Intermediate Energy Heavy Ion Collisions Research Articles

Short-time scale characteristics of intermediate energy heavy ion collisions

A microscopic model based on the molecular dynamics concept was applied to study the early phase ( t<300 fm/ c) of the 84Kr+ 159Tb reaction at 45 MeV/nucleon. The calculations agree with the experimental observation that the heavy ion reactions in the intermediate energy domain are predominantly of binary nature, even for collisions associated with a number of intermediate mass fragments. Collisions at small impact parameters ( b <3 fm) lead to the formation of a hot composite system.

Energy dependence of intermittency in nuclear reactions at intermediate energies

Using a hadronic transport model we study the beam energy dependence of intermittency in intermediate energy heavy-ion collisions by analysing the reduced scaled factorial moments of pion distributions in both pseudorapidity and kinetic energy. It is found that the anomalous dimension obtained from analysing pion pseudorapidity distributions decreases rapidly as the beam energy increases from about 0.5 to 3.0 GeV/nucleon. On the contrary, the anomalous dimension extracted from analysing pion kinetic energy distributions is almost independent of the beam energy.

Rotational behavior in intermediate energy heavy ion collisions.

The rotational behavior together with in-plane collective flow for the intermediate energy $^{40}\mathrm{Ar}$${+}^{27}$Al reaction is investigated by analyzing the rapidity-dependent azimuthal distributions with the Boltzmann-Uehling-Uhlenbeck (BUU) model. The azimuthal distributions are fitted by a Legendre polynomial expansion up to the second order. By incorporating the uncertainties in the experimental reaction plane determination into our calculations, quantitative agreement between the calculations and data is obtained. It is found that the rotational behavior at midrapidity depends strongly on the impact parameter. Information about the in-plane collective flow is also extracted from the azimuthal distributions. Both the rotational behavior and the in-plane collective flow depend strongly on the in-medium nucleon-nucleon cross section and nuclear equation of state.

Detecting multifragment disintegration of toroidal and disk-shaped nuclear configurations

Non-spherical breakup geometries have been predicted for intermediate-energy heavy-ion collisions. Through studying Coulomb trajectories of the outgoing particles, we show that two-body observables of intermediate mass fragments may contain an experimental signature of a non-spherical breakup configuration.

Universal characteristics of transverse momentum transfer in intermediate energy heavy ion collisions.

A microscopic optical model formalism for estimating momentum transfer in intermediate energy heavy ion collisions predicts universal behavior of the transverse component. In particular, for symmetric systems (Ap = AT) heavier than niobium, it appears that values of P perpendicular/A are independent of the mass and charge of the colliding nuclei and vary only with impact parameter and incident beam energy. This suggests that momentum transfer per nucleon saturates to some limiting value with increasing mass.

Widths of transverse momentum distributions in intermediate-energy heavy-ion collisions.

The need to include dynamical collision momentum transfer contributions, arising from interacting nuclear and Coulomb fields, to estimates of fragment momentum distributions is discussed. Methods based upon an optical potential model are presented. Comparisons with recent experimental data of the Siegen group for variances of transverse momentum distributions for gold nuclei at 980 A MeV fragmenting on silver foil and plastic nuclear track detector targets are made. The agreement between theory and experiment is good.

Kinetic-model description of collective effects in heavy-ion collisions: transverse momentum, stopping, and flow

Symmetric intermediate-energy heavy-ion collisions are studied in the Boltzmann-Nordheim-Vlasov theory, where the collision dynamics is determined by the mean field, the Pauli principle, and the NN scattering cross sections. Differences between the parallel-ensemble and full-ensemble approaches are enumerated. The effect of the model equation of state on the transverse momentum transfer is delineated and shown to be dramatically energy dependent. The mass dependence of the transverse momentum transfer, rapidity distribution, and collective flow angle is discussed.

Fragment emission time scale in intermediate energy heavy ion collisions

Complete two-fragment velocity systematics have been measured over the entire range of fragmentation products for the system $^{56}\mathrm{Fe}$ ${+}^{197}$Au at 50 and 100 MeV per nucleon. Fragment emission lifetimes are estimated from the reduced velocity (${\mathit{V}}_{\mathrm{rel}}$/ \ensuremath{\surd}${\mathit{Z}}_{1}$+${\mathit{Z}}_{2}$ ) correlation functions by a direct comparison with results from a classical three-body trajectory calculation. Furthermore, we examine the validity of the reduced velocity, ${\mathit{V}}_{\mathrm{red}}$, as a scaling parameter for constructing mixed-fragment (${\mathit{Z}}_{1}$\ensuremath{\ne}${\mathit{Z}}_{2}$) correlation functions over the full measured range (5\ensuremath{\le}${\mathit{Z}}_{\mathit{i},}$j\ensuremath{\le}53) of fragment pairs excluding fission.

A microcanonical model of hot nuclei

We describe our Metropolis Monte Carlo computer code for the microcanonical calculation of the fragmentation of a hot nucleus. It is applicable for nuclei with excitation energies on the order of 1–20 MeV per nucleon, such as might be created in intermediate-energy heavy-ion collisions. The code creates complete, individual events in which both energy and mass are exactly conserved, and will write the individual events as well as perform analyses. The events are created by sampling the phase space available to the fragmenting nucleus, visiting each cell in phase space in proportion to its statistical weight.

