S. Wuenschel

Measuring the temperature of hot nuclear fragments

Abstract A new thermometer based on fragment momentum fluctuations is presented. This thermometer exhibited residual contamination from the collective motion of the fragments along the beam axis. For this reason, the transverse direction has been explored. Additionally, a mass dependence was observed for this thermometer. This mass dependence may be the result of the Fermi momentum of nucleons or the different properties of the fragments (binding energy, spin, etc.) which might be more sensitive to different densities and temperatures of the exploding fragments. We expect some of these aspects to be smaller for protons (and/or neutrons); consequently, the proton transverse momentum fluctuations were used to investigate the temperature dependence of the source.

Constraining the symmetry term in the nuclear equation of state at subsaturation densities and finite temperatures

Methods of extraction of the symmetry energy (or enthalpy) coefficient to temperature ratio from isobaric and isotopic yields of fragments produced in Fermi-energy heavy-ion collisions are discussed. We show that the methods are consistent when the hot fragmenting source is well characterized and its excitation energy and isotopic composition are properly taken into account. The results are independent of the mass number of the detected fragments, which suggests that their fate is decided very early in the reaction.

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

Symmetry energy and the isoscaling properties of the fragments produced in Ar 40 , Ca 40 + Fe 58 , Ni 58 reactions at 25, 33, 45, and 53 MeV/nucleon

\frac{N-Z}{A}

Isoscaling of fragments with Z = 1 − 17 from reconstructed quasiprojectiles

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.

Analysis of fragment yield ratios in the nuclear phase transition

The symmetry energy and the isoscaling properties of the fragments produced in the multifragmentation of {sup 40}Ar, {sup 40}Ca+{sup 58}Fe, {sup 58}Ni reactions at 25, 33, 45, and 53 MeV/nucleon were investigated within the framework of statistical multifragmentation model. The isoscaling parameters {alpha}, from the primary (hot) and secondary (cold) fragment yield distributions, were studied as a function of excitation energy, isospin (neutron-to-proton asymmetry), and fragment symmetry energy. It is observed that the isoscaling parameter {alpha} decreases with increasing excitation energy and decreasing symmetry energy. The parameter {alpha} is also observed to increase with increasing difference in the isospin of the fragmenting system. The sequential decay of the primary fragments into secondary fragments, when studied as a function of excitation energy and isospin of the fragmenting system, show very little influence on the isoscaling parameter. The symmetry energy, however, has a strong influence on the isospin properties of the hot fragments. The experimentally observed scaling parameters can be explained by symmetry energy that is significantly lower than that for the ground-state nuclei near saturation density. The results indicate that the properties of hot nuclei at excitation energies, densities, and isospin away from the normal ground-state nuclei could be significantly different.

Density determinations in heavy ion collisions

In heavy-ion collisions, isoscaling provides a method for studying the evolution of nuclear symmetry energy as a function of excitation energy. One challenge in using isoscaling is to accurately determine the neutron-to-proton ratio (N/Z) of the fragmenting source. Isoscaling results are presented for the reactions of {sup 86,78}Kr+{sup 64,58}Ni at 35 MeV/nucleon taken on the NIMROD-ISiS array at Texas A and M University. The N/Z of the source was calculated from the isotopically identified fragments and experimentally measured neutrons emitted from reconstructed quasiprojectiles. These data exhibit isoscaling for elements with Z=1-17 over a broad range of isotopes. The isoscaling parameter {alpha} is shown to increase with increasing difference in the neutron composition ({delta}) of the compared sources. For a selected {delta}, the ratio {alpha}/{delta} is also shown to decrease with increasing excitation energy. This may reflect a corresponding decrease in the nuclear symmetry energy.

Quantum suppression of fluctuations and temperatures of reconstructed A ∼ 30 quasi-projectiles

The critical phenomenon of the liquid-gas phase transition has been investigated in the reactions {sup 78,86}Kr+{sup 58,64}Ni at beam energy of 35 MeV/nucleon using the Landau free energy approach with isospin asymmetry as an order parameter. Fits to the free energy of fragments showed three minima, suggesting that the system is in the regime of a first-order phase transition. The relation m = -{partial_derivative}F/{partial_derivative}H, which defines the order parameter and its conjugate field H, has been experimentally verified from the linear dependence of the mirror nuclei yield ratio data on the isospin asymmetry of the source. The slope parameter, which is a measure of the distance from a critical temperature, showed a systematic decrease with increasing excitation energy of the source. Within the framework of the Landau free energy approach, isoscaling provided similar results as obtained from the analysis of mirror nuclei yield ratio data. In the present work, it is shown that the external field is primarily related to the minimum of the free energy, which implies a modification of the source concentration {Delta} used in isospin studies.

Nuclear expansion and symmetry energy of hot nuclei

The experimental determination of freeze-out temperatures and densities from the yields of light elements emitted in heavy ion collisions is discussed. Results from different experimental approaches are compared with those of model calculations carried out with and without the inclusion of medium effects, which become of relevance for baryon densities above