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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

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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. ...

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