G. D. Westfall

Radial flow in Au + Au collisions at E = (0.25-1.15)A GeV

A systematic study of energy spectra for light particles emitted at midrapidity from Au+Au collisions at {ital E}= (0.25--1.15){ital A} GeV reveals a significant nonthermal component consistent with a collective radial flow. This component is evaluated as a function of bombarding energy and event centrality. Comparisons to quantum molecular dynamics and Boltzmann-Uehling-Uhlenbeck models are made for different equations of state.

Prospects of intermediate energy nuclear collisions

Abstract We focus on central nuclear collisions--“multifragmentation events”--in the energy range 20–400 MeV/n. They seem well suited to study bulk properties of nuclear matter at moderate entropies. Various ways of extracting information on the produced entropy are discussed. We emphasize the importance of medium mass fragment production for this goal. The consequences of a first order liquid-vapor phase transition at low densities ϱ ϱ 0 are pointed out--the release of latent heat results in an increase of the entropy at energies E LAB ≲ 200 MeV/n. It is pointed out that a minimum in the mass distribution is indicative of the onset of condensation. Such a minimum has indeed been observed in multifragmentation events. The medium energy reactions also provide an enhanced sensitivity to the stiffness of the nuclear equation of state at high densities ϱ > ϱ 0 . This is discovered in a 4π exclusive energy flow analysis performed on the basis of the nuclear fluid dynamical model. A strong bombarding energy dependence of the flow effects is predicted, which is not found in cascade simulations. The flow analysis can also be used to reveal the presence of a high density abnormal state via a characteristic change of the flow pattern at a critical bombarding energy.

Energy dependence of nuclear matter disassembly in heavy ion collisions

Abstract Measurements of light charged particle spectra from 20 Ne + Au at 100 and 156 MeV/nucleon are compared with results for similar systems at 9, 13, 19, 43, 241, 393, and 800 MeV/nucleon. Spectra at each energy are fitted with a moving source model to extract the temperatures, cross sections and source velocities for protons and light nuclei in the intermediate rapidity region. The 4 He/p production ratio decreases drastically with incident energy, whereas the d/p and t/p ratios are almost constant.

The use of radioactive nuclear beams to study the equilibration of the N Z degree of freedom in intermediate-energy heavy-ion reactions

Abstract The effect of the neutron to proton ratio of the colliding system on the isotope ratio of the emitted fragments (1 ⩽ Z ⩽ 5) was studied for for a variety of constant mass beam-target combinations. To extend the N Z ratio as far as possible, one of the projectiles employed, 40Cl, was a radioactive secondary beam. The isotopic ratios depend on the N Z of the target and beam in a way which is not consistent with N Z > equilibration on the timescale of the emission of intermediate-mass fragments.

The impact parameter and isospin dependence of the disappearance of flow

Abstract The disappearance of collective transverse flow can be related to the nuclear equation of state and in-medium nucleon-nucleon scattering. Experimental and theoretical results from several groups are presented for collisions of Ar+Sc, Ar+Al, Zn+Al, 58 Fe+ 58 Fe, 58 Ni+ 58 Ni, and 58 Mn+ 58 Fe and incident energies ranging from 35 to 155 MeV/nucleon. By comparing experimental results for the balance energy with predictions from transport models, one can show that the description of the nuclear mean field must include momentum dependence. The balance energy can be shown to depend on the in-medium cross sections using transport models. A model independent verification of this dependence has been done experimentally by varying the N/Z of the system while holding all other parameters constant. The balance energy changes with the isospin of the system in qualitative agreement with a transport model incorporating isospin dependent nucleon-nucleon scattering and mean field.

Fluctuations and correlations in STAR

We report measurements for the balance function, pt fluctuations and net charge fluctuations from Au+Au collisions at , 130 and 200 GeV as well as p+p and d+Au collisions at GeV using STAR at RHIC. For Au+Au collisions at 200 GeV, we observe a narrowing of the balance function in central collisions. We observe dynamic pt fluctuations at all incident energies. Observables related to pt fluctuations and net charge fluctuations are similar for peripheral Au+Au collisions and inclusive p+p collisions while central Au+Au collisions deviate significantly from HIJING predictions.We report measurements for the balance function, transverse momentum fluctuations, and net charge fluctuations from Au+Au collisions at 20, 130, and 200 GeV as well as p+p and d+Au collisions at 200 GeV using STAR at RHIC. For Au+Au collisions at 200 GeV, we observe a narrowing of the balance function in central collisions. We observe dynamic transverse momentum fluctuations at all incident energies. Observables related to transverse momentum fluctuations and net charge fluctuations are similar for peripheral Au+Au collisions and inclusive p+p collisions while central Au+Au collisions deviate significantly from HIJING predictions.

Impact parameter dependence of high energy gamma ray production in heavy-ion collisions☆

Abstract High energy photons ( E γ ⩾30 MeV) have been measured in coincidence with light particles observed in a multidetector array for the 40 Ar+ 51 V system at a bombarding energy of E A =65 MeV . Double differential cross sections were obtained as a function of the midrapidity charge representation of the centrality of the collision. The inverse slope parameter increases with centrality more strongly than predicted by a BUU model.

Statistical hadronization phenomenology in K/ fluctuations at ultra-relativistic energies

We discuss the information that can be obtained from an analysis of fluctuations in heavy-ion collisions within the context of the statistical model of particle production. We then examine the recently published experimental data on ratio fluctuations, and use it to obtain constraints on the statistical properties (physically relevant ensemble, degree of chemical equilibration, scaling across energies and system sizes) and freeze-out dynamics (amount of reinteraction between chemical and thermal freeze-out) of the system.

PRODUCTION OF NEUTRONS FROM INTERACTIONS OF GCR-LIKE PARTICLES

In order to help assess the risk to astronauts due to the long-term exposure to the natural radiation environment in space, an understanding of how the primary radiation field is changed when passing through shielding and tissue materials must be obtained. One important aspect of the change in the primary radiation field after passing through shielding materials is the production of secondary particles from the breakup of the primary. Neutrons are an important component of the secondary particle field due to their relatively high biological weighting factors, and due to their relative abundance, especially behind thick shielding scenarios. Because of the complexity of the problem, the estimation of the risk from exposure to the secondary neutron field must be handled using calculational techniques. However, those calculations will need an extensive set of neutron cross section and thicktarget neutron yield data in order to make an accurate assessment of the risk. In this paper we briefly survey the existing neutron-production data sets that are applicable to the space radiation transport problem, and we point out how neutron production from protons is different than neutron production from heavy ions. We also make comparisons of one the heavy-ion data sets with Boltzmann-Uehling-Uhlenbeck (BUU) calculations.

Tracking fission-like processes in central collisions of 40Ar+232Th; E = 15–115 A MeV

Abstract Fission-like fragments and coincident charged particles have been measured in a 4π geometry over a wide energy range (15–115 A MeV) for the reaction 40 Ar+ 232 Th. The exclusive folding angle distribution data provide direct evidence that fission-like processes following incomplete-fusion are still an appreciable exit channel for beam energies as high as 115 A MeV.