No abstract

Based on the improved isospin-dependent Boltzmann-Langevin model which incorporates the dynamical fluctuations, we study the π production in central heavy ion collisions at different incident energies from 250 to 1200 A MeV. It is found that the π multiplicity is sensitive to the nuclear equation of state. At π subthreshold energy, the fluctuations have a larger effect on the π multiplicity. The π − /π + ratios as a probe of nuclear symmetry energy are calculated with different stiffness of symmetry energy. The results favor a supersoft symmetry energy of the potential term in comparison with the FOPI data, which supports the one obtained by the usual Boltzmann-Uehling-Uhlenbeck model.During the last few years, the study of nuclear symmetry energy E sym (ρ) has been a highly interesting subject. The constraining of E sym (ρ) is important for not only understanding of heavy-ion reactions [1] but also many issues in astrophysics [1,2].Unfortunately, the form of E sym (ρ) is very controversial, especially at supra-saturation density. At sub-saturation density, constraints on the E sym (ρ) were obtained by analyzing the isospin diffusion data [3]. At supra-saturation density, the main difference of the E sym (ρ) forms predicted by some microscopical or phenomenological many-body approaches is the trend of the E sym (ρ) with the density. One is the E sym (ρ) increases continuously with the increasing density, and the other is the E sym (ρ) increases

Here we report the observation of a linear carbon-atom chain, synthesized using an arc discharge from a graphite double anode. We use high-resolution TEM to evidence the formation of a carbon-atom chain along the axis of a needlelike structure consisting of concentric cylindrical graphitic shells. The atomic chain in the core of the innermost cylinder, by calculations on the basis of van der Waals interactions, demonstrates an extremely high thermal stability ͑up to 4200 K͒ and exhibits high mechanical strains.

The symmetry energy coefficients for nuclei with mass number A = 20 ∼ 250 are extracted from more than 2000 measured nuclear masses. With the semi-empirical connection between the symmetry energy coefficients of finite nuclei and the nuclear symmetry energy at reference densities, we investigate the density dependence of symmetry energy of nuclear matter at subnormal densities.The obtained results are compared with those extracted from other methods.