Isospin Equilibration in Fermi-energy Heavy-ion Nuclear Collisions

Abstract

The nuclear Equation of State and the density dependence of the asymmetry energy have been explored via heavy-ion collisions of 35 MeV/nucleon 70Zn,64Ni+64Zn and 64Zn+70Zn,64Ni. The experimental data were collected on the NIMROD-ISiS (Neutron-Ion Multidetector for Reaction Oriented Dynamics with Indiana Silicon Sphere) 4π charged particle detector array coupled with the TAMU Neutron Ball at the Texas A&M University Cyclotron Institute which provides excellent isotopic resolution, event characterization and coverage of charged particles along with event-by-event measurement of neutron multiplicities.

The nature of isospin equilibration was examined via the use of the iBUU04 transport model and the Constrained Molecular Dynamics (CoMD) model coupled with the GEMINI statistical decay model. Both models provided insight into the nature of the heavy-ion collisions studied but both models must also be better understood in order to replicate the effects seen in the experimental data. An improved method of experimental impact parameter determination was demonstrated with the CoMD results.

Experimental measurements of the isoscaling parameter α, isobaric yield ratio for the A=3 isobar and reconstructed quasi-projectile isospin asymmetry were conducted with respect to the centrality of the collision. A new signature of isospin equilibration was proposed and observed in the experimental data: convergence of the quasi-projectile and quasi-target isospin asymmetries to each other as the interaction time increases. Finally, a direct comparison was made to previous works and found that the reaction systems studied experienced an isospin equilibration of approximately 75%.