Search Results (25 total)

The fragment yields from the multifragmentation of gold, lanthanum, and krypton nuclei obtained by the EOS Collaboration are examined in terms of Fisher’s droplet formalism modified to account for Coulomb energy. The critical exponents σ and τ and the surface energy coefficient c₀ are obtained. Estimates are made of the pressure-temperature and temperature-density coexistence curve of finite neutral nuclear matter as well as the location of the critical point.

A systematic analysis of multifragmentation (MF) in fully reconstructed events from 1A GeV Au, La, and Kr collisions with C has been performed. These data are used to provide a definitive test of the variable volume version of the statistical multifragmentation model (SMM). A single set of SMM parameters directly determined by the data and the semi-empirical mass formula are used after the adjustable inverse level density parameter ε₀ is determined by the fragment distributions. The results from SMM for second stage multiplicity, size of the biggest fragment, and the intermediate mass fragments are in excellent agreement with the data. Multifragmentation thresholds have been obtained for all three systems using SMM prior to secondary decay. The data indicate that both thermal excitation energy E_th^* and the isotope ratio temperature T_He-DT decrease with increase in system size at the critical point. The breakup temperature obtained from SMM also shows the same trend as seen in the data. The SMM model is used to study the nature of the MF phase transition. The caloric curve for Kr exhibits back-bending (finite latent heat) while the caloric curves for Au and La are consistent with a continuous phase transition (nearly zero latent heat) and the values of the critical exponents τ, β, and γ, both from data and SMM, are close to those for a “liquid-gas” system for Au and La. We conclude that the larger Coulomb expansion energy in Au and La reduces the latent heat required for MF and changes the nature of the phase transition. Thus the Coulomb energy plays a major role in nuclear MF.

Multifragmentation MF results from 1A GeV Au on C have been compared with the Copenhagen statistical multifragmentation model (SMM). The complete charge, mass, and momentum reconstruction of the Au projectile was used to identify high momentum ejectiles leaving an excited remnant of mass A, charge Z, and excitation energy E^* which subsequently multifragments. Measurement of the magnitude and multiplicity (energy) dependence of the initial free volume and the breakup volume determines the variable volume parametrization of SMM. Very good agreement is obtained using SMM with the standard values of the SMM parameters. A large number of observables, including the fragment charge yield distributions, fragment multiplicity distributions, caloric curve, critical exponents, and the critical scaling function are explored in this comparison. The two stage structure of SMM is used to determine the effect of cooling of the primary hot fragments. Average fragment yields with Z>~3 are essentially unaffected when the excitation energy is ≤7 MeV/nucleon. SMM studies suggest that the experimental critical exponents are largely unaffected by cooling and event mixing. The nature of the phase transition in SMM is studied as a function of the remnant mass and charge using the microcanonical equation of state. For light remnants A<~100, backbending is observed indicating negative specific heat, while for A>~170 the effective latent heat approaches zero. Thus for heavier systems this transition can be identified as a continuous thermal phase transition where a large nucleus breaks up into a number of smaller nuclei with only a minimal release of constituent nucleons. Z<~2 particles are primarily emitted in the initial collision and after MF in the fragment deexcitation process.

A systematic analysis of the moments of the fragment size distribution has been carried out for the multifragmentation of 1A GeV Au, La, and Kr on carbon. The breakup of Au and La is consistent with a continuous thermal phase transition. The data indicate that the excitation energy per nucleon and isotopic temperature at the critical point decrease with increasing system size. This trend is attributed primarily to the increasing Coulomb energy with finite size effects playing a smaller role.

The transverse momenta (p_x,p_y) of projectile fragments produced by 1.0A GeV ¹⁹⁷Au nuclei incident on Au and C targets have been measured. The medium and heavy fragments have p_x and p_y distributions, which are wider than predicted by models. For the Au target the widths of the distributions are significantly larger than those for C, particularly for the heavy fragments. The C distributions show a different gross structure, which may be due to the target-projectile size difference.

Elliptic flow from nuclear collisions is a hadronic observable sensitive to the early stages of system evolution. We report first results on elliptic flow of charged particles at midrapidity in Au+Au collisions at sqrt[s_NN]  =  130  GeV using the STAR Time Projection Chamber at the Relativistic Heavy Ion Collider. The elliptic flow signal, v₂, averaged over transverse momentum, reaches values of about 6% for relatively peripheral collisions and decreases for the more central collisions. This can be interpreted as the observation of a higher degree of thermalization than at lower collision energies. Pseudorapidity and transverse momentum dependence of elliptic flow are also presented.

