J.P. Bondorf

Statistical Multifragmentation of Nuclei

After an introduction to the main subject: break-up of nuclear matter, some elementary concepts from heavy ion dynamics are introduced. We discuss the collision process within a three step scenario, including fragmentation into many pieces of the interacting nuclei. The partition of the multiparticle system into clusters is discussed. The various components of the energy of a fragment assembly at the break up stage are introduced. They are generalizations of the Weissacker-energies. We consider the thermodynamics of a gaseous mixture of fragments in equilibrium. The thermodynamic probability determining the weight of a given partition is calculated. Some results of calculations are discussed.

Shock waves in colliding nuclei

Abstract We consider the circumstances under which nuclear matter at high densities can be produced in heavy ion collisions. We argue that lab energies of a few hundred MeV per nucleon will be suitable : the matter velocity will exceed the speed of isentropic compression waves, while the nuclear matter has sufficient stopping power to generate a shock front. From the hydrodynamic conservation laws we show that there is a maximum attainable compression ratio ν ∞ , determined by the thermal properties of the high-density matter. In an independent-fermion model, ν ∞ = 4, but it can be much larger if the phase of the system changes, for example by excitation of nucleon isobars, production of π-mesons, or the scalar-field condensation conjectured by Lee and Wick. We discuss the propagation of the shock front and subsequent decompression of the dense, hot matter.

Thermodynamical features of multifragmentation in peripheral Au + Au Collisions at 35 A.MeV

Abstract The distribution of fragments produced in events involving the multifragmentation of excited sources is studied for peripheral Au + Au reactions at 35 A MeV. The Quasi-Projectile has been reconstructed from its de-excitation products. An isotropic emission in its rest frame has been observed, indicating that an equilibrated system has been formed. The excitation energy of the Quasi-Projectile has been determined via calorimetry. A new event by event effective thermometer is proposed based on the energy balance. A peak in the energy fluctuations is observed related to the heat capacity suggesting that the system undergoes a liquid-gas type phase transition at an excitation energy ∼ 5 A MeV and a temperature 4–6 MeV, dependent on the freeze-out hypothesis. By analyzing different regions of the Campiplot, the events associated with the liquid and gas phases as well as the critical region are thermodynamically characterized. The critical exponents, τ, β, γ, extracted from the high moments of the charge distribution are consistent with a liquid-gas type phase transition.

Statistical multifragmentation of nuclei: (III). Decay of the fragments

Abstract The process of nuclear fragmentation is described through a two-step statistical model. The primordial partition of a hot nuclear system is calculated with the help of the Monte Carlo technique developed in parts (I) and (II) of the present work, whereas the subsequent decay of the highly-excited primary fragments is treated by a simplified evaporation approach. This model describes the main features of the inclusive experimental mass spectra of high-energy protoninduced reactions.

Energy thresholds for fragmentation and vaporization of the atomic nucleus

Abstract The statistical thermodynamics of a system of drops is applied to the study of the nuclear disassembly process. The threshold excitation energies for the multifragmentation and liquid-gas phase transition in finite nuclei are estimated using the Monte Carlo method in a thermodynamical model based on a finite-temperature liquid-drop description of nuclear properties.

On the observation of a transition from fusion to multifragmentation in high multiplicity 16O induced reactions

Abstract We have studied high multiplicity 16 O + Br and 16 O + Ag collisions from 10 A to 220 A MeV on an event-by-event basis. The collisions with the highest multiplicity are fusion-like for energies below 35 A MeV whereas at higher energies the onset of multifragmentation (two or more medium mass fragments) is observed with a charge distribution which gets less and less wide with increasing energy. Correlations between high moments of the charge distribution and the maximum charge or the total multiplicity have been studied. These correlations are generally well described by statistical models. Predicted signals of critical behaviour are however not observed.

Projectile fragments from 86 MeV/nucleon 12C induced reactions

Abstract An 86 MeV/nucleon 12 C beam has been used to bombard targets of C, Ag and Au. Energy spectra of reaction products with Z = 3−6 and A = 6−12 have been measured at angles between 3° and 20°. A simple fragmentation model, while predicting approximately the gross features of the data, fails to describe the results in detail.

Production of charged pions in intermediate-energy heavy-ion collisions

Abstract Cross sections for the production of π + and π − have been measured over a wide range of angles for 12 C + 7 Li, 12 C + 12 C and 12 C + 208 Pb collisions at 60 A , 75 A and 85 A MeV. Energy distributions are less steep and absolute yields of pions larger than expected from a straightforward Fermi gas nucleon-nucleon scattering prescription. The apparent velocity of the system in which the π + emission is symmetric, is closer to the nucleus-nucleus c.m. velocity than to the mean speed system. The Coulomb-corrected π − π + ratio is close to unity for 12 C + 12 C. The corresponding ratio for 12 C + 208 Pb is, however, much larger than expected from the neutron excess in 208 Pb only.

An interpretable family of equations of state for dense hadronic matter

Abstract We present a new family of equations of state for hadronic matter which is consistent with nuclear matter properties at normal nuclear densities and at the high densities produced in heavy-ion collision. This description contains interacting nucleons with free masses and reasonable singleparticle distributions, thus allowing reliable hadrochemical calculations.

Chiral symmetry, scale invariance and properties of nuclear matter☆

Abstract We construct an effective meson-nucleon lagrangian obeying the QCD requirements of chiral symmetry, scale invariance and trace anomaly. It takes the form of the scaled sigma model supplemented by the gauged vector field and dilation (glueball) field. The lagrangian is applied for describing the properties of isosymmetric nuclear matter in the mean-field approximation. By reasonable choice of model parameters we are able to reproduce the equilibrium density and binding energy of normal nuclear matter but the compressibility modulus comes out unrealistically large. The chiral-symmetry restoration is predicted by the model at sufficiently high baryon density.