On the model for the diffusion of hydrogen in titanium hydride

The diffusion parameters of hydrogen in the gamma phase of titanium hydride are discussed in the light of nuclear magnetic resonance measurements and inelastic neutron scattering data. A comparison with a previously proposed electrostatic model for diffusion is made. It is shown that the electrostatic model is not satisfactory in explaining hydrogen diffusion in titanium. An alternate potential well system based on inelastic neutron scattering data is shown to be consistent with nuclear magnetic resonance measurements. The model can in principle be used for estimating the tunneling contribution to diffusion. The potential well shape is consistent with the notion that for hydrogen diffusion in titanium, the activation energy is given by the difference between the ground state and the well height, in contrast to the case of hydrogen diffusion in niobium where the activation energy is less than this value.     

Thermal neutron scattering from a hydrogen-metal system in terms of a general multi-sublattice jump diffusion model—I: Theory

A Multi-Sublattice Jump Diffusion Model (MSJD) for hydrogen diffusion through interstitial-site lattices is presented. The MSJD approach may, in principle, be considered as an extension of the Rowe et al.[1] model. Jump diffusion to any neighbours with different jump times which may be asymmetric in space is discussed. On the basis of the model a new method of calculating the diffusion tensor is advanced. The quasielastic, double differential cross section for thermal neutron scattering is obtained in terms of the MSJD model. The model can be used for systems in which interstitial jump diffusion of impurity particles occurs. In Part II the theoretical results are compared with those for quasielastic neutron scattering from the αNbH_x system.     

模拟短脉冲激光在介质中传输的扩散综合有限元法

建立有限元模型,通过求解瞬态辐射传输方程模拟短脉冲激光在半透明介质中的传输.针对散射占优性半透明介质内辐射传输求解效率较差的问题,采用扩散综合加速迭代算法,提高计算效率,缩短计算时间.结果表明:采用精确解析式描述脉冲激光散射源项的求解策略可以获得准确的计算结果,精确地模拟快速变化的波前,不会产生数值扩散和数值振荡.此外,扩散综合迭代算法的计算时间仅为源项迭代的50%~60%.     

Characterizing diffusive surfaces using scattering and diffusion coefficients

This paper presents the effects of height and coverage density of diffusive surfaces through the measurements of scattering and diffusion coefficients using scale models. Measurements were conducted using a 1:10 scale model. Scattering coefficients of 1D, 2D, and 3D diffuser profiles as well as diffusion coefficients of the 3D diffuser profile (hemisphere) were measured. It was found that the structural height of the diffuser is closely related to the frequency characteristics of the scattering coefficients, while the coverage density of the diffuser is closely related to the values of the scattering coefficients. Diffusion coefficients were less sensitive to structural height and coverage density than were the scattering coefficients. Using the measurement results, scattering and diffusion coefficients were estimated. In addition, the relationship between scattering and absorption coefficients was discussed.     

Precision measurement of the muonic 5-4 transitions in Pb and 4-3 transitions in ba as a test for the validity of QED

The real parts of the optical model potentials for 104 MeV alpha-particle and 156 MeV⁶Li ion scattering from^{40, 48}Ca are calculated in terms of folding model approaches. The validity of different procedures is tested by comparing the differential cross section predictions with experimental data measured with high angular accuracy. It is found that a refined folding potential accounting for density dependence of an effective nucleon-nucleon interaction is appropriate for alpha particle scattering without any parameter adjustment. However, for⁶Li ion scattering renormalization of the depth of the real potential is necessary.     

电解质对浓悬浮液中胶体颗粒扩散特性的影响

本文利用相位调制光纤低相干动态光散射装置, 研究了不同物质量浓度下电解质 (NaCl及 BaCl₂) 对浓悬浮液中聚苯乙烯胶体颗粒扩散特性的影响. 实验结果表明, 当电解质浓度低于0.01 mol/L 时, 恒温条件下浓悬浮液中聚苯乙烯胶体颗粒扩散系数随电解质离子浓度以及离子化合价的增大而增大, 实验测量得到的扩散系数与Stern模型所得到的扩散系数符合较好. 关键词： 测量 电解质 浓悬浮液 低相干动态光散射     

Mechanisms of ultrasonic modulation of multiply scattered incoherent light based on diffusion theory

An analytic equation interpreting the intensity of ultrasound-modulated scattering light is derived,based on diffusion theory and previous explanations of the intensity modulation mechanism.Furthermore,an experiment of ultrasonic modulation of incoherent light in a scattering medium is developed.This analytical model agrees well with experimental results,which confirms the validity of the proposed intensity modulation mechanism.The model supplements the existing research on the ultrasonic modulation mechanism of scattering light.     