The cluster distributions of three different systems are examined to search for signatures of a continuous phase transition. In a system known to possess such a phase transition, both sensitive and insensitive signatures are present; while in systems known not to possess such a phase transition, only insensitive signatures are present. It is shown that nuclear multifragmentation results in cluster distributions belonging to the former category, suggesting that the fragments are the result of a continuous phase transition.

It is shown that the Fisher droplet model, percolation, and nuclear multifragmentation share the common features of reducibility (stochasticity in multiplicity distributions) and thermal scaling (one-fragment production probabilities are Boltzmann factors). Barriers obtained, for cluster production on percolation lattices, from the Boltzmann factors show a power-law dependence on cluster size with an exponent of 0.42±0.02. The EOS Collaboration Au multifragmentation data yield barriers with a power-law exponent of 0.68±0.03. Values of the surface energy coefficient of a low density nuclear system are also extracted.

Multifragmentation in fully reconstructed events from 1A GeV Kr and La collisions with C has been studied. Results are compared with similar data for 1A GeV Au+C. The emitted charged particles and fragments are identified with emission from either a prompt first stage or a second stage in which the remnant resulting from the first stage breaks up. The nuclear charge, mass, and excitation energy distributions of the remnant are determined. The total charged multiplicity, as well as those of the first and second stages are obtained. Freeze-out temperatures and thermal excitation energy permit the determination of the caloric curve. The fragment charge distribution as well as the IMF multiplicity distribution and those of individual fragments are obtained. The various results are examined as to the extent of universal behavior when scaled for varying system size. Comparisons are made with intranuclear cascade and statistical multifragmentation model calculations.

The inclusive light fragment (Z<~7) yield data in Au+Au reactions, measured by the EOS Collaboration at the LBNL Bevalac, are presented as a function of multiplicity. Moving from central to peripheral collisions the measured charge distributions develop progressively according to a power law which can be fitted, within errors, by a single τ exponent independently of the bombarding energy except for the data at 250A MeV. In addition, the location of the maximum in the individual yields of different charged fragments, for a given beam energy, shifts towards lower multiplicity as the fragment charge increases from Z=3 to Z=7. This trend is common to all six measured beam energies. Moments of charge distribution are also reported. The universal features observed in the present Au + Au data are consistent with previous experimental findings in the Au + C multifragmentation reaction at 1A GeV.

The properties of the remnant resulting from the emission of prompt particles in the interaction of 1A GeV ¹⁹⁷Au+C interactions have been compared with intranuclear cascade and Boltzmann-Uehling-Uhlenback transport calculations. The number of first-stage particles and the energy spectra of first-stage protons are also compared. Both models can fit the general but not the detailed features of the data.

Transverse kinetic energies of individual fragments have been measured over a broad range of emitter excitation energies for the reaction 1A GeV Au+C. For excitation energies leading to large intermediate mass fragment multiplicities, these transverse energies require large collective radial expansion of the emitting systems. However, the traditional decomposition of the transverse energy into a thermal component and a Coulomb and collective component proportional to the fragment mass cannot account for this expansion. Expansion velocities show an increase with decreasing fragment Z and thus indicate fractionation of the collective energy for the expanding system. This collective energy increases with emitter excitation up to about 50% of the energy deposited for a nuclear system with total energy ∼12A MeV. The bulk of the collective energy is carried away by ejectiles of Z<~3.

A high-statistics exclusive study of the multifragmentation of 1A GeV gold on carbon has been performed. Particles with Z<~2 show evidence of emission in a first prompt stage as well as in a second equilibrium stage whereas fragments with Z>~3 appear to be emitted essentially only in the second stage. Two methods for the separation of the Z<~2 particles into the two stages are given and they are in agreement. The yields for each stage are determined as a function of the event charged particle multiplicity m. The mass, nuclear charge, excitation energy per nucleon, and temperature of the remnant left after the first stage and their fluctuations have been determined as a function of m. The expansion of the remnant to fragment freeze-out is examined. The freeze-out temperature is determined from double isotope ratios as a function of m and isentropic trajectories are obtained in the temperature-density plane. The caloric curve shows a monotonic increase with excitation energy. Some of the energy is in the form of radial flow. Overall, the results are consistent with a previous statistical analysis of the data which suggests that, over a certain range of excitation energies, multifragmentation involves a continuous phase transition.

