Multifragmentation of XE(50A MEV)+SN Confrontation of Theory with Data

Abstract

We compare in detail central collisions Xe(50A MeV) + Sn, recently measured by the INDRA collaboration, with the Quantum Molecular Dynamics (QMD) model in order to identify the reaction mechanism which leads to multifragmentation. We find that QMD describes the data quite well, in the projectile/target region as well as in the midrapidity zone where also statistical models can be and have been employed. The agreement between QMD and data allows to use this dynamical model to investigate the reaction in detail. We arrive at the following observations: a) the in medium nucleon nucleon cross section is not significantly different from the free cross section, b) even the most central collisions have a binary character, c) most of the fragments are produced in the 1 central collisions and d) the simulations as well as the data show a strong attractive in-plane flow resembling deep inelastic collisions e) at midrapidity the results from QMD and those from statistical model calculations agree for almost all observables with the exception of d 2 σ dZdE . This renders it difficult to extract the reaction mechanism from midrapidity fragments only. According to the simulations the reaction shows a very early formation of fragments, even in central collisions, which pass through the reaction zone without being destroyed. The final transverse momentum of the fragments is very close to the initial one and due to the Fermi motion. A heating up of the systems is not observed and hence a thermal origin of the spectra cannot be confirmed.

Characteristics of the fragments produced in central collisions of129Xe+natSnfrom 32Ato 50AMeV

“…It increases slightly with the charge of the fragment for the four incident energies and in particular for proton and alpha particles. Notice that the kinetic energies of 3 He are high compared to the values of the other particles. The observed effect may be due to the higher identification threshold energy for 3 He.…”

“…In Quantum Molecular Dynamics [1,2,3,4] simulations or Microcanonical Metropolis Monte-Carlo model [5] calculations the primary fragments are rather cold i.e. they are almost unaffected by subsequent secondary decays and arrive unchanged in the detectors.…”

Model-independent tracking of criticality signals in nuclear multifragmentation data

“…The increase of < Z max > for the most central collisions at these energies suggests a contribution from (incomplete) fusion reactions where the heaviest fragment detected is an evaporation residue. The disappearance of this phenomenon for 39A MeV can be interpreted as signalling the disappearance of fusion-evaporation residues, and marking the onset either of fusion-multifragmentation [34] or of transparency in central collisions (incomplete stopping) [30,56,57].…”

“…Signals of the phase transition in experimental data on multifragmentation may be revealed by anomalously * deceased large fluctuations of fragment observables [19,20,21]. The main obstacles to such endeavours insofar as experimental data are concerned are the huge statistical fluctuations inherent to small systems such as atomic nuclei, and the still-open question of the mechanism(s) of fragment production in heavy ion collisions at intermediate energies [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39]. Indeed in all experimental studies of the question, at the same time as one is searching for evidence of the phase transition of a piece of hot nuclear matter [35], one is (implicitly or not) obliged to elucidate the manner in which such an excited system may be formed in the course of certain reactions [34].…”

Quasiclassical model of intermediate velocity particle production in asymmetric heavy ion reactions

“…For example, a Landau-Vlasov semiclassical transport model was used to analyze the presence [7] and the temporal behavior [12] of the particle emission process that populates the mid-rapidity range. Neck instabilities, dynamical fluctuations, and particle production at mid-velocities were also studied using the quantum molecular dynamics (QMD) model in semi-peripheral [13], and central heavy ion collisions [14]. In addition, in a recent contribution, [15], two mechanisms of IMF production were suggested (neck, and fission-like surface emission), and two different models were introduced in order to describe them.…”