Searching for Neutron-Proton Equilibration at Fermi Energies

Abstract

The density dependence of the symmetry term of the nuclear Equation of State (nEoS) has been studied using reactions of 70Zn+70Zn, 64Zn+64Zn and 64Ni+64Ni at 35 MeV/nucleon. The experiment was performed using the NIMROD (Neutron-Ion Multidetector for Reaction Oriented Dynamics), which is a 4π charged-particle array housed inside the TAMU Neutron Ball. The detector was chosen due to its fantastic isotopic resolution and large angular coverage.

Neutron-proton (NZ) equilibration between the two largest fragments originating from the excited projectile-like fragment was used as a probe to study the density dependence of the nEoS. The NZ equilibration was observed to be exponentially decreasing for the second heaviest fragment and exponentially increasing for the heaviest fragment. A mean equilibration lifetime of 0.3 zs was extracted for the heaviest and second heaviest fragments in all three reaction systems.

The results were compared to the Constrained Molecular Dynamics (COMD) and Anti-symmetrized Molecular Dynamics (AMD) models in conjunction with the GEMINI statistical model. The results suggest a soft density dependence of the asymmetry energy term.

Additionally, the NIMROD array and the Neutron Ball were recommissioned and upgraded. New detectors were tested, characterized and implemented. The results from the Neutron Ball indicate the background is approximately 1.5 neutrons lower per event than previously measured results.