Invariant mass analyses of (p,π^±) pairs in ⁵⁸Ni+Cu collisions at 1.97A GeV have been performed and show correlations resulting from the decays of the Δ resonance, the Λ baryon, and possibly the N^*(1440) resonance. A reduction in the Δ mass is observed and the mass reduction increases with collision centrality. Events generated by the relativistic cascade model (ARC) also reveal a mass reduction. The mass reduction is related to the size of the reaction volume and the details of Δ production mechanisms in heavy ion collisions.

Transverse flow has been studied as a function of impact parameter for pions and protons from the reaction 1.15A GeV ¹⁹⁷Au+¹⁹⁷Au. We observe an “antiflow” behavior for both π⁺ and π^- in peripheral collisions.

We study the energy dependence of collective (hydrodynamic-like) nuclear matter flow in (400–1970)A MeV Ni+Au and (1000–1970)A MeV Ni+Cu reactions. The flow increases with energy, appears to reach a maximum, and then to decrease at higher energies. A way of comparing the energy dependence of flow values for different projectile-target mass combinations is introduced, which demonstrates a more-or-less common scaling behavior among flow values from different systems.

An exclusive study of the interaction of 1A GeV Au with C shows a separability into two stages: a prompt stage involving emission of most Z  =  1 and some Z  =  2 particles and a second stage involving the decay of an equilibrated remnant, which typically undergoes multifragmentation. The mean mass, charge, excitation energy, and the initial temperature T_i of the remnant have been determined as a function of the total charge multiplicity, m, as has the freeze-out temperature T_f. Both T_i and T_f increase linearly with m and their values at the critical point have been determined. T_f rises monotonically with excitation energy as expected for a continuous phase transition.

Using charged-particle-exclusive measurements of Au+Au collisions in the Bevalac's EOS time projection chamber, we demonstrate the advantages of an alternative representation of the squeeze-out phenomenon where the speed of collective expansion is slowest in the plane of the reaction, and is modulated sinusoidally according to fragment azimuth relative to this plane. This simpler representation facilitates a highly comprehensive description of light fragment spectra and the three main categories of collective motion: sideward flow, squeeze-out, and radial expansion.

Measurements of the forward and the transverse energy in 158 GeV per nucleon ²⁰⁸Pb + Pb collisions are presented. A total transverse energy of about 1 TeV is created in central collisions. An energy density of about 3 GeV/fm³ is estimated for near head-on collisions. Only statistical fluctuations are seen in the ratio of electromagnetic to hadronic transverse energy.

A systematic study of energy spectra for light particles emitted at midrapidity from Au+Au collisions at E  =   (0.25–1.15) 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.

Exclusive measurements have been made of Au +Au reactions with beam energies ranging from 0.25 A to 1.15 A GeV. We present measurements of directed collective flow averaged over all light fragments with masses up to alphas, as well as separate measurements for protons, deuterons, tritons, ³He, ⁴He, and Li. The results show a strong increase of the directed flow with fragment mass at all energies measured. Experimental results are compared with a quantum molecular dynamics model. We find that neither the “soft” nor the “hard” equation of state can describe the data over the entire range of beam energies.

Using charged-particle-exclusive measurements of Au+Au collisions in the LBL Bevalac's EOS time projection chamber, we investigate momentum-space densities of fragments up to ⁴He as a function of fragment transverse momentum, azimuth relative to the reaction plane, rapidity, multiplicity, and beam energy. Most features of these densities above a transverse momentum threshold are consistent with momentum-space coalescence, and, in particular, the increase in sideward flow with fragment mass is generally well described by a momentum-space power law.

Using reverse kinematics, we have studied the breakup of 1.0A GeV gold nuclei incident on a carbon target. The detector system permitted exclusive event reconstruction of nearly all charged reaction products. The moments of the resulting charged fragment distribution provide strong evidence that nuclear matter possesses a critical point observable in finite nuclei. We have determined values for the critical exponents γ, β, and τ. These values are close to those for liquid-gas systems and clearly different than those for 3D percolation and the liquid-gas mean field limit.

The fragment momentum distributions in the projectile rest frame are, typically, Gaussian shaped, narrow, consistent with isotropy, depend on fragment and projectile, and have no significant correlation with target mass or beam energy. The nuclear temperature is inferred from the momentum distributions of the fragments and is approximately equal to the projectile nuclear binding energy, indicative of small energy transfer between target and fragment.

We report the first results of a Bevatron heavy-ion experiment on the inclusive spectra of isotopically identified nuclei, 3<~Z<~7, produced by the fragmentation of 29-GeV ¹⁴N ions in carbon and hydrogen. The preliminary values of the partial differential cross sections at 0° give evidence that the modes of fragmentation of ¹⁴N projectiles are independent of the target nucleus.