Effects of isospin dynamics on neck fragmentation in isotopic nuclear reactions

The neck dynamics in Fermi-energy heavy-ion collisions, to probe the nuclear symmetry energy in the domain of subsaturation densities, is investigated within an isospin-dependent transport model. The single and double ratios of neutrons to protons from free nucleons and light clusters (complex particles) in the isotopic reactions are analyzed systematically. Isospin effects of particles produced from the neck fragmentations are explored. It is found that the ratios of the energetic isospin particles strongly depend on the stiffness of the nuclear symmetry energy and the effects increase with softening of the symmetry energy, which would be a nice probe for extracting the symmetry energy below the normal density in experiments. A flat structure appears at the tail spectra from the double ratio distributions. The neutron to proton ratio of light intermediate-mass fragments with charge number $Z\le 8$ is related to the density dependence of the symmetry energy with less sensitivity in comparison to the isospin ratios of nucleons and light particles.

Figure 1

Nuclear symmetry energy as a function of baryon density for the soft, linear, and hard cases denoted by the solid, dashed, and dot-dashed lines, respectively.

Figure 2

Fragment distributions as functions of charge and mass numbers in central ${}^{197}\mathrm{Au}+{}^{197}\mathrm{Au}$ collisions.

Figure 3

Kinetic-energy spectra of neutron to proton ratios from the yields of free nucleons [panels (a) and (b)] and gas-phase nucleons (nucleons, hydrogen and helium isotopes) [panels (c) and (d)] from neck fragmentations in the ${}^{112}\mathrm{Sn}+{}^{58}\mathrm{Ni}$ and ${}^{124}\mathrm{Sn}+{}^{64}\mathrm{Ni}$ reactions at a Fermi energy of 35 MeV/nucleon within the collision centralities of 6–8 fm.

Figure 4

The same as in Fig. 3, but for the reactions of ${}^{112}\mathrm{Sn}+{}^{112}\mathrm{Sn}$ and ${}^{124}\mathrm{Sn}+{}^{124}\mathrm{Sn}$.

Figure 5

The average neutron to proton ratio of IMFs emitted within the rapidity range of $|y/y_{\mathrm{proj}}|<0.3$ as a function of the charge number with the different symmetry energies. The data from the CHIMERA detector [24] are shown for comparison.

Figure 6

The double neutron to proton ratios of free nucleons and gas-phase particles in collisions of ${}^{124}\mathrm{Sn}+{}^{64}\mathrm{Ni}$ over ${}^{112}\mathrm{Sn}+{}^{58}\mathrm{Ni}$ and ${}^{124}\mathrm{Sn}+{}^{124}\mathrm{Sn}$ over ${}^{112}\mathrm{Sn}+{}^{112}\mathrm{Sn}$ at an incident energy of 35 MeV/nucleon and with the impact parameters of 6–8 fm.