A fully microscopical simulation of nuclear collisions by a new QMD model

Abstract

Nucleon-ion and ion-ion collisions at non relativistic bombarding energies can be described by means of Monte Carlo approaches, such as those based on the Quantum Molecular Dynamics (QMD) model. We have developed a QMD code, to simulate the fast stage of heavy-ion reactions, and we have coupled it to the de-excitation module available in the FLUKA Monte Carlo transport and interaction code. The results presented in this work span the projectile bombarding energy range within 200–600 MeV/A, allowing to investigate the capabilities and limits of our non-relativistic QMD approach. 1 General framework and motivation of this work A fully microscopical simulation of nucleon-ion and ionion collisions, at nucleon level, can be performed, among several different approaches [1], by means of Quantum Molecular Dynamics (QMD) models [2]. They are dynamical models which allow to study the phase-space evolution of the projectile-target colliding systems, from their initial mutual trajectory influence and eventually their overlap, depending on the impact parameter, to the compression phase, accompanied by a temperature and density increase, up to the following expansion stage, characterized by the formation of hot excited fragments (pre-fragments). The whole phase occurs on a time scale within a few hundreds fm/c, depending on the size of the colliding systems and the bombarding energy, and is called the “fast” stage of the reaction. Additionally, improved versions of QMD models (e.g., the CoMD one, developed by Papa et al. [3]) allow to compute the system evolution even for a longer time, up to thousands fm/c, and thus have also been used to describe pre-fragment de-excitation, at least in its initial stage. This has led to direct comparisons of the results of improved QMD simulations to experimental data concerning fragment emission distributions. On the other hand, pre-fragment de-excitation can occur on a time scale even larger (up to ∼10−15 s). Thus, a complete treatment of this slow stage can be covered by different models, generally based on statistical considerations. The underlying assumption in applying one of these statistical models to nuclear systems is that they are thermalized. While at the lowest energies the colliding ions stay close to each other for a time long enough for thermalization to occur, to define a temperature for the whole system at higher energies (several tens MeV/A) can be very problematic, since the expansion phase can begin before a global thermalization process is completed. Anyway, in the last case, at advanced time in the expansion stage pre-fragments are well separated and each of them is supposed to be thermalized. Whereas theoretical models allow to compute an excitation energy for each pre-fragment, from the experimental point of view the problem of the determination of hot fragment temperatures a Presenting author, e-mail: maria.garzelli@mi.infn.it 1e-14 1e-12 1e-10 1e-08 1e-06 1e-04 0.01 1 10