Explosion and Limiting Excitation Energy for 20Ne+20Ne Central Collision at Intermediate Energy

The incomplete fusion-explosion/multifragmentation transition in intermediate energy heavy ion collision has been investigated within the framework of the Boltzmann-Uehling-Uhlenbeck theory. A detailed analysis for central 20Ne+20Ne collision from 10 MeV/u to 100 MeV/u is performed. Quantitative estimates of the excitation energy limit and the explosion threshold are given for 20Ne+20Ne central collision.

Phenomenology of flow disappearance in intermediate-energy heavy ion collisions

The disappearance of collective flow effects in heavy ion collisions is investigated using a microscopic optical model formalism for estimating collision momentum transfers. Phenomenological expressions for the balance energy are obtained which agree very well with measurements for various experimental collision pairs and with results obtained from Boltzmann-Uehling-Uhlenbeck simulations.

Dynamical instability and nuclear multifragmentation in BUU model for heavy-ion collisions

By simulating numerically the reaction dynamics of intermediate-energy heavy-ion collisions within the Boltzmann-Uehling-Uhlenbeck (BUU) transport model, we study systematically the onset of dynamical instabilities and nuclear multifragmentation in central collisions of 96Mo + 96Mo at beam energies of 55 and 100 MeV/nucleon. It is found that dynamical instabilities characterized by an imaginary adiabatic sound velocity in the central region of the reaction develop during the expansion phase of the reaction. Thermalization can be reached at about the same time when the dynamical instabilities set in. Multifragmentation patterns, geometries of final-fragment distributions as well as their dependence on the nuclear equation of state and the beam energy are also studied by making scatter plots of the one-body density distributions.

AMD Approach to the Reaction Mechanisms of Intermediate Energy Heavy Ion Collisions(Microscopic Cluster Models of Light Nuclei and Related Topics)

AMD Approach to the Reaction Mechanisms of Intermediate Energy Heavy Ion Collisions(Microscopic Cluster Models of Light Nuclei and Related Topics)

Multifragment source lifetime in intermediate energy heavy ion collisions

Multifragment source lifetime in intermediate energy heavy ion collisions

Particle-particle correlations and the space-time structure of heavy ion collisions

The present status of the use of two-particle intensity interferometry as a diagnostic tool to study the space-time dynamics of intermediate energy heavy ion collisions is examined. Calculations for the two-proton and two-pion correlation functions are presented and compared to experiment. The calculations are based on the nuclear Boltzmann-Uehling-Uhlenbeck transport theory.

Multifragmentation of thermalized residual nuclei in intermediate-energy heavy-ion collisions

Nucleus-nucleau reactions leading to the copious production of intermediate mass fragments are considere as a two-stage process. The first nonequilibrium stage of the reactions is simulated by the intranuclear cascade model and (preequilibrium emission) exciton model. At the equilibrium stage of the reaction thermalized residual nuclei (TRN) undergo a multifragment break-up which is described within the framework of the statistical multifragmentation model. The comparison to experimental correlation characteristics of intermediate mass fragments allows us to extract the important information concerning the actual masses and excitation energies of TRN.

Nuclear dynamics and intensity interferometry

The present status of the use of two-particle intensity interferometry as a diagnostic tool to study the dynamics of intermediate energy heavy ion collisions is examined. Calculations for the two-proton correlation function are presented and compared to experiment. These calculations are based on the nuclear Boltzmann-Uehling-Uhlenbeck transport theory. At heavy ion beam energies around 100 MeV per nucleon it is found that there is only a weak sensitivity of the results on the nuclear compressibility, but a strong dependence on the in-medium nucleon-nucleon cross section.

Fragment Mass Distribution in Intermediate Energy Heavy-Ion Collisions and the Reaction time Scale

Quantum Molecular Dynamics method is combined with multi-step statistical decay calculation and they are applied to the investigation of fragment mass distribution and the reaction time scale in intermediate energy heavy-ion collisions. The fragment mass distributions in 14N(35 MeV /u)+ C , and 4°Ar(44 MeV/u)+ Al collisions are well reproduced by our framework. We discuss the time evolution of fragment mass yield and contribution of statistical decay process in these two systems.

Fragment Mass Distribution in Intermediate Energy Heavy-Ion Collisions and the Reaction time Scale

Quantum Molecular Dynamics method is combined with multi-step statistical decay calculation and they are applied to the investigation of fragment mass distribution and the reaction time scale in intermediate energy heavy-ion collisions. The fragment mass distributions in 14N(35 MeV/u) + 12C and 40Ar(44 MeV/u) + 27Al collisions are well reproduced by our framework. We discuss the time evolution of fragment mass yield and contribution of statistical decay process in these two systems.

A case study in intermittency: The Thermodynamic Model for intermediate energy heavy ion collisions

The Thermodynamic Model for heavy ion collisions explains, at least semi-quantitatively, many, though not all, features of the experimental data at Bevalac energies. We use this model as a case study for intermittency. A composite with n nucleons and total momentum P will be registered at the detector as n nucleons each with momentum P/ n. We find that in this model the scaled factorial moments grow indefinitely with M where M is the number of divisions in the p z window. In so far as composites are droplets of various sizes (free nucleons constitute a gas), intermittency is seen to be linked to the existence of droplets. The role of resonances is examined by including excited “deuterons” whose mass determines the width of the momentum distribution of the decay nucleons. For nucleons and resonances only, the scaled factorial moments saturate with M when the bin size becomes smaller than the width. These results do not depend upon the size of the colliding systems thus the model cannot explain why, for example, e + e − shows stronger intermittency than the currently available heavy ion data.