The researchers in the Theoretical Physics Group at Institute of Modern Physics, Chinese Academy of Sciences (IMP) have explored the density dependence of symmetry energy of nuclear matter at subsaturation densities from nuclear mass differences.  

The density dependence of nuclear symmetry energy is a hot topic in nuclear physics, which is related to many important issues, such as heavy ion collisions, exotic nuclei, stability of superheavy nuclei, fusion cross sections, the structures, composition and cooling of neutron stars. Much theoretical and experimental effort has been made to constrain the density dependence of symmetry energy. However, this problem remains unsolved due to the difficulty of nucleon-nucleon interactions and many-body problems.  

With the help of experimental nuclear mass differences, the symmetry energy coefficient of heavy nuclei is determined in the present work. The result was applied to analyze the density dependence of symmetry energy of nuclear matter at subsaturation densities. The slope parameter at the saturation density is determined to be L = 50.5±15.5 MeV in the present work. Fig. 1 shows the calculated symmetry energy as a function of density. According to some existing investigations, the neutron skin thickness of heavy nuclei is uniquely fixed by the slope parameter L(ρ) of symmetry energy at a subsaturation density of ρ≈0.11fm^-3  rather than that at the saturation density. At this subsaturation density, the symmetry energy and its slope parameter obtained in the present work are 25.98±0.01 MeV and L = 49.6 ± 6.2 MeV, respectively, being in agreement with the results given by the other independent analysis. The present results are expected to be useful to further determine the neutron skin thickness and to explore the cooling of neutron stars.  

The work has been published in Phys. Rev. 89, 017305 (2014) 

Fig. 1 Symmetry energy as a function of density (Image by IMP)