Nucleon mean free path in nuclear matter

In calculations of nuclear reaction yields at incident energies of some tens of MeV consistently better agreement with experiments is obtained by assuming a nucleon mean free path in nuclear matter longer than that deduced from the Fermi gas model and free nucleon-nucleon cross sections.     

Memory effects in the energy dissipation for slow collective nuclear motion

The energy dissipation in slow collective nuclear motion is considered as a combined effect of the diabatic production of particle-hole excitations and the subsequent equilibration by two-body collisions. Memory effects due to the long mean free path of the nucleons are treated analytically for an interacting Fermi gas within moving walls leading to a friction kernel (frequency-dependent friction coefficient) in the classical equation of collective motion.     

Comments on the state densities and the transition rates in the pre-equilibrium exciton model

It is proposed to renormalize the exciton-state densities used in models for precompound decay such that the summed state densities agree with the expressions employed in equilibrium statistical models. In this way a close fit can be guaranteed between preequilibrium model calculations and the results of equilibrium statistical models for the evaporative stage of the reaction. The consequences of this proposal for the internal transition rates of the pre-equilibrium exciton model are analyzed. The matrix element for the residual interaction is obtained not from a phenomenological parametrization, but from the nucleon mean free path in nuclear matter. It is demonstrated that the proposed renormalization, from one-component Fermi-gas formulas to two-fermion expressions for the state densities, leads to strongly improved agreement of the effective exciton-model values for the nucleon mean free path in nuclear matter with realistic estimates. It is proved that the particle-hole state densities for a two-component Fermi gas, summed over the allowed exciton-state numbers, agree with the phenomenological state-density expressions used in statistical Hauser-Feshbach models.     

Shortening of the nucleon''s mean free path in heavy ion collisions

The mean free path of nucleons in heavy ion collisions is most essential for the development of nuclear collective phenomena. We discuss the effect of the nucleon Fermi motion in nuclei for shortening the mean free path. This Fermi motion together with the prior Pauli effect makes nuclei nontransparent for heavy ions in the medium energy domain.     

Kinetic parameters in the nuclear Fermi gas model at finite temperatures

The validity of the dilute Fermi gas model for the evaluation of transport parameters in nuclear matter is examined in the framework of quantal kinetic theory. The consistency of the approximations involved in the calculations of the collision rate between weakly interacting nucleons is analyzed, considering several ways of representing the residual interaction, namely via zero-range, medium-range, and short-momentum-spread forces. The theoretical mean free path is derived, with a proper handling of the collision kernel in a nuclear kinetic equation, and computed as a function of temperature and single-particle energy for the interactions in the weak-coupling approximation. The competition among interaction range and quantal and kinetic length scales is discussed.     

Emission and absorption of neutrinos by nonrelativistic neutrons in a magnetic field

The neutrino luminosity of a nonrelativistic nondegenerate neutron gas in a magnetic field owing to the flip of an anomalous magnetic moment, as well as the mean free path of the neutrino due to the absorption in a magnetized neutron gas, has been calculated using the neutron density matrix in the magnetic field obtained in this work. The Fermi energy and partial concentrations of the degenerate neutron gas in the magnetic field have been determined. The astrophysical applications are discussed.     

Single-particle occupation function and indirect ion-ion interaction in liquid metals

Using Bardeen's disorder scattering model we calculate the single-particle occupation function for liquid metals. Our results demonstrate that the blurring of the Fermi surface is considerable in liquids with short mean free path. Finally, using an appropriate dielectric function, we obtain the effective ion-ion interaction which shows how the Friedel oscillations are damped in the liquid.     

Nuclear fluid dynamics

For energies where the mean free nucleon path is comparable to the nuclear size, there is no fast local relaxation in nuclear collective motion. Therefore, in a fluid-dynamical approach, the intrinsic energy-density functional must take the distortion of the local Fermi surface into account. This can be achieved by a dynamical Thomas Fermi method which leads to additional restoring forces for the collective modes, except for the casesL=0 andL=1 where usual hydrodynamics is sufficient.     

Ladungstransport in dünnen Filmen

The electronic charge transport in thin indium films of high purity is investigated (direct current, eddy current and magnetoresistance in a magnetic field perpendicular to the surface) at low temperatures where surface scattering of the electrons becomes important. A method is described which allows the determination of the mean free path (corresponding to a Sommerfeld-model for the conduction electrons) for a single specimen by measuring both the direct current conductivity and the decay time of eddy currents. For a multivalent metal like indium, with a Fermi surface extending over different Brillouin zones, the “mean free path” evaluated in this way can no longer be regarded as the mean value of the electronic free path calculated over the whole Fermi surface. It is shown, that for indium the ?mean free path” evaluated from our size effect investigations will mainly correspond to the electrons of the second zone, and that the electrons of the third zone only neglectingly contribute to the electronic charge transport.     

Nuclear transparency due to the pauli exclusion principle and the study of densities in the nuclear interior

24.5 MeV proton elastic scattering data for ³²S, ¹¹⁸Sn and ²⁰⁸Pb are analyzed in the framework of an optical-model potential. This potential is calculated by a folding procedure using interactions corrected for Pauli effects and Fermi motion. Neutron densities are extracted in a model-independent fashion. The sensitivity of low-energy proton scattering to the density in the nuclear interior due to the long mean free path is established.     

