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The microscopic origin of Stokes’ law for a hard sphere system is examined from two points of view. First, we derive a set of hydrodynamic equations with source terms, which account for the presence of a Brownian particle in the fluid, from the generalized Langevin equation. The friction coefficient can be expressed in terms of averages of the hydrodynamic fields over the Brownian particle. The solution of the source hydrodynamic equations for these averaged fields leads to the slip Stokes’ law result. Second, we consider the problem from kinetic theory and show that the same source hydrodynamic equations are obtained when projections onto hydrodynamic states are made.

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Production rates of volatile elements following spallation reaction of 1.4 GeV protons on a liquid Pb/Bi target have been measured. The experiment was performed at the ISOLDE facility at CERN. These data are of interest for the developments of targets for accelerator driven systems such as MEGAPIE. Additional data have been taken on a liquid Pb target. Calculations were performed using the FLUKA and MCNPX Monte Carlo codes coupled with the evolution codes ORIHET3 and FISPACT using different options for the intra-nuclear cascades and evaporation models. Preliminary results from the data analysis show good comparison with calculations for Hg and for noble gases. For other elements such as I it is apparent that only a fraction of the produced isotopes is released. The agreement with the experimental data varies depending on the model combination used. The best results are obtained using MCNPX with the INCL4/ABLA models and with FLUKA. Discrepancies are found for some isotopes produced by fission using the MCNPX with the Bertini intranuclear cascade model coupled with the Dresner evaporation model.

A microscopic derivation of the translational and rotational friction coefficients for a Browian particle in a dilute rough sphere fluid is presented. A projection operator method is used to couple the test particle’s motion to the collective fluid fields, and the resulting generalized Langevin equations are averaged over fluctuations to yield source hydrodynamic equations. After averaging over the surface and volume of the spherical Brownian particle, these equations are algebraically manipulated to give expressions for the force and torque exerted on the tagged particle by the surrounding fluid. The Stokes’ law relations, for partial stick boundary conditions, are obtained.