Coupling multimodelling with local mesh refinement for the numerical computation of laminar flames

In this work, we propose a twofold adaptive method for the simulation of steady reactive flows. On the one hand, locally refined meshes are used. On the other hand, two types of diffusion models are applied: a simple Fick law and a more accurate multicomponent diffusion model. The diffusion model is changed locally throughout the computational domain. An analytically derived a posteriori error estimator provides reliable information on where to refine the mesh and where to choose the appropriate diffusion model. During the adaptation process, discretization and modelling errors are equilibrated. Numerical results are presented for ozone and hydrogen laminar flames.     

Transport of quantum noise through random media

We present an experimental study of the propagation of quantum noise in a multiple scattering random medium. Both static and dynamic scattering measurements are performed: the total transmission of noise is related to the mean free path for scattering, while the noise frequency correlation function determines the diffusion constant. The quantum noise observables are found to scale markedly differently with scattering parameters compared to classical noise observables. The measurements are explained with a full quantum model of multiple scattering.     

Dynamics of water molecules adsorbed by silica and resin SGK-7

This paper has presented neutron spectroscopy data on the dynamics of light water molecules adsorbed in the cation exchanger (ion-exchange resin) SGK-7 and on the surface of aerosils (highly dispersed pyrogenic silica) with different levels of hydration. The measurements have been performed on a DIN-2PI spectrometer (Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research, Dubna, Russia). The characteristics of the diffusive and vibrational motions of adsorbed water molecules have been determined from the experimental neutron scattering spectra. The data obtained in the quasi-elastic neutron scattering region have been analyzed using a model accounting for the effects of restricted translational and rotational diffusion. The results have demonstrated a significant decrease in the diffusion mobility of adsorbed water molecules as compared to conventional (bulk) water. In particular, the self-diffusion coefficient decreases several times, and the diffusion rate is the lower, the smaller is the thickness of the hydration layer. The dependences of the intensity and half-width of the quasi-elastic scattering peak on the magnitude of the neutron momentum transfer q in the scattering process exhibit a nonmonotonic character. This indicates manifestation of the effects of restricted translational diffusion, rotational diffusion, and jump diffusion. The partial distributions of vibrational frequencies of hydrogen atoms of water molecules adsorbed by the cation exchanger and aerosils have been obtained from the inelastic neutron scattering data.     

Quantitative theory of channeling particle diffusion in transverse energy in the presence of nuclear scattering and direct evaluation of dechanneling length

A refined equation for channeling particle diffusion in transverse energy taking into consideration large-angle scattering by nuclei is suggested. This equation is reduced to the Sturm–Liouville problem, allowing one to reveal both the origin and the limitations of the dechanneling length notion. The values of the latter are evaluated for both positively and negatively charged particles of various energies. New features of the dechanneling dynamics of positively charged particles are also revealed. First, it is demonstrated that the dechanneling length notion is completely inapplicable for their nuclear dechanneling process. Second, the effective electron dechanneling length of positively charged particle varies more than twice converging to a constant asymptotic value only at the depth exceeding the latter.     

Effect of the spatial distribution of probe beam on the results of measurements of the disperse composition of nanoparticles by dynamic light scattering method

The model of dynamic scattering on a finite ensemble of Brownian particles in liquid is considered. It is shown that an artifact characteristic relaxation time appears in the autocorrelation function of the scattered light intensity, which is much longer than the correlation time controlled by particle diffusion in the scattering volume.     