Asymptotic Behavior for Rayleigh Problem Based on Kinetic Theory

We investigate the dynamics of the gas bounded by an infinite flat plate which is initially in equilibrium and set at some instant impulsively into uniform motion in its own plane. We use the Boltzmann equation to describe intermolecular collisions and assume the diffuse reflection to describe the interaction of the gas with the boundary. The Mach number of the plate is assumed to be small so that we can linearize the Boltzmann equation as well as the boundary condition. We show that the asymptotic behavior of the gas represents a perturbation to the free molecular gas when the time is much less than the mean free time. On the other hand, if the time is much greater than the mean free time, we show that the gas dynamics is governed by the linearized Navier–Stokes equation with a slip flow on the boundary and establish a boundary layer correction with thickness of the order of the mean free path. We also establish the singularity of velocity distribution function along the particle trajectory near the boundary.     

Influence of the separation energy on the radius of neutron rich nuclei

The intensive studies of equilibration processes in heavy ion reactions have produced a need for information on nuclear level densities at high energies. In a recent paper, it was concluded that standard Fermi gas formulas will be incorrect by exponential factors at energies above 100 MeV. Exact calculations of the nuclear level density in bases as large as 10³⁸ have been made and are compared with Fermi gas formulas. Two possible alternative forms are considered. Both forms produce much better agreement at high energies than does the Fermi gas model. All calculations reported are for non-interacting Fermions, but the effects expected from the two-body interaction are briefly examined. These considerations have consequences not only in heavy ion physics but also in astrophysics.     

核子相对论微观光学势、薛定谔等效势和平均自由程对核密度及温度的依赖关系

在Walecka模型的基础上,应用热动力学理论和Dirac-Bruckner-Hartree-Fock方法,研究了有限温度不同密度下核子相对论微观光学势及其相应的薛定谔等效势和平均自由程.计算结果表明,核子薛定谔等效势和平均自由程对核密度的依赖相当敏感,当核密度增大时对核密度的依赖变得更为敏感.     

Systematics of nuclear level density parameters of nuclei deficient in one neutron

Photoneutron mean energies of 38 elements were measured as a function of peak bremsstrahlung energy for elements with 23 ≦ Z ≦ 83. Results are compared with neutron mean energies calculated from statistical theory, using for nuclear level densities modified Fermi gas formulae with and without pairing corrections and a constant temperature formula. Except near closed shells the Fermi gas formula with pairing corrections gives reasonable to good correlation between experimental and theoretical data. Derived values of the nuclear level density parameter a-except near Z = 82-are in quantitative agreement with those from recent neutron resonance data.     

Calculation of the residual resistivity based on multiple-plane-waves and iterative solutions to the Boltzmann equation: Application to aluminium alloys

We describe a multiple-plane-wave pseudopotential calculation for the residual resistivity in alloys. The electronic wavefunctions, the Fermi surface, and the scattering rates are all determined from a 4-OPW calculation, while the mean free path is determined from an iterative technique which is developed for solving the Boltzmann equation. Numerical results obtained for Al alloys reveal that in many cases the OPW-mixing leads to dramatic enhancement of the residual resistivity over the free electron value, and that this enhancement improves agreement with experiment.     

Nuclear matter theory revisited

Claiming that the usual Fermi sea of plane waves Hartree-Fock vacuum state cannot be the true (or stablest) ground vacuum state for nuclear matter, since it violates a monotonicity requirement, we discuss some recent attempts in facing the problem of how to find such a (zero-order) state. The one-dimensional Fermi gas with zero- and finite-range interactions, as well as self-consistent alpha-clustering effects in three-dimensional nuclear matter, are employed as examples to illustrate the concepts involved in such a search.     

Comments on nucleon mean free paths in nuclear matter

It is suggested that recent evidence cited for a fourfold increase in the mean free path of nucleons in nuclear matter results from an error in formulation of the exciton model. The literature cited as being in support of the longer mean free path is reviewed and found to be in disagreement with the new value, and in quite reasonable agreement with results used over the past 30 years.     

Studying Validity of Single-Fluid Model in Inertial Confinement Fusion

The validity of single-fluid model in inertial confinement fusion simulations is studied by comparing the results of the multi- and single-fluid models. The multi-fluid model includes the effects of collision and interpenetration between fluid species. By simulating the collision of fluid species, steady-state shock propagation into the thin DT gas and expansion of hohlraum Au wall heated by lasers, the results show that the validity of single-fluid model is strongly dependent on the ratio of the characteristic length of the simulated system to the particle mean free path. When the characteristic length L is one order larger than the mean free path A, the single-fluid model＇s results are found to be in good agreement with the multi-fluid model＇s simulations, and the modeling of single-fluid remains valid. If the value of L/A is lower than 10, the interpenetration between fluid species is significant, and the single-fluid simulations show some unphysical results; while the multi-fluid model can describe well the interpenetration and mix phenomena, and give more reasonable results.     

与核密度及温度相关的核子相对论微观光学势

在Walecka模型、热场动力学和相对论Dirac-Bruckner-Hartree-Fock计算结果基础上,研究了在不同核密度和各种温度情况下的核子相对论微观光学势及其相应的薛定谔等效势和平均自由程,计算结果表明,对于不同的温度,核子的薛定谔等效势与平均自由程随核密度变化较为敏感,而核温度对核子的薛定谔等效势和平均自由程的影响随着核密度的增加变大.     

Effect of ellipsodial fermi surfaces and relaxation processes on the indirect exchange in the Bloembergen-Rowland approximation

We investigate the effect of relaxation processes (mean free path) and ellipsodial Fermi surfaces on the indirect exchange interaction in the Bloembergen-Rowland approximation and obtain a simple analytic expression which oscillates with mean free path and emphasizes the relative importance of this parameter on the oscillatory behaviour of the indirect exchange.