Search Results (14 total)

We have experimentally studied small impact parameter heavy-ion collisions in the (nearly) symmetry entrance channels ¹²C+¹²C, ²⁰Ne+²⁷Al, ⁴⁰Ar+⁴⁵Sc, ⁸⁴Kr+⁹³Nb, and ¹²⁹Xe+¹³⁹La, each at many intermediate beam energies. The results from a number of analyses based on a projection of the ‘‘shapes’’ of the experimental events called the sphericity are presented. Comparisons of the relative efficiencies of various experimental methods for the selection of central events are made. The importance of autocorrelations between the sphericity and the various impact-parameter–dependent variables is evaluated. Searches for beam energy-dependent transitions from sequential binary disassembly to multifragmentation in the central events are described. Comparisons to dynamic and hybrid model code calculations will be discussed. The average sphericities of the intermediate mass fragments (IMF’s, for which 3≤Z≲20), are presented. The possibility that the IMF emission occurs following the formation of transient toroidal or disk-like geometries in the central events is explored. Increases in the average sphericities of the central events for increasing beam energies are observed which is attributed to transitions from sequential binary disassembly to multifragmentation. The transitional beam energies for the central ⁴⁰Ar+⁴⁵Sc, ⁸⁴Kr+⁹³Nb, and ¹²⁹Xe+¹³⁹La reactions are near ∼50, ∼40, and ∼40 MeV/nucleon, respectively.

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.

The average multiplicities of intermediate mass fragments (IMFs) for central heavy-ion collisions in the (nearly) symmetric entrance channels ²⁰Ne+²⁷Al, ⁴⁰Ar+⁴⁵Sc, ⁸⁴Kr+⁹³Nb, and ¹²⁹Xe+¹³⁹La, are systematically studied over a wide range of intermediate beam energies. Cuts on experimental variables commonly assumed to be correlated with the impact parameter are used to select the most central collisions. The results for six different centrality variables are compared, and the extent to which measurements of the multiplicities of IMFs in small impact parameter collisions are affected by the variable used to select the central events is discussed. General methods for locating such ‘‘autocorrelations’’ are described. The two centrality observables that are the least autocorrelated with the number of intermediate mass fragments are identified, and these variables are used to select the most central collisions. The entrance channel mass and beam energy dependence of the experimental IMF multiplicities are presented and compared to a variety of model predictions. The models picturing the disassembly as a sequential binary process always underpredict the experimental IMF multiplicities. A generally more accurate reproduction of these multiplicities is provided by several similar chemical equilibrium models commonly assumed to be the theoretical description of multifragmentation.

Charge, velocity, and angular correlations between intermediate mass fragments (IMF) are presented for 50 and 100 MeV/nucleon Fe bombardments of Ta, Au, and Th targets. Correlation functions generated as a function of the relative velocity and the opening angle between two IMF’s are qualitatively independent of the projectile energy and target mass and show a suppression at small relative velocities and opening angles due to the Coulomb repulsion between the fragments. The correlations are consistent with IMF’s emitted primarily from a highly excited target residue following a rapid preequilibrium cascade. The correlation data are compared to model calculations using the event generator meneka and the quantum molecular dynamics (QMD) code with a statistical deexcitation of residual fragments utilizing the multifragmentation code smm. All data are consistent with a simultaneous multifragmentation at a freeze-out density of 0.1–0.3 times normal nuclear matter density or a more sequential emission with time constant τ≤500 fm/c.

The relationship between observed intermediate mass fragment and total charged particle multiplicities has been measured for ⁸⁴Kr + ¹⁹⁷Au collisions at energies between E/A=35 and 400 MeV. Fragment multiplicities are greatest for central or near-central collisions. For these collisions, fragment production increases up to E/A≊100 MeV, and then decreases at higher energies.

