Andrew Raphelt

Asymmetry Dependence of the Nuclear Caloric Curve

Abstract A basic feature of the nuclear equation of state is not yet understood: the dependence of the nuclear caloric curve on the neutron–proton asymmetry. Predictions of theoretical models differ on the magnitude and even the sign of this dependence. In this work, the nuclear caloric curve is examined for fully reconstructed quasi-projectiles around mass A = 50 . The caloric curve extracted with the momentum quadrupole fluctuation thermometer shows that the temperature varies linearly with quasi-projectile asymmetry N − Z A . An increase in asymmetry of 0.15 units corresponds to a decrease in temperature on the order of 1 MeV. These results also highlight the importance of a full quasi-projectile reconstruction in the study of thermodynamic properties of hot nuclei.

Using light charged particles to probe the asymmetry dependence of the nuclear caloric curve

Recently, we observed a clear dependence of the nuclear caloric curve on neutron-proton asymmetry

Experimental determination of the quasi-projectile mass with measured neutrons

\frac{N-Z}{A}

CRITICAL SCALING OF TWO-COMPONENT SYSTEMS FROM QUANTUM FLUCTUATIONS

through examination of fully reconstructed equilibrated quasi-projectile sources produced in heavy ion collisions at E/A = 35 MeV. In the present work, we extend our analysis using multiple light charged particle probes of the temperature. Temperatures are extracted with five distinct probes using a kinetic thermometer approach. Additionally, temperatures are extracted using two probes within a chemical thermometer approach (Albergo method). All seven measurements show a significant linear dependence of the source temperature on the source asymmetry. For the kinetic thermometer, the strength of the asymmetry dependence varies with the probe particle species in a way which is consistent with an average emission-time ordering.

Particle-particle correlation functions as an experimental probe of the nuclear asymmetry energy

Abstract The investigation of the isospin dependence of multifragmentation reactions relies on precise reconstruction of the fragmenting source. The criteria used to assign free emitted neutrons, detected with the TAMU Neutron Ball, to the quasi-projectile source are investigated in the framework of two different simulation codes. Overall and source-specific detection efficiencies for multifragmentation events are found to be model independent. The equivalence of the two different methods used to assign experimentally detected charged particles and neutrons to the emitting source is shown. The method used experimentally to determine quasi-projectile emitted free neutron multiplicity is found to be reasonably accurate and sufficiently precise as to allow for the study of well-defined quasi-projectile sources. Experimental QP neutron multiplicity distributions for three similar reactions with different isospin content are also presented. An increase in neutron emission is found for more n-rich systems.

Experimental signals of a nuclear liquid-gas phase transition

The thermodynamics of excited nuclear systems allows the exploration of a phase transition in a two-component quantum mixture. Temperatures and densities are derived from quantum fluctuations of fermions. The pressures are determined from the grand partition function of Fishers model. Critical scaling of observables is found for the first time for fragmenting systems which differ in neutron to proton concentrations thus constraining the equation of state (EOS) of asymmetric nuclear material. The derived critical exponent, β = 0.35 ±0.01, belongs to the liquid–gas universality class. The critical compressibility factor Pc/ρcTc increases with increasing neutron concentration, which could be due to finite-size and/or Coulomb effects.

Temperature Measurements in Low Excitation Energy Reactions to Probe a Possible Phase Transition

The shapes of correlation functions are known to give information about the spatial and temporal extent of an emitting source. Since these two properties can be sensitive to the density-dependence of the asymmetry energy, these functions have been constructed using transport models in the past. In this work, correlation functions extracted using Constrained Molecular Dynamics (CoMD) are used to investigate the possibility of experimentally probing the density-dependence of the asymmetry energy using these correlation functions and detectors with high angular resolution.

Equation of State Effects on Nucleon Transport

The critical phenomenon of the nuclear liquid-gas phase transition has been investigated in the reactions 64Zn+64Zn, 64Ni+64Ni and 70Zn+70Zn at beam energy of 35 MeV/nucleon. Yields of fragments arising from fragmenting quasi-projectiles (QPs) with different neutron-proton asymmetries were analyzed within the framework of the Landau free energy approach. Fits to the free energy of fragments as a function of fragment asymmetry showed three minima, indicating the system to be in a regime of a first-order phase transition. The QP temperature estimates were extracted from the analysis of N=Z fragment data. Additionally, we make use of a recent method based on quantum fluctuations of fermions to derive temperatures and densities of selected QPs. Critical scaling of these observables is found for systems which differ in neutron to proton asymmetry. The derived critical exponent β = 0.35 ± 0.01, belongs to the liquid-gas universality class.

Source-Specific Neutron Detection Efficiencies of the TAMU Neutron Ball

Several methods have been used to determine the temperatures of systems formed in multifragmentation reactions. From these temperatures, caloric curves can be constructed and possible nuclear phase transitions can be explored. This work presents a previously observed, low temperature phase transition predicted by a theoretical simulation, and an experimental proposal to observe this transition. The proposed experiment will explore what types of reactions produce fragments near the phase transition temperature, and expand the range of fragments that can be detected in this low temperature region.

Asymmetry Energy Effects on Reaction Break-up Mechanisms Near the Fermi Energy

Previous studies have examined the nucleon transport in charge symmetric beam-on-target reaction systems as a probe for the asymmetry energy of nuclear matter. Using the iBUU transport model, isospin transport in charge symmetric as well as mass symmetric systems of Zn and Ni isotopes at 35 MeV/nucleon will be explored. Comparisons of the behavior of charge symmetric (70,64Zn + 70,64Zn) to mass symmetric (64Zn,64Ni + 64Zn,64Ni) reaction systems will be studied. The effect of different nucleon interactions will also be studied. This set of reaction systems will be used to determine what effect the difference in charge between projectile and target may have on the proton transport as compared to the neutron transport. These studies will be carried out with a view for use in the analysis of experimental data acquired using the NIMROD-ISiS array and TAMU Neutron Ball at the Cyclotron Institute at Texas A&M.