Research Overview

The decay properties of hot nuclear matter prior to reaching equilibrium are of fundamental importance in understanding the nuclear equation-of-state (EOS), as well as examining the interplay between statistics and dynamics in a finite, strongly-interacting, two-component quantal system. Our goal in this work is a better understanding of complex nuclei and the in-medium many-body correlations within these systems. In particular, our focus is the response of the many-body system and the correlations within it to changes in temperature, density, shape, and isospin (neutron-proton asymmetry). In addition to the impact on our fundamental knowledge of nuclear matter in finite systems, understanding the response of nuclear matter to extremes of temperature, density, shape, and isospin  is also important to the description of astrophysical phenomena such as the exotic nuclei in the crust of neutron stars [Mo97] or the aggregation of low density nuclear matter in supernovae to form neutron stars and black holes [La00].    

The spinodal decomposition of nuclear matter in which the bulk matter disassembles due to a volume instability has been predicted for some years [Bo85,Gr90,Fr90]. Observation and characterization of this instability would provide significant information for the nuclear EOS (the size of the of the instability region) and the timescale involved in the growth of fluctuations within this region. Recent claims of observing this instability [Bor00] are intriguing but remain to be well established.

Alternatively, the role of deformation (well established in the fission process) for short-lived systems at high excitation remains poorly understood.  While progress in our understanding of the influence of deformation on cluster emission, through the thermal distribution of shapes, has recently been made [Cha00], at high excitation energy the shape degree of freedom is not expected to be equilibrated prior to cluster/fragment emission.  For example, in the case of mid-central collisions between symmetric nuclei, exit channel dynamics may play a significant role in determining the deformation of the system that emits the fragments.  Decay characteristics of the two reaction partners on each other through their mutual Coulomb interaction has been theoretically calculated [Bo99] but has not yet been rigorously confronted with experimental data. 

Both of these paradigms focus on the decay of a well prepared system in a defined initial state.  In reality, the nuclear collision processes used to prepare these non-equilibrium systems itself provides a rich testing ground for microscopic theories (e.g. quantum molecular dynamics) of the interaction phase between the colliding nuclei [Li98]. 

One of the relatively unexplored degrees-of-freedom in the nuclear EOS is isospin (neutron-to-proton asymmetry).  Isospin fractionation of nuclear matter has been predicted on theoretical grounds [Mu95] to undergo a fractionation into two phases.  Such a neutron-proton separation instability depends critically on the structure of the symmetry term in the nuclear equation-of-state [Ku94,Ba97].  Observation and characterization of fractionation into two phases with different isospin content would represent a major step forward in our understanding of the nuclear phase diagram as well as aid our understanding of supernova explosions and neutron star formation [Bo94,Bo97].  Preliminary evidence by the LASSA collaboration on the isotopic composition of fragments produced at mid-rapidity in heavy-ion collisions [Xu00] may possibly be a signature of such a fractionation process.  In general, increasing isospin asymmetry may amplify the fragility of isoscalar volume modes resulting in enhanced fragility of unstable nuclei [Co98]. 

 Over the past few years we have learned that:

  1. The emission of fragments is a rapid process [Cor95].
  2. The emission process is evolutionary [Cor96] and can be related to the excitation of the system at the time of emission [Be00].
  3. Near E*/A » 5 MeV one observes a transition in the decay mechanism from one that is dominated by evaporation and fission at lower excitation [Go96] to multifragmentation above this value [Be00, Du98,Po96].
  4. The "caloric curve" behavior of these hot systems has parallels to that of a liquid-gas phase transition [Po96].
Despite this progress, many fundamental questions remain.
  1. In heavy-ion collisions, what is the influence of deformation (shape degrees of freedom) on fragment formation? How quickly can shape degrees of freedom assert themselves in an excited, short-lived system?
  2. What role do dynamical/surface instabilities play in contrast to spinodal/volume instabilities, particularly for peripheral and mid-central symmetric heavy-ion collisions?
  3. Does isospin play a role in the multifragmentation process? Does it modify the size and location of the region subject to the instability?
  4. What is the process by which non-equilibrium cluster emission occurs? For p+A collisions, which are not subject to the "contamination" of projectile breakup processes, a significant forward peaked yield of complex fragments is observed. What is the origin of these clusters? Driven by these questions, we intend to study the following research questions.

    A. Importance of shape/surface instabilities on fragment formation

    B. Influence of isospin on thermally driven multifragmentation

    C. Cluster Emission from Hot, Dilute Nuclear Matter

    D. Design and construction of a High Resolution Array (HiRA)

    E. The ISiS Program

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