We consider the reaction ³⁶Ar+¹⁹⁷Au at incident ³⁶Ar energies of 35, 50, 80, and 110A MeV, comparing calculations of precompound decay using the Boltzmann master equation (BME) and quantum molecular dynamics (QMD) models. We then estimate quasiequilibrated nuclei and excitations using the BME, and use these values as input into statistical multifragmentation models. For the latter we compare sequential binary decay as an extension of the Weisskopf-Ewing evaporation model, and a simultaneous multifragmentation for an expanded low density gas. The exclusive multiplicities predicted by these models are compared with experimental results.

Multifragmentation has been measured for ¹⁹⁷Au+¹⁹⁷Au collisions at E/A=100, 250, and 400 MeV. The mean fragment multiplicity increases monotonically with the charged particle multiplicity at E/A=100 MeV, but decreases for central collisions with incident energy, consistent with the onset of nuclear vaporization. Molecular dynamics calculations follow some trends but underpredict the observed fragment multiplicities. Including the statistical decay of excited residues improves the agreement for peripheral collisions but worsens it for central collisions.

A quasiclassical Pauli potential is used to simulate the Fermi motion of nucleons in a molecular dynamical simulation of heavy ion collisions. The thermostatic properties of a Fermi gas with and without interactions are presented. The inclusion of this Pauli potential into the quantum molecular dynamics (QMD) approach yields a model with well defined fermionic ground states, which is therefore also able to give the excitation energies of the emitted fragments. The deexcitation mechanisms (particle evaporation and multifragmentation) of the new model are investigated. The dynamics of the QMD with Pauli potential is tested by a wide range of comparisons of calculated and experimental double-differential cross sections for inclusive p-induced reactions at incident energies of 80 to 160 MeV. Results at 256 and 800 MeV incident proton energy are presented as predictions for completed experiments which are as yet unpublished.

Experimental results are presented on the charge, velocity, and angular distributions of intermediate mass fragments (IMFs) for the reaction Fe+Au at bombarding energies of 50 and 100 MeV/nucleon. Results are compared to the quantum molecular dynamics (QMD) model and a modified QMD which includes a Pauli potential and follows the subsequent statistical decay of excited reaction products. The more complete model gives a good representation of the data and suggests that the major source of IMFs at large angles is due to multifragmentation of the target residue.

We investigate the onset of multifragmentation employing an improved version of the N-body ‘‘quantum’’ molecular-dynamics approach. We study in detail the reaction ¹⁸O+¹⁹⁷Au at 84 MeV/nucleon and find good agreement between the calculated results and the data for the double-differential proton cross section, the mass yield, the multiplicity, the kinetic energy of the fragments, and even for the kinematic correlations between intermediate mass fragments (IMF’s), which have been measured in this experiment for the first time. We observe a strong correlation between the impact parameter and both the size of the target remnant as well as the average proton multiplicity. Hence both observables can be used to determine the impact parameter experimentally. The IMF’s come from the most central collisions. The calculations confirm the experimental result that they are not emitted from an equilibrated system. Although the inclusive energy spectra look thermal, we cannot identify an impact parameter-independent isotropically emitting source. Even in central collisions global equilibrium is not observed. We find that multifragment emission at this bombarding energy is caused by a process very similar to that proposed in the macroscopic cold multifragmentation model. Thus it has a different origin than at beam energies around 1 GeV/nucleon, although the mass yield has an almost identical slope.

The statistical model is used to illustrate the consequences of a successive binary decay mechanism as the initial nuclear excitation is pushed towards the limits of stability. The partition of the excitation energy between light and heavy fragments is explicitly calculated, as are the consequences of the decay of the primary light fragments to particle-bound residual nuclei which would be observed experimentally. The test nucleus ₄₄¹⁰⁰Ru is considered at initial excitations of 100, 200, 400, and 800 MeV. Exit channels of n, p, and α and 100 clusters of 3≤Z≤20, 6≤A≤48 are considered from all nuclides in the deexcitation cascade. The total primary and final cluster yields are shown versus Z and initial excitation. The primary versus final yields are also shown individually for ¹²C, ²⁶Mg, and ⁴⁸Ca. We show how multifragmentation yields will change with the excitation energy due to a successive binary decay mechanism. Measurements that may be prone to misinterpretation are discussed, as are those that should be representative of initial nucleus excitation.

