Thermodynamics of nuclei in thermal contact

The behaviour of a di-nuclear system in the regime of strong pairing correlations is studied with the methods of statistical mechanics. It is shown that the thermal averaging is strong enough to assure the application of thermodynamical methods to the energy exchange between the two nuclei in contact. In particular, thermal averaging justifies the definition of a nuclear temperature.Comment: 9 pages, 1 figur

Final excitation energy of fission fragments

We study how the excitation energy of the fully accelerated fission fragments is built up. It is stressed that only the intrinsic excitation energy available before scission can be exchanged between the fission fragments to achieve thermal equilibrium. This is in contradiction with most models used to calculate prompt neutron emission where it is assumed that the total excitation energy of the final fragments is shared between the fragments by the condition of equal temperatures. We also study the intrinsic excitation-energy partition according to a level density description with a transition from a constant-temperature regime to a Fermi-gas regime. Complete or partial excitation-energy sorting is found at energies well above the transition energy.Comment: 8 pages, 3 figure

Modelling of compound nucleus formation in fusion of heavy nuclei

A new model that includes the time-dependent dynamics of the single-particle (s.p.) motion in conjunction with the macroscopic evolution of the system is proposed for describing the compound nucleus (CN) formation in fusion of heavy nuclei. The diabaticity initially keeps the entrance system around its contact configuration, but the gradual transition from the diabatic to the adiabatic potential energy surface (PES) leads to fusion or quasifission. Direct measurements of the probability for CN formation are crucial to discriminate between the current models.Comment: 4 pages,2 figures,1 table, Submitted to PR

Liquid-Drop Model and Quantum Resistance Against Noncompact Nuclear Geometries

The importance of quantum effects for exotic nuclear shapes is demonstrated. Based on the example of a sheet of nuclear matter of infinite lateral dimensions but finite thickness, it is shown that the quantization of states in momentum space, resulting from the confinement of the nucleonic motion in the conjugate geometrical space, generates a strong resistance against such a confinement and generates restoring forces driving the system towards compact geometries. In the liquid-drop model, these quantum effects are implicitly included in the surface energy term, via a choice of interaction parameters, an approximation that has been found valid for compact shapes, but has not yet been scrutinized for exotic shapes.Comment: 9 pages with 3 figure

Mechanical and chemical spinodal instabilities in finite quantum systems

Self consistent quantum approaches are used to study the instabilities of finite nuclear systems. The frequencies of multipole density fluctuations are determined as a function of dilution and temperature, for several isotopes. The spinodal region of the phase diagrams is determined and it appears that instabilities are reduced by finite size effects. The role of surface and volume instabilities is discussed. It is indicated that the important chemical effects associated with mechanical disruption may lead to isospin fractionation.Comment: 4 pages, 4 figure

Memory effects on descent from nuclear fission barrier

Non-Markovian transport equations for nuclear large amplitude motion are derived from the collisional kinetic equation. The memory effects are caused by the Fermi surface distortions and depend on the relaxation time. It is shown that the nuclear collective motion and the nuclear fission are influenced strongly by the memory effects at the relaxation time $\tau \geq 5\cdot 10^{-23}{\rm s}$. In particular, the descent of the nucleus from the fission barrier is accompanied by characteristic shape oscillations. The eigenfrequency and the damping of the shape oscillations depend on the contribution of the memory integral in the equations of motion. The shape oscillations disappear at the short relaxation time regime at $\tau \to 0$, which corresponds to the usual Markovian motion in the presence of friction forces. We show that the elastic forces produced by the memory integral lead to a significant delay for the descent of the nucleus from the barrier. Numerical calculations for the nucleus $^{236}$U shows that due to the memory effect the saddle-to-scission time grows by a factor of about 3 with respect to the corresponding saddle-to-scission time obtained in liquid drop model calculations with friction forces.Comment: 22 pages, 8 figures, submitted to Phys. Rev.

Modeling Complex Nuclear Spectra - Regularity versus Chaos

A statistical analysis of the spectrum of two particle - two hole doorway states in a finite nucleus is performed. On the unperturbed mean-field level sizable attractive correlations are present in such a spectrum. Including particle-hole rescattering effects via the residual interaction introduces repulsive dynamical correlations which generate the fluctuation properties characteristic of the Gaussian Orthogonal Ensemble. This signals that the underlying dynamics becomes chaotic. This feature turns out to be independent of the detailed form of the residual interaction and hence reflects the generic nature of the fluctuations studied.Comment: 8 pages of text (LATEX), figures (not included, available from the authors), Feb 9

Characterization of Landau-Zener Transitions in Systems with Complex Spectra

This paper is concerned with the study of one-body dissipation effects in idealized models resembling a nucleus. In particular, we study the quantum mechanics of a free particle that collides elastically with the slowly moving walls of a Bunimovich stadium billiard. Our results are twofold. First, we develop a method to solve in a simple way the quantum mechanical evolution of planar billiards with moving walls. The formalism is based on the {\it scaling method} \cite{ver} which enables the resolution of the problem in terms of quantities defined over the boundary of the billiard. The second result is related to the quantum aspects of dissipation in systems with complex spectra. We conclude that in a slowly varying evolution the energy is transferred from the boundary to the particle through Landau$-$Zener transitions.Comment: 24 pages (including 7 postcript figures), Revtex. Submitted to PR

Random Matrices and Chaos in Nuclear Physics

The authors review the evidence for the applicability of random--matrix theory to nuclear spectra. In analogy to systems with few degrees of freedom, one speaks of chaos (more accurately: quantum chaos) in nuclei whenever random--matrix predictions are fulfilled. An introduction into the basic concepts of random--matrix theory is followed by a survey over the extant experimental information on spectral fluctuations, including a discussion of the violation of a symmetry or invariance property. Chaos in nuclear models is discussed for the spherical shell model, for the deformed shell model, and for the interacting boson model. Evidence for chaos also comes from random--matrix ensembles patterned after the shell model such as the embedded two--body ensemble, the two--body random ensemble, and the constrained ensembles. All this evidence points to the fact that chaos is a generic property of nuclear spectra, except for the ground--state regions of strongly deformed nuclei.Comment: 54 pages, 28 figure

Thermal multifragmentation in p + Au interactions at 2.16, 3.6 and 8.1 GeV incident energies

Multiple emission of intermediate-mass fragments has been studied for the collisions p + Au at 2.16, 3.6 and 8.1 GeV with the FASA setup. The mean IMF multiplicities for events with at least one IMF are equal to 1.7, 1.9 and 2.1 (±0.2) respectively. The multiplicity, charge distributions and kinetic energy spectra of IMF are described in the framework of a intranuclear cascade model followed by the statistical multifragmentation model. However, between the two parts of the calculation the excitation energies and the residual masses and charges are modified to take into account the losses during expansion. The results support a scenario of true thermal multifragmentation of a hot and expanded target spectator