The cross sections and velocity distributions of projectile-like fragments from the reaction of 25 MeV/nucleon 86 Kr + 64 Ni have been measured using the MARS recoil separator at Texas A&M, with special emphasis on the neutron rich isotopes. Proton-removal and neutron pick-up isotopes have been observed with large cross sections. A model of deep-inelastic transfer (DIT) for the primary interaction stage and the statistical evaporation code GEMINI for the deexcitation stage have been used to describe the properties of the product distributions. The results have also been compared with the EPAX parametrization of high-energy fragmentation yields. The experimental data show an enhancement in the production of neutron-rich isotopes close to the projectile, relative to the predictions of DIT/GEMINI and the expectations of EPAX. We attribute this enhancement mainly to the effect of the extended neutron distribution (neutron "skin") of the 64 Ni target in peripheral interactions of 86 Kr with 64 Ni. The large cross sections of such reactions near the Fermi energy, involving peripheral nucleon exchange, suggest that, not only the N/Z of the projectile and the target, but also the N/Z distribution at the nuclear surface may properly be exploited in the production of neutron-rich rare isotopes. This synthesis approach may offer a fruitful pathway to extremely neutron-rich nuclei, towards the neutron-drip line.

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The isoscaling parameter $\alpha$, from the fragments produced in the multifragmentation of $^{58}$Ni + $^{58}$Ni, $^{58}$Fe + $^{58}$Ni and $^{58}$Fe + $^{58}$Fe reactions at 30, 40 and 47 MeV/nucleon, was compared with that predicted by the antisymmetrized molecular dynamic (AMD) calculation based on two different nucleon-nucleon effective forces, namely the Gogny and Gogny-AS interaction. The results show that the data agrees better with the choice of Gogny-AS effective interaction, resulting in a symmetry energy of $\sim$ 18-20 MeV. The observed value indicate that the fragments are formed at a reduced density of $\sim$ 0.08 fm$^{-3}$.Comment: 5 pages, 5 figures, Accepted for publication in Phys. Rev. C (Rapid Communication

The evolution of the symmetry energy coefficient of the binding energy of hot fragments with increasing excitation is explored in multifragmentation processes following heavy-ion collisions below the Fermi energy. In this work, high-resolution mass spectrometric data on isotopic distributions of projectile-like fragments are systematically compared to calculations involving the Statistical Multifragmentation Model (SMM). Within the SMM picture, the present study suggests a gradual decrease of the symmetry energy coefficient of the hot primary fragments from 25 MeV at the compound nucleus regime towards 15 MeV in the multifragmentation regime. The isotopic distributions of the hot primary fragments are found to be very wide and extend towards the neutron drip-line. These findings are expected to have important implications to the modeling of the composition and the evolution of hot and dense astrophysical environments, such as those of core-collapse supernova.PACS numbers: 25.70.Hi,25.70.Lm,26.30.+k Nuclear multifragmentation is one of the most interesting phenomena in nuclear physics as it holds promise for understanding nuclear matter properties at the extreme conditions of high excitation energy and large isospin (N/Z) asymmetry [1,2,3,4,5]. The latter, in particular, plays a profound role in the dynamics of various astrophysical environments [6,7,8,9]. It is well established (e.g. [4]) that nuclear systems with relatively low excitation energy (E * /A ≤ 2 MeV) form the traditional compound nucleus, whereas at higher excitation energy, the hot nuclear system expands and, subsequently, disassembles into an ensemble of hot primary fragments. This extremely complicated process, namely the multifragmentation, occurs on a timescale of 100 fm/c (3.3×10 −22 sec) during which the system can sample a large number of configurations. For this reason, statistical calculations (e.g., [10,11]) have been very successful in describing this process.Recently, a remarkable similarity has been pointed out between the thermodynamic conditions (temperature, density, isospin asymmetry N/Z) reached in nuclear multifragmentation and the collapse/explosion of massive stars [12,13,14]. This observation opens up the possibility of applying well-established models of nuclear reactions to describe matter distribution and evolution during supernova explosions [12]. In addition, statistical calculations suggest that in multifragmentation [12,15] and in hot astrophysical environments (e.g. supernova) [12,16], the ensemble of primary fragments includes neutron-rich nuclei towards or beyond the neutron drip-line.The primary fragments are expected to be hot (with excitation energies approaching 2-3 MeV/nucleon [17]) and, initially, in close proximity to neighboring fragments or nucleons. These conditions render their properties, e.g. binding energy, different from those of cold (ground state) isolated nuclei. In particular, recent studies [18,19,20,21] give evidence for a significant decrease of the symmetry energy of hot primary fragments. ...

A large enhancement in the production of neutron-rich projectile residues is observed in the reactions of a 25 MeV/nucleon 86 Kr beam with the neutron rich 124 Sn and 64 Ni targets relative to the predictions of the EPAX parametrization of high-energy fragmentation, as well as relative to the reaction with the less neutron-rich 112 Sn target. The data demonstrate the significant effect of the target neutron-to-proton ratio (N/Z) in peripheral collisions at Fermi energies. A hybrid model based on a deep-inelastic transfer code (DIT) followed by a statistical de-excitation code accounts for part of the observed large cross sections. The DIT simulation indicates that the production of neutron-rich nuclides in these reactions is associated with peripheral nucleon exchange in which the neutron skins of the neutron-rich 124 Sn and 64 Ni target nuclei may play an important role. From a practical viewpoint, such reactions between massive neutron-rich nuclei offer a novel synthetic avenue to access extremely neutron-rich rare isotopes towards the neutron-drip line.PACS numbers: 25.70.Hi,25.70.Lm Exploration of the nuclear landscape towards the neutron-drip line [1] is currently of great interest in order to elucidate the evolution of nuclear structure with increasing neutron-to-proton ratio (N/Z) [2,3] and understand important nucleosynthesis pathways [4], most notably the r-process [5]. Reactions induced by neutronrich nuclei provide invaluable information on the isospin dependence of the nuclear equation of state [6,7]. Extremely neutron-rich nuclei offer the unprecedented opportunity to extrapolate our knowledge to the properties of bulk isospin-rich matter, such as neutron stars [8,9]. The efficient production of very neutron-rich nuclides is a key issue in current and future rare isotope beam facilities around the world [10,11,12] and, in parallel, the search for new synthetic approaches is of exceptional importance.Neutron-rich nuclides have traditionally been produced in spallation reactions, fission, and projectile fragmentation [13]. In high-energy fragmentation reactions, the production of the most neutron-rich isotopes is based on a "clean-cut" removal of protons from the projectile. The world's data on fragmentation cross sections are well represented by the empirical parametrization EPAX [14]. EPAX is currently the common basis for predictions to plan rare beam experiments and facilities. In addition to the widely used projectile fragmentation approach, neutron-rich nuclides can be produced in multinucleon transfer reactions [15] and deep-inelastic reactions near the Coulomb barrier (e.g. [16,17,18]). In such reactions, the target N/Z significantly affects the production cross sections, but the low velocities of the fragments and the ensuing wide angular and ionic charge state distributions render practical applications rather limited. The Fermi energy regime (20-40 MeV/nucleon) [19] offers the unique opportunity to combine the advantages of both low-and high-energy reactions. At this energy, the synergy...