Alfred R. DeAngelis

Statistical model analysis of ALADIN multifragmentation data

Abstract Using a statistical model for nuclear multifragmentation we analyze the ALADIN multifragmentation data from Au induced reactions on various targets at a beam energy of 600 MeV/nucleon. By adjusting excitation energies of fragmenting heavy residues such that the correlation between the average multiplicity of intermediate mass fragments versus Z bound can be well reproduced, we find that all experimental observables which we have studied can be well reproduced simultaneously. The extracted excitation energies of heavy residues provide a strong constraint on the reaction dynamics leading to the formation of the residues.

Intermittency in and the fractal nature of nuclear fragmentation: A critical examination

Abstract We discuss the use of factorial moments to search for intermittency in nuclear fragmentation, and relate it to the fractal nature of the fragmenting system. We show thatthe horizontally scaled moments measure the deviation of the fluctuations from a poissonian distribution. The intermittency index is directly related to the anomalous or fractal dimension of the system, and its dependence on the order of the moments is different for mono- and multifractals. We calculate the moments and intermittency indices for the charge distribution from the statistical decay of an equilibrated hot nucleus, which we model by sampling the microcanonical phase space for nuclear fragmentation. The model is used to realistically study the conditions leading to the phenomenon of intermittency in heavy-ion reactions. The results are compared to those measured in 1 GeV/amu 197 Au 79 on emulsion data. They indicate possible biasing of the experimental data towards events of low multiplicity. We are able to reproduce both the resolution and order scaling as given by the data while generating many more events. We show that order scaling can be related to the form of the fluctuations as well as signal intermittency.

IMF-IMF correlations

Abstract The correlation function of two intermediate-mass fragments (IMF) is derived from first principles. Care is taken of the final-state Coulomb interaction of the observed IMF with the rest source and of the initial correlations in the source at the moment of emission. These correlations are of various origins. Due to basic conservation laws of mass, charge, momentum and energy the emission probabilities of IMF are strongly correlated. Secondly, near to the onset of multifragmentation dynamical correlations in the emitting source are of crucial importance. These IMF-IMF correlation functions carry the main information on the transition from evaporation or very hot asymetric fission to multifragmentation. All these features are ignored in the usual analysis starting with the Koonin-Pratt formula. The information on the decaying source contained in the IMF-IMF correlation function is analyzed within the Berlin multifragmentation model. The rise seen at low excitation energies signalizes very asymmetric hot fission. It disappears within the region of multifragmentation when the largest fragment is also an IMF.

Multifragmentation of finite many-body systems under long-range forces and an exactly solvable model of fragmentation with strict mass-conversation and quantum-symmetry

Abstract During the last 2 years multifragmentation of hot nuclei has become a well established phenomenon. Model calculations suggest that this is likely connected to the the occurence of new, well defined, phase-transitions driven by the repulsive long-range Coulomb force. The origin of the transitions and the possible significance for other systems in physics is emphasised. In these transitions the critical fluctuations give further insight into the relevant physical mechanisms. For a detailed discussion of these fluctuation an exactly solvable model offragmentation is presented. The effect of mass-conservation and quantum-symmetry can rigorously be considered. As a function of a tuning parameter self similar mass-distributions with realistic power-laws and/or fluctuations which show intermittency can be generated. Thus the relation of both critical phenomena can be studied analytically. It is found that they are unliked. There are cases of the model where the mass distribution has a power-law with realistic exponents between τ = −2 and τ = −3 but no intermittency, or vice versa.

A microcanonical model of hot nuclei

Abstract 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.

Charge-conservation, quantum symmetry, and Metropolis sampling in an exactly solvable model of nuclear fragmentation

Abstract An exactly solvable model of fragmentation is presented. The method is very simplified and is not intended to describe the process of nuclear fragmentation in detail. Nevertheless, it has in a crude way many of the features required by the nuclear problem (except for the Coulomb interaction, which is completely ignored) and can be used to test the various methods of microcanonical Monte Carlo sampling of nuclear fragmentation. It is found that the Monte Carlo methods currently available lead to fragment charge distributions that agree with the exact results if care is taken to sample the phase space in an efficient way. Otherwise the sampling may fail to locate the important regions of phase space and an erroneous distribution is produced. The Campi plots for the events generated by the model are investigated. The factorial moments of the events are also studied in order to gain insight into the role mass conservation has on intermittency and critical fluctuations in nuclear fragmentation.

Saturation and nucleation in hot nuclear systems

We investigate nuclear fragmentation in a supersaturated system using classical nucleation theory. This allows us to go outside the normally applied constraint of chemical equilibrium. The system is governed by a virial equation of state, which we use to find an expression for the density as a function of pressure and temperature. The evolution of the system is discussed in terms of the phase diagram. Corrections are included to account for the droplet surface and all charges contained in the system. Using this model we investigate and discuss the effects of temperature and saturation, and compare the results to those of other models of fragmentation. We also discuss the limiting temperatures of the system for the cases with and without chemical equilibrium. We find that large nuclei will be formed in saturated systems, even above the limiting temperature as previously defined. We also find that saturation and temperature dominate surface and Coulomb effects. The effects are quite large, thus even a qualitative inspection of the yields may give an indication of the conditions during fragmentation.