Littérature scientifique sur le sujet « Isospin transport ratio »

Abstract The isospin diffusion of the quasi-projectile formed in the Ni 64 , 58 on Ni 64 , 58 reactions in the Fermi energy domain is investigated in the framework of the Boltzmann–Uehling–Uhlenbeck transport model. The well known isospin transport ratio observable is revisited, with the aim of insuring an optimal comparison between experimental data and theoretical calculations and reducing the present uncertainties in the extraction of empirical equation of state parameters. We show that isospin transport ratios are sensitive to all the low order isovector parameters (E sym, L sym and K sym, but the quantitative results depend on the choice of the isospin sensitive observable. We demonstrate that realistic models of the equation of state, covering the uncertainty that presently affects the theoretical description of neutron stars static observables, can be effectively discriminated by isospin diffusion experiments, provided the neutron to proton ratio of the projectile remnant is precisely measured as a function of centrality.

The production/absorption rate of particles in compressed and heated asymmetric matter is studied using a Relativistic Mean Field (RMF) transport model with an isospin dependent collision term. We show that the K+/K° ratio reflects the isospin effects on the production rates just because of the large sensitivity around the threshold. The results are very promising with respect to the possibility of a direct link between particle production data in exotic Heavy Ion Collisions (HIC) and the isospin dependent part of the Equation of State (EoS) at high baryon densities.

The reaction mechanism of the central collisions and peripheral collisions for 112,124 Sn + 112,124 Sn at E/A = 50 MeV is investigated within the framework of the Improved Quantum Molecular Dynamics model. The results show that multifragmentation process is an important mechanism at this energy region, and the influence of the cluster emission on the double n / p ratios and the isospin transport ratio is important. Furthermore, three observables, double n / p ratios, isospin diffusion and the rapidity distribution of the ratio R7 for 112,124 Sn +112,124 Sn at E/A = 50 MeV are analyzed with the Improved Quantum Molecular Dynamics model. The results show that these three observables are sensitive to the density dependence of the symmetry energy. By comparing the calculation results to the data, the consistent constraint on the density dependence of the symmetry energy from these three observables is obtained.

The neutron–proton effective mass splitting in neutron-rich nucleonic matter reflects the spacetime nonlocality of the isovector nuclear interaction. It affects the neutron/proton ratio during the earlier evolution of the Universe, cooling of proto-neutron stars, structure of rare isotopes and dynamics of heavy-ion collisions. While there is still no consensus on whether the neutron–proton effective mass splitting is negative, zero or positive and how it depends on the density as well as the isospin-asymmetry of the medium, significant progress has been made in recent years in addressing these issues. There are different kinds of nucleon effective masses. In this mini-review, we focus on the total effective masses often used in the non-relativistic description of nuclear dynamics. We first recall the connections among the neutron–proton effective mass splitting, the momentum dependence of the isovector potential and the density dependence of the symmetry energy. We then make a few observations about the progress in calculating the neutron–proton effective mass splitting using various nuclear many-body theories and its effects on the isospin-dependence of in-medium nucleon–nucleon cross-sections. Perhaps, our most reliable knowledge so far about the neutron–proton effective mass splitting at saturation density of nuclear matter comes from optical model analyses of huge sets of nucleon–nucleus scattering data accumulated over the last five decades. The momentum dependence of the symmetry potential from these analyses provide a useful boundary condition at saturation density for calibrating nuclear many-body calculations. Several observables in heavy-ion collisions have been identified as sensitive probes of the neutron–proton effective mass splitting in dense neutron-rich matter based on transport model simulations. We review these observables and comment on the latest experimental findings.

Heavy-ion collisions in the Fermi energy regime have been widely employed to probe the properties of nuclear matter far from equilibrium conditions. More specifically, they allow to investigate various phenomena (e.g. isospin transport phenomena) that can be interpreted in the framework of the Nuclear Equation of State (NEoS), which describes the properties of nuclear matter in terms of thermodynamic variables such as temperature, pressure, density. In particular, this work is focused on semiperipheral and peripheral collisions, for which a binary output channel is the most probable result, characterised by the production of two heavy fragments, called quasiprojectile (QP) and quasitarget (QT), together with some lighter ejectiles. This thesis concerns the results of the analysis of the E789 experiment, the first one exploiting the recently coupled INDRA-FAZIA apparatus: FAZIA, placed at forward polar angles, provides optimal (Z,A) identification for the ejectiles mostly belonging to the QP phase space, while INDRA covers most of the remaining solid angle. In the E789 experiment, we investigate the four different reactions 64,58Ni+64,58Ni at two different beam energies 32AMeV and 52AMeV. The availability of all the four possible combinations of 58Ni and 64Ni allows to exploit the isospin transport ratio technique, that enables to inspect the isospin equilibration process by comparing the results of the two asymmetric systems with both the neutron rich and neutron deficient symmetric systems. Thanks to the data for two different beam energies we can investigate possible differences in the isospin transport mechanisms associated with two different reaction dynamics with associated different interaction times. In this work, two main reaction channels are selected and examined: the QP evaporation channel (with a QP-like fragment accompanied only by lighter particles) and the QP breakup channel (with two fragments compatible with QP fission products). The latter selection shows the typical features of a QP dynamical fission, or breakup, i.e. the anisotropy of the emission pattern for the most asymmetric splits, with the light breakup fragment (LF) preferentially emitted backwards with respect to the heavy one (HF). The experimental results are also compared to the predictions of the transport model AMD, coupled with GEMINI as afterburner: the AMD+GEMINI++ simulations, filtered according to the apparatus acceptance, nicely reproduce the general features of the reaction products, in both the selected reaction channels. By exploiting the isospin transport ratio, studied as a function of a selected centrality estimator, namely the reduced QP momentum along the beam axis, clear indications of isospin diffusion between projectile and target are found in the two asymmetric reactions. In the QP evaporation channel, the action of isospin equilibration is evidenced on the neutron content of both the QP remnant and of the light charged particles emitted forward with respect to the QP (and hence presumably produced in its statistical decay): we obtain an evident and regular evolution towards isospin equilibration for increasing reaction centrality, with a stronger effect in the reactions at 32AMeV than at 52AMeV, perhaps due to shorter projectile-target contact times in the latter case. In the QP breakup channel, we study the isospin composition of the QP reconstructed as the sum of the two breakup fragments: a clear trend towards isospin equilibration is found for increasing centrality, again stronger at 32AMeV than at 52AMeV. Quite interestingly, by comparing the results for the isospin transport ratio in the two channels, we observe an unexpected stronger tendency to isospin equilibration on the QP in the breakup channel than in the evaporative channel. The interpretation of this result is not straightforward and deserves deeper investigation. Also some indications of neutron enrichment of fragments at midvelocity, possibly related to isospin drift, are found in this work: we show an evident neutron enrichment of all the light species emitted backward with respect the QP, compared to those emitted forward, for all the investigated reactions. Lastly, in the QP breakup channel, we investigate the evolution of the isospin composition of the HF and the LF, which, assuming that the fission process is fast enough, is expected to retain some information on the isospin transport (both drift and diffusion) prior to the breakup, also depending on the time elapsed between the QP-QT split and the QP breakup itself. We observe a relaxation of the isospin imbalance between the LF and the HF for increasing angle between the QP-QT separation axis and the QP fission axis: this result is qualitatively, and in some respect also quantitatively, compatible with what reported in the literature.