Nuclear isospin diffusivity

Abstract

The isospin diffusion and other irreversible phenomena are discussed for a twocomponent nuclear Fermi system. The set of Boltzmann transport equations, such as employed for reactions, are linearized, for weak deviations of a system from uniformity, in order to arrive at nonreversible fluxes linear in the nonuniformities. Besides the diffusion driven by a concentration gradient, also the diffusion driven by temperature and pressure gradients is considered. Diffusivity, conductivity, heat conduction and shear viscosity coefficients are formally expressed in terms of the responses of distribution functions to the nonuniformities. The linearized Boltzmann-equation set is solved, under the approximation of constant form-factors in the distributionfunction responses, to find concrete expressions for the transport coefficients in terms of weighted collision integrals. The coefficients are calculated numerically for nuclear matter, using experimental nucleon-nucleon cross sections. The isospin diffusivity is inversely proportional to the neutron-proton cross section and is also sensitive to the symmetry energy. At low temperatures in symmetric matter, the diffusivity is directly proportional to the symmetry energy. PACS numbers: 21.65.+f,

Reaction dynamics with exotic nuclei

“…A good degree of isospin equilibration corresponds to a "convergence" to 0 of both R P (upper curves) and R T (lower curves). From our results we conclude that an asystiff-like EOS provides a better agreement to the experimental observations, shown as arrows in Fig.5-20, [198,216]. In fact in the MSU experiment there is no particular selection on binary events.…”

“…In Refs. [198,216] a possible explanation was proposed related to the observation that below normal density the asysoft EOS has a larger value of symmetry energy. Therefore an enhanced isospin equilibration has to be triggered if the diffusion takes place at lower density.…”

“…When the ions in the entrance channel have a different N/Z ratio, we expect that isospin diffusion will lead the system towards a more uniform asymmetry distribution. The degree of equilibration, correlated to the interaction time, may provide insights on transport properties of fermionic systems [213,214], in particular on the diffusion coefficient of asymmetric nuclear matter, [215,216].…”

Influence of in-mediumNNcross sections, symmetry potential, and impact parameter on isospin observables

“…Previous studies have shown that larger in-medium NN cross sections enhance isospin diffusion [48], and so the value of R i in case 1 (free cross sections, inverted triangles) should be smaller than the R i values for case 2 (reduced cross sections, open squares). But as Fig.…”

Double neutron-proton differential transverse flow as a probe for the high density behavior of the nuclear symmetry energy

“…Several potentially useful observables, such as, the free neutron/proton ratio [9], the isospin fractionation [10,11,12,13,14,15], the neutron-proton correlation function [16], t/ 3 He [17,18], the isospin diffusion [19,20], the proton differential elliptic flow [21] and the π − /π + ratio [22,23,24,25] have been proposed in the literature. The concept of the neutron-proton differential flow was first introduced by one of us [26] several years ago.…”