IMF isotopic properties in semi-peripheral collisions at Fermi energies

Abstract

We study the neutron and proton dynamical behavior along the fragmentation path in semi-peripheral 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 spin-odal 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 asym-metric 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 ...

Transverse collective flow and midrapidity emission of isotopically identified light charged particles

“…The 3 H-3 He flow results discussed above can be interpreted in terms of a more neutron-rich emission from the neck region in the stiff case [28]. The lower value of the stiff symmetry potential, with respect to the soft, during the expansion phase of the system leads to less repulsive dynamics, associated with a generally larger flow of the LCPs (see Figure 7).…”

“…The results demonstrate that the increased relative mid-rapidity emission (a decrease in (R Mid Y ield ) −1 ) coincides with a decreased transverse flow. A possible explanation for the correlation based on the isospin migration phenomenon [28,30] is presented below in context of the SMF model results.…”

“…At the same time, the neutron excess is transferred towards the low-density neck region, from which light charged particles eventually emerge. The latter mechanism (isospin migration), that is sensitive to the derivative of the symmetry energy just below normal density, is more effective in the stiff case [28] producing a more neutron-rich neck region. This is confirmed by examining the R Mid Y ield values of the LCPs calculated from both the stiff and soft symmetry potential, as shown in Figure 3.…”

“…For example, Lukasik et al showed that 65-70% of the total 3 H production can be attributed to the mid-rapidity region [18]. Furthermore, it has been suggested that the neck-like structure represents a low-density region of nuclear matter between the higher density QP and QT [27,28]. Thus, the neck region can provide an opportunity to examine dilute neutron-rich nuclear matter.…”

Correlations between emission timescale of fragments and isospin dynamics in124Sn+64Ni and112Sn+Filippo1, Pagano2, Russotto3 et al. 2012Phys. Rev. CSelf Cite806580View full textAdd to dashboardCite

“…1a), the average N/Z isotopic distributions for all charges between Z=3 and Z=8. scale, but it is linked to the reaction dynamics in a consistent way; ii) the <N/Z> of the fragments can be correlated with the alignment properties of different emission scenario, so enhancing the experimental sensitivity to select those genuine effects due to isospin dynamics [4,38].…”