the prescription used for the Pauli principle in the molecular dynamics calculations. @ 1997 Elsevier Science B.V. PACS: : 25.7O.Pq Keywords: Charge equilibration; Heavy-ions collisions; Isotopic distributions In central heavy-ion collisions at low energies, it is quite reasonable to assume instantaneous charge equi- libration. This is because the velocity of the nucleons in nuclear matter is much larger than the relative ve- locity of the ions, so that the time scale associated with charge equilibration is much shorter than the one corresponding to the reaction. This could be no longer true for more energetic collisions and, therefore, it be- comes important to ascertain the time scale for charge equilibration. In particular, it should be interesting to compare this quantity with the duration of the reaction processes under consideration. Many experimental investigations carried out over the last years revealed that the emision of complex fragments (Z > 3) is an important reaction mecha- nism in heavy-ion collisions at energies ranging from ’ E-mail address: donangel@if.ulij.br. * E-mail address: srsouza@if.ufrj.br. a few tens to a few hundreds MeV per nucleon (see [ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO 1 ] for a recent review on the subject). The theoret- ical study of this phenomenon, i.e. the nuclear multi- fragmentation process, is a very challenging task due to the complex many-body features of the interaction between the nucleons, which cause the system to fol- low an intricate dynamical path. Since the exact treatment of the actual quantum many-body problem is beyond the present day com- putational capabilities, a large number of models has been used to explain the great variety of aspects ob- served in those reactions [ zyxwvutsrqponmlkjihgfedcbaZ 11. In spite of being much more tractable than the exact formulation of the prob- lem, those models are sometimes very computation- ally demanding. For this reason, sometimes they are further simplified so as not to distinguish between pro- tons and neutrons (see for example [Z-5] ) , assum- ing, in some cases, that each nucleon has an effective 0370-2693/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO 370- 2693(97>00848- 4