Search

The mimimum requirement for a meaningful comparison of the 1 GeV Au on C data with the SMM1 predications is that the mass AR the charge Z R , and the excitation energy E x of the multifragmenting system be determined on an event by event basis. This can only be done if a full reconstruction of the collision is possible for each event. The EOS data2–4 provides a complete charge and momentum reconstruction for every event. Average fragment masses are determined. The basic remnant information A R (ZR, EX) is also used as input information for the SMM calculation. Thus a fragmentation event produced by SMM consists of M2s fragments with masses A F and charge Z F. A data fragmentation event consists of M2D fragments with mass A F and charge Z F . Since the remnant mass A R = A AU - M1 where M1 is the collisional particle multiplicity, it is convenient to introduce the total charged particle multiplicity M T for both the SMM predictions and the data. M T = M 2s + M1 for the SMM events and M T = M 2D + M1 for the Au on C events.

We supplement a recent study of the dynamics of the multifragmentation of the remnant of 1A GeV Au + C collisions by (1) providing a more complete characterization of the remnant and (2) performing a more detailed analysis of collective radial flow and exploring its consequences for the caloric curve.

Multifragmentation (MF) is the break-up of colliding nuclei into many species of lighter nuclei, particularly intermediate mass fragments (IMF’s) with 3 ≤ Z IMF ≤ 30 [1]. MF occurs in many different kinds of nuclear reactions when the excitation energy per nucleon is comparable to the nucleon binding energy. At lower excitation energy, a compound nucleus is formed, which decays by evaporation of a few light particles (primarily neutrons, protons, and alphas), leaving a large residual nucleus that contains most of the original mass. At higher energy, the excited system decomposes entirely into light particles. As excitation energy increases, the final mass yields start at low excitation with a double structure peaked at the lowest and highest masses, progress through a power law mass distribution of IMF’s at intermediate excitation, and end at high excitation with an exponential distribution of only light particles. This evolution strongly resembles the progression of a heated fluid from the liquid state through the critical point into the gaseous phase. Since pioneering studies in the early 1980’s [2], intense experimental and theoretical effort has focused on this behavior, attempting to understand the mechanism of MF. Although it is not universally accepted, the idea of a nuclear liquid-gas phase transition has become the leading paradigm used to interpret MF phenomena. Many important questions remain unresolved, including: does MF exhibited by different nuclear reactions have a common underlying physical mechanism? do the excited systems equilibrate sufficiently to apply thermal concepts? is the phase transition first order or continuous, and does this vary for different reactions? what are the thermodynamic properties (e.g. temperature, density, entropy) of these systems? what are the trajectories in the temperature-density plane for various reactions? can the physical properties of nuclear matter, including the equation of state, be extracted from MF data?

The breakup of 1A GeV gold nuclei incident on carbon has been studied with a detector system that permitted exclusive event reconstruction of nearly all charged reaction products. The moments of the resulting charged fragment distribution and fluctuations therein provide strong evidence that nuclear matter possesses a critical point observable in finite nuclei. Values of the critical exponents γ, β, and τ have been determined. These values are close to those for liquid-gas systems but differ significantly from those for other 3-dimensional systems. Assumptions in our analysis have been examined and confirmed by dynamical information.

In two recent publications [1, 2], the EOS Collaboration has presented the first model independent determination of four critical exponents for nuclear matter based on the analysis of exclusive nuclear multifragmentation data obtained in 1 GeV/nucleon collisions of gold on a carbon target at the Lawrence Berkeley Bevalac. These studies were motivated in part by the striking resemblance nuclear multifragmentation data has with many aspects of critical phenomena [3, 4, 5]. In reference [2], the critical exponent σ was determined assuming that the multifragmentation data would exhibit the type of scaling expected for systems possessing critical behavior. In this paper, we demonstrate that this is indeed the case, and we determine for the first time the scaling function of nuclear matter.

Experiments at the Bevalac using the Plastic Ball and streamer chamber initiated the study of nucleus-nucleus collisions with close to 4π acceptance for charged particles. [l] Much was learned as a result of these experiments, and by the mid 1980s, there was a widespread belief that it would soon be possible to constrain unambiguously the equation of state of the compressed hadron gas that exists for a few fm/c during the early stage of the collision. During the years since then, we have come to a better understanding of the difficulties involved in inferring properties of the nuclear equation of state, and while these difficulties are surmountable, there are some indications that experimental observables are less strongly influenced by the state variables of interest than was initially assumed. [2,3] Such considerations motivate new measurements with good statistics and the smallest possible observational biases.

The study of giant resonances in nuclei via inelastic electron and hadron scattering is currently of great interest.1,2 These resonances are outstanding manifestations of nuclear vibrational motion in which an appreciable number of nucleons participate. Charge exchange reactions such as (p,n) and (n,p) offer powerful options for studying isovector excitations without interference from isoscalars. In general, if the target has isospin T then an isovector transition leaves the residual nucleus with isospin T-1, T, or T+1. (Refer to fig. 1.) The T-1 excitation requires charge exchange, as in (p,n), so that the 3-component T3=T (the charge-component) of isospin is also changed to T-1. Only for N=Z (T=0) nuclei are all isovector transitions to states of isospin T+l. In such reactions as (p,p’) and (e,e’) the isoscalar mode can also be excited and, indeed, these tend to be the strongest transitions at medium energies (≲ 100 MeV).