Excitation energy, temperature and pre-equilibrium separation in the multifragmentation of gold nuclei

Joseph Andrew Hauger, Purdue University

Abstract

Results from a high statistics exclusive experiment performed at the Lawrence Berkeley Laboratory Bevalac are presented. Using a gold beam at 1A GeV bombarding a carbon target, several hundred thousand nuclear multifragmentation events were recorded. The events have been analyzed for dynamical information. The excitation energy of the events is computed using an energy balance between the nuclear remnant immediately after the collision and the final non-interacting stage of the reaction. The energy balance for each event involves a sum of the kinetic and removal energies of each fragment. The fragment kinetic energies in the moving reference frame of the nuclear remnant are computed by transforming from the laboratory system to the moving system. The velocity of the decaying remnant is determined using a mass weighted average of the velocity of several light fragments emitted from the excited nuclear remnant. Since the excitation energy calculation refers to the fragmenting system, particles produced in the initial fast stage of the reaction must be separated from the fragments produced by the fragmenting nuclear system. A method to separate the reaction stages based on particle kinematics is presented. Some evidence that the pre-equilibrium clusters are formed through the mechanism of coalescence is also shown. Nuclear remnant temperatures are also computed as a function of event multiplicity. The initial temperature immediately following the collision but prior to any volume expansion is calculated by assuming the degenerate Fermi gas picture of the nucleus. A freeze-out temperature is calculated using a chemical and thermal equilibrium model. These temperatures are both linearly increasing functions of multiplicity and differ by about a factor of two. Some evidence for expansion between the initial stage and the freeze-out stage is presented. The freeze-out temperature is also shown as a function of the excitation energy per nucleon. The temperature increases steadily with excitation energy per nucleon and is consistent with a continuous phase transition.

Degree

Ph.D.

Advisors

Tincknell, Purdue University.

Subject Area

Nuclear physics

COinS