Experimental verification and refinement of the model of light-beam brightness fluctuations in a turbid medium with randomly nonuniform absorption

We study experimentally the brightness fluctuations of the light beam at the output of a layer of model turbid medium with strongly anisotropic scattering and randomly nonuniform distribution of an absorbing substance. The earlier developed theory of the light-field fluctuations is refined to broaden the limits of its applicability. It is shown that the refined model of fluctuations agrees well with the experiment and can be used for determining the parameters of absorbing irregularities of a turbid medium from the measured characteristics of brightness fluctuations of the light beam transmitted through the medium. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 3, pp. 247–263, March 2008.     

牙齿组织光学穿透深度有限元仿真分析

利用有限元方法对光在二维牙齿双层有限尺寸模型中传输的扩散方程进行求解,获取了光能在组织体内部的分布情况,并对牙釉质和牙本质在不同光学参量模型下的光学穿透深度进行仿真分析.结果发现,穿透深度随牙釉质散射系数的增大而减小,随牙本质的散射系数增大而增大.但牙釉质的穿透深度随散射系数的变化率（βe=0.007 97）要远远大于牙本质（βd=0.000 828）.采用Monte Carlo随机统计方法验证了本文有限元求解扩散方程的正确性.     

Positron diffusion in metals

Calculations are presented of the positron diffusion constant, mobility and other diffusion related quantities in simple metals. The mobility is found to be largely limited by the positron-phonon interaction which is treated in the deformation potential model. The effect of positron conduction electron scattering is also evaluated and found to be small. The effect of positron impurity scattering is discussed and detailed estimates given for Li in Al. In dilute alloys at moderate temperatures the positron phonon interaction will dominate also over this effect.     

Particle Transport through Scattering Regions with Clear Layers and Inclusions

This paper introduces generalized diffusion models for the transport of particles in scattering media with nonscattering inclusions. Classical diffusion is known as a good approximation of transport only in scattering media. Based on asymptotic expansions and the coupling of transport and diffusion models, generalized diffusion equations with nonlocal interface conditions are proposed which offer a computationally cheap, yet accurate, alternative to solving the full phase-space transport equations. The paper shows which computational model should be used depending on the size and shape of the nonscattering inclusions in the simplified setting of two space dimensions. An important application is the treatment of clear layers in near-infrared (NIR) spectroscopy, an imaging technique based on the propagation of NIR photons in human tissues.     

Theoretical and experimental study of the intensity distribution in biological tissues

Based on the diffusion approximate theory （DA）, a theoretical model about the distribution of the intensity of a narrow collimation beam illuminating on a semi-lnfinite biological tissue is developed. In order to verify the correctness of the model, a novel method of measuring the distributions of the intensity of light in Intralipid-10% suspension at 650 nm is presented and ts of the distributions of the distance-dependent intensity of scattering light in different directions are made. The investigations show that the results from our diffusion model are in good agreement with the experimental results beyond and in the areas around the light source, and the distance-dependent intensity in the incident direction attenuates approximately in the exponential form. Furthermore, our theoretic results indicate the anisotropic characteristics of the intensity in different directions of scattering light inside the biological tissue.     

Characterization and imaging in optically scattering media by use of laser speckle and a variable-coherence source

We demonstrate the application of laser-speckle statistics formed by a variable-coherence source illuminating a scattering medium, for determining the scattering parameter mu;(s)>(?) of a diffusion model for the medium. Furthermore, we apply this technique to visualize laterally localized inhomogeneities embedded within a highly scattering sample.     

A theoretical study of rotational and translational diffusion dynamics of molecules with a six-fold point symmetry adsorbed on a hexagonal lattice by neutron scattering

A complete analytical model for the rotational and translational diffusion of molecules with a six-fold point symmetry on a hexagonal lattice is presented. It can be applied, in particular, to the diffusion of benzene molecules adsorbed flat on the basal plane of graphite in the case of incoherent scattering. Under the weak hindered approximation, the classical mechanics framework and making use of the van Hove formalism of correlation functions, the intermediate scattering function and its Fourier transform, the scattering law, are both obtained. They can be expressed as sums of exponential decays or Lorentzian functions, respectively, containing the contribution of each of the dynamical processes taking place. In the case of benzene lying flat on the substrate we expect translational diffusion, continuous rotations of isolated molecules and hindered rotations of molecules within clusters. Each particular diffusive mechanism can be recognized owing to its particular signature in the dependence of the quasi-elastic broadening on the momentum transfer.