Abstract:A statistical treatment of finite unbound systems in the presence of
collective motions is presented and applied to a classical Lennard-Jones
Hamiltonian, numerically simulated through molecular dynamics. In the ideal gas
limit, the flow dynamics can be exactly re-casted into effective time-dependent
Lagrange parameters acting on a standard Gibbs ensemble with an extra total
energy conservation constraint. Using this same ansatz for the low density
freeze-out configurations of an interacting expanding system, … Show more
“…Observed first by the FOPI collaboration [5][6][7][8] and interpreted as an "extra push" with respect to the thermal pressure of an equilibrated composite system, a collective expansion energy has been extracted from experimental data over a wide range of beam energy [9]. Several studies of the multifragmentation process, and of the corresponding role of the collective expansion, have been undertaken in the framework of transport theories [4, 10-12] and some analyses [13][14][15] have pointed out the importance of this effect on fragment formation, looking at, for example, the balance between the amount of radial collective flow and recombination probability [13].…”
Section: Introductionmentioning
confidence: 99%
“…In this case two-body correlations and fluctuations are expected to provide the seeds for fragment formation leading to the occurrence of multifragmentation phenomena which can be described in the framework of a liquid-gas-type phase transition [4].Observed first by the FOPI collaboration [5][6][7][8] and interpreted as an "extra push" with respect to the thermal pressure of an equilibrated composite system, a collective expansion energy has been extracted from experimental data over a wide range of beam energy [9]. Several studies of the multifragmentation process, and of the corresponding role of the collective expansion, have been undertaken in the framework of transport theories [4, 10-12] and some analyses [13][14][15] have pointed out the importance of this effect on fragment formation, looking at, for example, the balance between the amount of radial collective flow and recombination probability [13].However, to our knowledge, estimates of the radial collective energy present in experimental multifragmentation data have been mostly obtained by employing statistical models [16][17][18] which treat separately fragment production and collective expansion effects [19][20][21][22][23][24]. The main justification is the small contribution of the collective expansion energy [25] with respect to the total excitation energy characterizing the Fermi energy domain (around 20-30%).…”
We present an analysis of multifragmentation events observed in central Xe+Sn reactions at Fermi energies. Performing a comparison between the predictions of the Stochastic Mean Field (SMF) transport model and experimental data, we investigate the impact of the compression-expansion dynamics on the properties of the final reaction products. We show that the amount of radial collective expansion, which characterizes the dynamical stage of the reaction, influences directly the onset of multifragmentation and the kinematic properties of multifragmentation events. For the same set of events we also undertake a shape analysis in momentum space, looking at the degree of stopping reached in the collision, as proposed in recent experimental studies. We show that full stopping is achieved for the most central collisions at Fermi energies. However, considering the same central event selection as in the experimental data, we observe a similar behavior of the stopping power with the beam energy, which can be associated with a change of the fragmentation mechanism, from statistical to prompt fragment emission.
“…Observed first by the FOPI collaboration [5][6][7][8] and interpreted as an "extra push" with respect to the thermal pressure of an equilibrated composite system, a collective expansion energy has been extracted from experimental data over a wide range of beam energy [9]. Several studies of the multifragmentation process, and of the corresponding role of the collective expansion, have been undertaken in the framework of transport theories [4, 10-12] and some analyses [13][14][15] have pointed out the importance of this effect on fragment formation, looking at, for example, the balance between the amount of radial collective flow and recombination probability [13].…”
Section: Introductionmentioning
confidence: 99%
“…In this case two-body correlations and fluctuations are expected to provide the seeds for fragment formation leading to the occurrence of multifragmentation phenomena which can be described in the framework of a liquid-gas-type phase transition [4].Observed first by the FOPI collaboration [5][6][7][8] and interpreted as an "extra push" with respect to the thermal pressure of an equilibrated composite system, a collective expansion energy has been extracted from experimental data over a wide range of beam energy [9]. Several studies of the multifragmentation process, and of the corresponding role of the collective expansion, have been undertaken in the framework of transport theories [4, 10-12] and some analyses [13][14][15] have pointed out the importance of this effect on fragment formation, looking at, for example, the balance between the amount of radial collective flow and recombination probability [13].However, to our knowledge, estimates of the radial collective energy present in experimental multifragmentation data have been mostly obtained by employing statistical models [16][17][18] which treat separately fragment production and collective expansion effects [19][20][21][22][23][24]. The main justification is the small contribution of the collective expansion energy [25] with respect to the total excitation energy characterizing the Fermi energy domain (around 20-30%).…”
We present an analysis of multifragmentation events observed in central Xe+Sn reactions at Fermi energies. Performing a comparison between the predictions of the Stochastic Mean Field (SMF) transport model and experimental data, we investigate the impact of the compression-expansion dynamics on the properties of the final reaction products. We show that the amount of radial collective expansion, which characterizes the dynamical stage of the reaction, influences directly the onset of multifragmentation and the kinematic properties of multifragmentation events. For the same set of events we also undertake a shape analysis in momentum space, looking at the degree of stopping reached in the collision, as proposed in recent experimental studies. We show that full stopping is achieved for the most central collisions at Fermi energies. However, considering the same central event selection as in the experimental data, we observe a similar behavior of the stopping power with the beam energy, which can be associated with a change of the fragmentation mechanism, from statistical to prompt fragment emission.
“…It is an interesting question whether the equivalence between the multifragmentation reaction and the equilibrium system still holds under such circumstances. It is also interesting to compare observables such as the momentum distribution of fragments and the system size of multifragmentation reactions with those of the corresponding equilibrium systems in the explicit presence of expansion and flow effects [57,58].…”
International audienceRelavence of equilibrium in a multifragmentation reaction is studied by simulating both reaction and equilibrium systems with the antisymmetrized molecular dynamics. Fragment observables in reaction is well explained by an equilibrium assumption, though there are other observables which do not agree between reaction and equilibrium
“…Such extended ensembles can be coherently modelled by accounting for the experimental constraints including time-odd observables and collective flows [8,9], and lead to predictions that can interpolate between the standard canonical, microcanonical and grandcanonical ensembles of macroscopic (N, V, T ) systems [10,11].…”
The origin of bimodal behavior in the residue distribution experimentally measured in heavy ion reactions is reexamined using Boltzmann-Uehling-Uhlenbeck simulations. We suggest that, depending on the incident energy and impact parameter of the reaction, both entrance channel and exit channel effects can be at the origin of the observed behavior. Specifically, fluctuations in the reaction mechanism induced by fluctuations in the collision rate, as well as thermal bimodality directly linked to the nuclear liquid-gas phase transition are observed in our simulations. Both phenomenologies were previously proposed in the literature, but presented as incompatible and contradictory interpretations of the experimental measurements. These results indicate that heavy ion collisions at intermediate energies can be viewed as a powerful tool to study both bifurcations induced by out-of-equilibrium critical phenomena, as well as finite size precursors of thermal phase transitions.
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