[en] The linear Boltzmann equation for the transport of neutral particles is investigated with the objective of generating a benchmark-quality calculation for the three-dimensional searchlight problem in a semi-infinite medium. The derivation assumes stationarity, one energy group, and isotropic scattering. The surface and interior scalar fluxes are the quantities of interest. The source considered is a pencil-beam incident at a point on the surface of a semi-infinite medium. The scalar flux will have two-dimensional variation only if the beam is normal; otherwise it is three-dimensional. The solutions are obtained by using Fourier and Laplace transforms and standard numerical techniques. Comparisons of these numerical solutions to results from the probabilistic code MCNP are also provided

[en] Classical transport of neutral particles in a binary, scattering, stochastic media is discussed. It is assumed that the cross-sections of the constituent materials and their volume fractions are known. The inner structure of the media is stochastic, but there exist a statistical knowledge about the lump sizes, shapes and arrangement. The transmission through the composite media depends on the specific heterogeneous realization of the media. The current research focuses on the averaged transmission through an ensemble of realizations, frm which an effective cross-section for the media can be derived. The problem of one dimensional transport in stochastic media has been studied extensively [1]. In the one dimensional description of the problem, particles are transported along a line populated with alternating material segments of random lengths. The current work discusses transport in two-dimensional stochastic media. The phenomenon that is unique to the multi-dimensional description of the problem is obstacle bypassing. Obstacle bypassing tends to reduce the opacity of the media, thereby reducing its effective cross-section. The importance of this phenomenon depends on the manner in which the obstacles are arranged in the media. Results of transport simulations in multi-dimensional stochastic media are presented. Effective cross-sections derived from the simulations are compared against those obtained for the one-dimensional problem, and against those obtained from effective multi-dimensional models, which are partially based on a Markovian assumption

[en] Magnetization relaxation rate in HTSC Y-Ba-Cu-O monocrystal is locally measured for the first time using the Faraday effect in Bi-containing ferrite garnet film, its light magnetization axis (LMA) lying in its plane. It is shown that the temperature dependence of the normalized rate of residual magnetization lozarithmic relaxation may be different for different parts of one and the same Y-Ba-Cu-O monocrystal

[en] Time-dependent one-speed neutron behavior is assessed for one-dimensional multiregion cylindrical geometry with multiplication and without delayed neutrons using an exact solution to the diffusion equation that supplements those reported previously for Cartesian and spherical geometries. Selected illustrative benchmark calculations are presented to illustrate the space-time evolution of the neutron flux. This trio of solutions creates the capability to examine the one-speed space-time neutron prompt behavior in an arbitrary number of neutronically dissimilar material regions for the three basic one-dimensional geometries using analytic solutions. (author)

[en] The Quasi-Diffusion (QD) method is a nonlinear algorithm for solving linear transport problems. Because the QD method utilizes both a transport sweep and a diffusion calculation within each iteration, it is operationally more complex than the Source Iteration (SI) method, which utilizes only a transport sweep within each iteration. However, the QD method often converges rapidly and with high accuracy, especially for optically thick regions with scattering ratios close to unity; these are the regions for which acceleration is most needed. A difficulty with the QD method is that because it is nonlinear, every scalar flux iterate must be positive at each point in the system. Also, the formulation of diffusion boundary conditions to optimize accuracy and speed of convergence is not obvious. In this paper, both of these issues is considered. Specifically, a new formulation of the QD method in spherical geometry is proposed to guarantee positivity of the analytic solution, and new diffusion boundary conditions are proposed in planar and spherical geometry that lead to more accurate and efficient solution algorithms. Also, ways to accurately and positively discretize the transport and diffusion equations, and extensive numerical results are presented. 14 refs., 1 fig., 9 tabs

[en] The authors introduce a new approach for characteristic based S_n transport on arbitrary polygonal meshes in XY geometry. They approximate a general surface as an arbitrary polygon and rotate to a coordinate system aligned with the direction of particle travel. They use exact moment balance equations on whole cells and subregions called slices and close the system by analytically solving the characteristic equation. The authors assume spatial functions for boundary conditions and cell sources and formulate analogous functions for outgoing edge and cell angular fluxes which exactly preserve spatial moments of the analytic solution. In principle, their approach provides the framework to extend characteristic methods formulated on rectangular grids to arbitrary polygonal meshes. The authors derive schemes based on step and linear spatial approximations. Their step characteristic scheme is mathematically equivalent to the Extended Step Characteristic (ESC) method but their approach and scheme differ in the geometry rotation and in the solution form. Their solutions are simple and permit edge-based transport sweep ordering

[en] For more than a decade, it has been known that exponential convergence on discrete transport problems was possible using adaptive Monte Carlo techniques. An adaptive Monte Carlo method that empirically produces exponential convergence on a simple continuous transport problem is described

[en] This chapter discussed the following subjects: Interactions of particle radiation i.e. neutron, alpha particles, beta particles, positrons; electromagnetic radiations;

[en] The scaling behavior of the closed trajectories of a moving particle generated by randomly placed rotators or mirrors on a square or triangular lattice is studied numerically. On both lattices, for most concentrations of the scatterers the trajectories close exponentially fast. For special critical concentrations infinitely extended trajectories can occur which exhibit a scaling behavior similar to that of the perimeters of percolation clusters. At criticality, in addition to the two critical exponents τ=15/7 and d_f=7/4 found before, the critical exponent σ=3/7 appears. This exponent determines structural scaling properties of closed trajectories of finite size when they approach infinity. New scaling behavior was found for the square lattice partially occupied by rotators, indicating a different universality class than that of percolation clusters. Near criticality, in the critical region, two scaling functions were determined numerically: f(x), related to the trajectory length (S) distribution n_S, and h(x), related to the trajectory size R_S (gyration radius) distribution, respectively. The scaling function f(x) is in most cases found to be a symmetric double Gaussian with the same characteristic size exponent σ=0.43 ∼3/7 as at criticality, leading to a stretched exponential dependence of n_s on S, n_s∼exp(-S^6/7). However, for the rotator model on the partially occupied square lattice an alternative scaling function is found, leading to a new exponent σ'=1.6±0.3 and a superexponential dependence of n_s on S. h(x) is essentially a constant, which depends on the type of lattice and the concentration of the scatterers. The appearance of the same exponent σ=3/7 at and near a critical point is discussed

[en] The compact white dwarf x-ray binary AM her(culis) objects differ from the mass transfer white dwarf binary systems by containing a strongly magnetic white dwarf, B* ≅10-60 MG. The AM Her systems being relatively faint are not easily modeled. Thus the discovery of rapid, 1-3 s, optical quasi-periodic oscillations (QPOs) in some of the Am Her systems is quite interesting (Middleditch 19982). Middleditch suggested that QPOs are due to an instability of radiative shock waves discovered by langer. Chanmugam, and shaviv (1981) during time-dependent calculations of accretion flows onto magnetic white dwafts. If this is correct, then the OPOs can be used to study the radiating plasmas under conditions similar to those found in terrestrial magnetically confined fusion devices. Also they serve to place constraints on the properties of the AM-Her systems (e.g., see wood, imamura, and wolff 1993). The QPOs are observed in the optical emission, thus they are thought to be due to cyclotron emission from the radiating strongly magnetic shocked plasma. In this work, the effect of funnel's extension on the QOPs is studied. (author). 24 refs., 5 figs