Isospin Dynamics in Fragmentation Reactions at Fermi Energies

Abstract

We study the neutron and proton dynamical behavior along the fragmentation path in semiperipheral collisions:58 Fe+ 58 Fe (charge asymmetric, N/Z = 1.23) and 58 Ni+ 58 Ni (charge symmetric, N/Z = 1.07), at 47 AMeV. We observe that isospin dynamics processes take place also in the charge-symmetric system 58 Ni+ 58 Ni, that may produce more asymmetric fragments. A neutron enrichment of the neck fragments is observed, resulting from the interplay between pre-equilibrium emission and the phenomenon of isospin-migration. Both effects depend on the EoS (Equation of State) symmetry term. This point is illustrated by comparing the results obtained with two different choices of the symmetry energy density dependence. New correlation observables are suggested, to study the reaction mechanism and the isospin dynamics. Collisions between heavy ions with large isospin asymmetries, made possible by the recent radioactive beam developments, represent a very efficient way to probe the structure of nuclear-EoS (Equation of State) symmetry term. In particular the symmetry energy behavior is influencing very dissipative reaction mechanisms, such as fragmentation processes, leading to important effects on fragment composition.In central heavy ion collisions at intermediate energies the spinodal decomposition has been proposed as a possible mechanism for fragment formation, see Ref.[1] for a recent review. According to this description, fragments should reflect the properties of the low density phase, where they are formed. In charge asymmetric systems, the isospin distillation, i.e. the formation of more symmetric fragments surrounded by a neutron richer dilute phase, takes place. Here we will focus on fragmentation in semi-peripheral collisions, where intermediate mass fragments (IM F ) are mostly produced in the overlap zone (the neck region) between projectile-like and target-like fragments (P LF/T LF , the spectator region), [2, 3, 4, 5, 6, 7] [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. We will discuss the influence of the symmetry energy on the features of this fragmentation mechanism. The presence of a density gradient between the neck (low density) and the spectator (high density) regions affects the N/Z of fragments in a different way with respect to the spinodal decomposition mechanism [20,21,22]. Moreover since the isoscalar density gradients are ruling the isospin transfer through the density dependence of the symmetry energy, we see that measurements of isospin observables in semiperipheral collisions will directly probe the slope of the symmetry term around saturation, of large impor- * On leave from HH-NIPNE and University of Bucharest, Romania.tance for the structure of neutron-rich nuclei [23].We consider the reactions 58 Fe + 58 Fe (charge asymmetric N/Z = 1.23) and 58 Ni + 58 Ni (charge symmetric N/Z = 1.07). In fact in these collisions, due to the uniform N/Z distributions, we do not have isospin gradients initially. We show that the study of the full reaction dynamics and of the possible occurrence of densit...

Mean-Field Instabilities and Cluster Formation in Nuclear Reactions

“…44,45 The neck fragmentation with a peculiar intermediate mass fragment (2 < Z < 20) distribution and an entrance channel memory was observed experimentally and predicted by various transport models. 36,[46][47][48] In this case, the low-density neck region triggers an isospin migration from the higher density regions corresponding to the projectilelike fragment (PLF) and targetlike fragment (TLF). Therefore, the isospin content of the IMFs is expected to reflect the isospin enrichment of the midvelocity region.…”

Collision dynamics at medium and relativistic energies

“…The neck fragmentation with a peculiar intermediate mass fragment (2 < Z < 20) distribution and an entrance channel memory was observed experimentally and predicted by various transport models [46,128,129,130,131,132,133,134,135,136,137,138,139,140,141]. An interplay between statistical and dynamical emission mechanisms can be expected; however it is possible to identify clear dynamical signatures, such as the appearance of hierarchy effects of the IMF size vs. transverse velocity, which keep track of the cluster formation time scale and of the related impact of many-body correlations.…”

Isospin in Reaction Dynamics. The Case of Dissipative Collisions at Fermi Energies

“…The analysis of the N/Z distribution versus the emission angle reveals that larger fluctuations are present close to forward and backward angles, suggesting that IMF's that are more correlated to the spectator matter may become more neutron-rich, see Fig. 5 for the F e, F e/N i, N i systems at 47AM eV 36 . We stress again that the neck − IM F always present a neutron enrichment, even in the case of a n-poor system.…”

“…Moreover, moving from mid-to "spectator"-rapidities an increasing hierarchy in the mass and N/Z of the fragments is 48 . An interesting related observable is the corresponding angular correlation due to the driving force of the Projectile(Target)-like partners 36 . The analysis of the N/Z distribution versus the emission angle reveals that larger fluctuations are present close to forward and backward angles, suggesting that IMF's that are more correlated to the spectator matter may become more neutron-rich, see Fig.5 for the F e, F e/N i, N i systems at 47AM eV 36 .…”

“…Neutron and/or light isobar measurements in different rapidity regions appear important. Moreover, moving from mid-to "spectator"rapidities an increasing hierarchy in the mass and N/Z of the fragments is expected 36 . Some experimental evidences are in ref.…”