Search

Our current understanding of the structure of nuclei with up to A=8, including energy spectra, electromagnetic form factors, and capture reactions, is critically reviewed within the context of a realistic approach to nuclear dynamics based on two- and three-nucleon interactions and associated electromagnetic currents., 10 pages, 5 figures, proceedings of the XVIIth European Conference on Few-Body Problems in Physics, Evora, Portugal

The relativistic mean-field theory provides a framework in which the nuclear many-body problem is described as a self-consistent system of nucleons and mesons. In the mean-field approximation, the self-consistent time evolution of the nuclear system describes the dynamics of collective motion: nuclear compressibility from monopole resonances, regular and chaotic dynamics of isoscalar and isovector collective vibrations., LaTeX, 10 pages, 5 figures, Invited Talk, Topical Conference on Giant resonances, Varenna, May 1998, to be published in Nucl. Phys. A

The time scale of fusion-fission reactions was found to be in the range from τ f = 5 · 10 −20 to 5 · 10 −19 s. This result was obtained from the analysis of the prescission neutron multiplicities with a new method combining the time-dependent statistical cascade calculations with the nuclear dynamics model of Feldmeier.

The density depedence of the K− optical potential Vopt(r) at zero kinetic energy is studied by fitting it to a comprehensive set of atomic data across the periodic table. Two families of solutions offering improved fits over that for a teffϱ(r) parameterization are identified, one containing solutions for Vopt with r.m.s. radii larger than those of the matter density ϱ, as expected from finite-range folding corrections. The other gives rise to Re Vopt of a size smaller than that of ϱ. The latter family offers substantially improved fits to the data, with particularly strong attraction inside the nucleus (200 ± 20 MeV). For a subfamily of solutions, Vopt becomes repulsive at large distances, and solutions satisfying the low-density limit V opt → t K N ϱ can be found. Comments are made on the relevance of Λ(1405) nuclear dynamics to these findings.

Many macroscopic nuclear properties, both static and dynamical, can be well described on the basis on semi-classical models, in which nucleons moving in a self-consistent effective mean field experience occasional two-body scatterings. In general, and especially when instabilities are encountered, it is necessary to incorporate the stochastic character of the elementary scattering processes, thus gaining an ability to treat catastrophic developments. Some recent advances in this direction are reviewed: The Fokker-Planck treatment of the Boltzmann-Langevin theory, the equilibration dynamics and the triggering of catastrophies in nuclear matter, and the consequences for nuclear multifragmentation dynamics.

The question “Is the idea of quantum chaos productive for nuclear physics?” is answered positively. Generic features of complicated dynamics are expressed in terms of“ many-body chaos”. As applications, dynamic enhancement of weak interaction, damping of single and multiple giant resonances, anharmonicity of soft nuclei and continuum effects are discussed.

Shapes consisting of a series of pear-like bodies surrounding an inner joined sphere are assumed to be those associated with symmetric fission or fragmentation into two or more fragments. The shapes evolve smoothly from that of an initial spherical nucleus to the scission point. The shapes are calculated using a generalization of the cassinian ovaloid shape parameterization, which has been demonstrated to be a good approximation for binary fission. Using this parameterization, fission barriers have been calculated for 2-, 3-, 4-, 6- and 8-fold multi-fragment final states. Proximity effects are included. The temperature dependence of these fission barriers is also calculated. The importance of nuclear dynamics in determining the range in which static barriers are sampled is discussed.

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.

Effective theories of nuclear dynamics must be able to treat trajectory branching at the macroscopic level, a salient feature of nuclear collisions at intermediate energies. Such generalization is possible for Boltzmann-type one-body models. Idealized applications can provide economic model characterizations and thus facilitate model comparisons.

We compare several methods for incorporating the effect of the fluctuating collision integral in Boltzmann-Uehling-Uhlenbeck simulations of nuclear dynamics: the method of correlated pseudo-particle collisions proposed by Bayer, Bertsch and Das Gupta, the method of projection onto collective variables (specifically the local quadrupole moment of the momentum distribution) proposed by Ayik and Gregoire, and the method of direct simulation on a lattice in phase space recently proposed by Burgio, Chomaz and Randrup. Considering a periodic two-dimensional nucleon gas in a constant one-body field, we examine the evolution of the fluctuations of the phase-space occupancy and their correlations.

The transition amplitudes for various direct nuclear reactions are formulated as finite-order expansions based on the coupled-channels equations. The validity of the present formulation is successfully tested for the Ca(16O, 15N)Sc transfer reactions at 56 MeV, which are typical angular-momentum-mismatched transitions. The nuclear dynamics of how the matching condition can be recovered through the coupled-channels calculation is clarified using this expansion method.

Three nucleon scattering is reviewed with emphasis on recent rigorous Faddeev calculations using meson-theoretical two-nucleon interactions and their comparison with measurements. Recent progress in the Coulomb problem and configuration-space Faddeev equations are discussed.

This report attempts to survey our present knowledge of the nuclear muon capture reactions. Starting from the formation of the muonic atom, various phenomena, having a bearing on the nuclear capture, are reviewed. The nuclear reactions are then studied from two angles — to learn about the basic muon-nucleon weak interaction process, and to obtain new insights on the nuclear dynamics. Future experimental prospects with the newer generation muon “factories” are critically examined. Possible modification of the muon-nucleon weak interaction in complex nuclei remains the most important open problem in this field.

A review on dynamical models and their validity limits in the energy range 100MeV

In previous publications, we have presented a nonrelativistic model for nuclear dynamics in which the one-body density matrix is evolved in time in a mean-field theory and where two-body collisions are calculated by a relaxation-time method. To further investigate this model, the rate of two-nucleon collisions in nuclei is calculated for collisions between nuclei at laboratory energies from 30–84 MeV/ A . Nucleon-nucleon collisions are found to be enhanced in regions where nucleons collide with the nuclear wall and hot spots are formed. There appears to be a focusing by the mean field. Nuclear densities, mass currents, temperatures and Wigner functions are shown, in particular, as they relate to nucleon-nucleon collisions. Other topics discussed and illustrated by calculations are promptly emitted particles (Fermi jets) and linear momentum transfer.

We introduce for the first time the effective finite-range interaction of Gogny in the semi-classical description of heavy-ion reactions based on the Landau-Vlasov equation. The characteristics of the flow for heavy-ion collisions are studied as functions on the incident energy, the impact parameter and the mass number. The Gogny force reproduces the energy dependence of the real optical potential up to 200 MeV; we show that this energy dependence increases the flow and reproduces the values obtained with stiff local Skyrme forces. These results demonstrate the need of a proper account of the momentum dependence of the nuclear force, to extract informations on the nuclear equation of state, from high-energy heavy-ion collisions.

Several aspects of nuclear collective motion are explored. A unified shell-model picture of nuclear deformation valid throughout the periodic table is outlined. Some open problems related to the development of a unified microscopic theory of nuclear collective motion are discussed.

First two major classes of models for nuclear dynamics are considered, namely onebody (mean-field) models and A -body (molecular-dynamics) models, and some areas in need of further progress are identified; a brief description is also given of a recently developed quasi-classical molecular-dynamics model that incorporates a Pauli potential and mimics nuclear systems well. Then the attention is turned towards statistical models of multifragmentation, and recent work on contrasting sequential binary breakup with prompt multifragmentation is described.

The wall-and-window formula, describing the macroscopic energy dissipation for two nuclei in relative motion, is generalized to include the dissipation associated with a time rate of change of the mass asymmetry degree of freedom. The additional term is crucial for the possibility of understanding the existence of deep-inelastic nuclear reactions.