The quantum molecular dynamic method is used to study multifragmentation and fragment flow and their dependence on in-medium cross sections, momentum dependent interactions, and the nuclear equation of state, for collisions of ¹⁹⁷Au+¹⁹⁷Au and ⁹³Nb+⁹³Nb in the bombarding energy regime from 100 to 800A MeV. Time and impact parameter dependence of the fragment formation and their implications for the conjectured liquid-vapor phase transition are investigated. We find that the inclusive fragment mass distribution is independent of the equation of state and exhibits a power-law behavior Y(A)∼A^-τ with an exponent τ≊-2.3. True multifragmentation events are found in central collisions for energies E_{lab∼30–200} MeV/nucleon. The associated light fragment (d,t,α) to proton ratios increase with the multiplicity of charged particles and decrease with energy, in agreement with recent experiments. The calculated absolute charged particle multiplicities, the multiplicities of intermediate mass (A>4) fragments, and their respective rapidity distributions do compare well with recent 4π data, but are quite insensitive to the equation of state.

On the other hand, these quantities depend sensitively on the nucleon-nucleon scattering cross section, and can be used to determine σ experimentally. The transverse momentum flow of the complex fragments increases with the stiffness of the equation of state. Reduced (in-medium) n-n scattering cross sections reduce the fragment flow. Momentum dependent interactions increase the fragment flow. It is shown that the measured fragment flow at 200A MeV can be reproduced in the model. We find that also the increase of the p_x/A values with the fragment mass is in agreement with experiments. The calculated fragment flow is too small as compared to the plastic ball data, if a soft equation of state with in-medium corrections (momentum dependent interactions plus reduced cross sections) is employed. An alternative, most intriguing resolution of the puzzle about the stiffness of the equation of state could be an increase of the scattering cross sections due to precritical scattering in the vicinity of a phase transition.

We present a detailed microscopic quantum molecular dynamic analysis of fragment formation in the reaction Ne(1.05 GeV/nucleon) + Au. The theoretical predictions of the total mass yield, the multiplicity distribution of clusters, their average momentum, and their angular distribution agree well with the available data. We find a rather localized hot participant zone, which predominantly emits protons and neutrons. The multiplicity of light clusters depends strongly on the impact parameter whereas the heavier fragments A≥40 result from the decay of spectator residues. Their yield can provide a good measure for the impact parameter. The hypothesis of a compound system of A_P and A_T nucleons which is globally heated and equilibrated is not supported by our results. Light and massive fragments occupy different regions in phase space. Semiperipheral reactions do not lead to a stopping of the projectile. We observe a power law behavior of the inclusive mass yield distribution. Its form, however, is caused by averaging over different impact parameters. This rules out inclusive mass yield distributions as candidates for revealing a possible liquid gas phase transition. Light and intermediate mass fragments are formed during the early compressional stage of the reaction. We find that the projectile causes a high density wave to travel through the target. It causes the target to fragment and transfers transverse momentum to the intermediate mass fragments. Lighter fragments receive additional momentum transfer due to n-n collisions.

We demonstrate that momentum-dependent nuclear interactions (MDI) have a large effect on the dynamics and on the observables of high-energy heavy-ion collisions: A soft potential with MDI suppresses pion and kaon yields much more strongly than a local hard potential and results in transverse momenta intermediate between soft and hard local potentials. The collective-flow angles and the deuteron-to-proton ratios are rather insensitive to the MDI. Only simultaneous measurements of these observables can give clues on the nuclear equation of state at densities of interest for supernova collapse and neutron-star stability.