Jørgen Randrup

One-body dissipation and the super-viscidity of nuclei☆

Abstract This is a study of a type of fluid dynamics dominated by a “one-body” dissipation mechanism expected to be relevant for an assembly of particles whose mean free paths are comparable to or larger than the size of the system. Two simple dissipation formulas are derived, one relevant for the process of nuclear fission and the other for nuclear collisions. The resulting predictions, free of adjustable parameters, are compared quantitatively with measured fission-fragment kinetic energies and qualitatively with nucleus-nucleus collision data. The one-body dissipation concept is also tested against classical and quantal computer studies of particles in a deforming potential well. This brings out special effects associated with the symmetries of the well and points to a macroscopic dynamics of nuclear deformations which, except for super-fluidity at very low temperatures, consists of a smooth background dominated by one-body dissipation (“super-viscidity” of nuclei), on which are superposed modifications due to symmetries and quantization.

The Disassembly of Nuclear Matter

A statistical model is applied for multi-fragment final states in nuclear collisions with bombarding energies E/A ≈ 100 MeV. A portion of the intermediate system formed is assumed to decay according to the available classical non-relativistic phase space, calculated in a grand canonical ensemble. The model correlates and predicts many experimental observables in terms of three parameters: the available energy per nucleon, the isospin asymmetry, and the effective interaction volume.

Fluctuations in one-body dynamics☆

Abstract With a view towards nuclear dynamics at intermediate energies, we develop a transport treatment for systems whose reduced one-body phase-space density exhibits a markovian time evolution. A moment expansion of the corresponding Fokker-Planck equation leads to closed equations for the temporal evolution of the ensemble average of the phase-space occupancy f( r, p ) and its correlation function. The general properties of the associated relaxation processes are illustrated and the utility of the formulation for studies of nuclear collisions is discussed.

Baryon-strangeness correlations: a diagnostic of strongly interacting matter.

The correlation between baryon number and strangeness elucidates the nature of strongly interacting matter, such as that formed transiently in high-energy nuclear collisions. This diagnostic can be extracted theoretically from lattice QCD calculations and experimentally from event-by-event fluctuations. The analysis of present lattice results above the critical temperature severely limits the presence of qq bound states, thus supporting a picture of independent (quasi)quarks.

Nuclear one-body proximity friction

We study the friction between two juxtaposed leptodermous systems in relative motion, arising from the exchange of particles between them. For gently curved geometries a proximity treatment is valid and leads to a factorization into a simple geometrical factor and a universal key function related to the flux between two parallel surfaces as a function of their separation. On the basis of the nuclear Thomas-Fermi approximation, this key function is calculated and tabulated; a simple analytical approximation is also given.

Explosion-evaporation model for fragment production in medium-energy nuclear collisions☆

Abstract Statistical considerations are applied to the multifragment disassembly of a piece of hot nuclear matter created in medium-energy nuclear collisions. A two-stage model, consisting of a quick explosion and a slower evaporation, is presented. Results are compared to those of previous simpler calculations and to recent experimental data. The agreement is encouraging for a realistic range of the main parameters of the model: the available energy per nucleon, the isospin asymmetry, and the extension of the primary explosion in space and time.

MICROCANONICAL SIMULATION OF NUCLEAR DISASSEMBLY

Abstract We formulate a model for the disassembly of a highly excited finite nuclear source into interacting nuclear fragments. Monte Carlo sampling of the exact microcanonical and canonical ensemble provides many-fragment configurations at the effective freeze-out stage. The effect of including the interaction between the fragments is significant and an elaboration of the model that allows for a nucleon vapor suggests the existence of a first-order phase transition.

STATISTICAL SIMULATION OF COMPLETE EVENTS IN ENERGETIC NUCLEAR COLLISIONS

Abstract A statistical model is developed for the generation of complete multifragment events in medium-energy nuclear collisions. Based on simple geometrical considerations, the collision system is divided into a few (participant/spectator) sources that disassemble independently. The sufficiently excited sources quickly explode into pions, nucleons, and composite, possibly particle unstable, nuclei. The different final states compete according to their microcanonical weight. The less excited sources, and the unstable explosion products, deexcite by sequential light-particle emission. The model has been implemented as a Monte Carlo computer code that is sufficiently efficient to permit generation of large event samples. The analysis of such multifragment events is addressed and some illustrative applications are discussed.

New developments in the calculation of β-strength functions

Abstract We have further developed a QRPA model that uses single-particle levels and wave-functions as the starting point for calculating Gamow-Teller β-strength functions. In our enhanced version Nilsson, Woods-Saxon, or folded-Yukawa wave functions and single-particle energies may serve as the starting point for determining the wave functions of the mother and daughter nuclei involved in the β-decay. Pairing may be treated in either the BCS or the Lipkin-Nogami approximation. To account for the retardation of low-energy GT decay rates we add, as in the earlier model, a simple residual interaction specific to GT decay, namely V GT = : β 1− · β 1+ :, to the hamiltonian. This residual interaction is studied in the RPA approximation. In the case of odd-mass nuclei the Δν = 0 transitions are generally treated in a first-order perturbation expansion. We found that these expansions occasionally break down, and have modified them to avoid the singularities. The odd-odd case is treated in a way analogous to the odd- A case by considering one or the other of the odd particles as a spectator for Δν = 0 and both as spectators for Δν = 2. As a final extension of the earlier model, we also allow the unpaired odd particle to be in an excited state. We use the enhanced model to calculate Gamow-Teller β-strength functions, β-decay half-lives, and β-delayed neutron emission probabilities for nuclei in several regions of the periodic system, but with the main emphasis on the rare-earth region.

Early recognition of clusters in molecular dynamics

Abstract We propose a novel method for recognizing bound clusters in molecular dynamics simulations of nuclear collisions. Demanding that each particle in a given cluster is bound with respect to that cluster, we employ a Metropolis procedure (akin to simulated annealing) to maximize the cluster binding. Remarkably, the cluster can be identified approximately well before spatial separation occurs.